Betelgeuse
Overview
 
Betelgeuse, also known by its Bayer designation
Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name...

 Alpha Orionis (α Orionis, α Ori), is the eighth brightest star in the night sky and second brightest star in the constellation of Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...

, outshining its neighbour Rigel
Rigel
Rigel is the brightest star in the constellation Orion and the sixth brightest star in the sky, with visual magnitude 0.18...

 (Beta Orionis) only rarely. Distinctly reddish-tinted, it is a semiregular variable star
Semiregular variable star
Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light...

 whose apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 varies between 0.2 and 1.2, the widest range of any first magnitude star.
Encyclopedia
Betelgeuse, also known by its Bayer designation
Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name...

 Alpha Orionis (α Orionis, α Ori), is the eighth brightest star in the night sky and second brightest star in the constellation of Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...

, outshining its neighbour Rigel
Rigel
Rigel is the brightest star in the constellation Orion and the sixth brightest star in the sky, with visual magnitude 0.18...

 (Beta Orionis) only rarely. Distinctly reddish-tinted, it is a semiregular variable star
Semiregular variable star
Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light...

 whose apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 varies between 0.2 and 1.2, the widest range of any first magnitude star. The star marks the upper right vertex
Vertex (geometry)
In geometry, a vertex is a special kind of point that describes the corners or intersections of geometric shapes.-Of an angle:...

 of the Winter Triangle and center of the Winter Hexagon
Winter Hexagon
The Winter Hexagon or Winter Circle/Oval is an asterism appearing to be in the form of a hexagon with vertices at Rigel, Aldebaran, Capella, Pollux/Castor, Procyon, and Sirius. It is mostly upon the Northern Hemisphere's celestial sphere...

.

Classified as a red supergiant
Red supergiant
Red supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...

, Betelgeuse is one of the largest and most luminous stars known. If it were at the center of our Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, its surface would extend past the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

 possibly to the orbit of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and beyond, wholly engulfing Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

. However, with distance estimates in the last century that have ranged anywhere from 180 to 1,300 light years from Earth, calculating its diameter, luminosity and mass have proven difficult. Betelgeuse is currently thought to lie around 640 light years away, yielding a mean absolute magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...

 of about −6.05.

In 1920, Alpha Ori was the first star (after the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

) to have its angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

 measured. Since then, researchers have used a number of telescopes to measure this stellar giant, each with different technical parameters, often yielding conflicting results. Current estimates of the star's apparent diameter range from about 0.043 to 0.056 arcseconds. This is a moving target at best, as Betelgeuse appears to change shape periodically. Because of limb darkening
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...

, variability
Stellar pulsation theory – Regular versus irregular variability
Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. These fluctuations in stellar radius causes corresponding changes in the luminosity of the star. Astronomers are able to deduce this mechanism by measuring the spectrum and...

, and angular diameters that vary with wavelength
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

, the star remains a perplexing mystery. To complicate matters further, Betelgeuse has a complex, asymmetric envelope
Circumstellar envelope
Circumstellar envelope is the part of the star,having roughly spherical shape and not gravitationally bound to the star core.Usually circumstellar envelopes are formed from the dense stellar wind or presentbefore formation of the star...

 caused by colossal mass loss
Stellar mass loss
Stellar mass loss is a phenomenon observed in some massive stars. It occurs when a triggering event causes the ejection of a large portion of the star's mass. Stellar mass loss can also occur when a star gradually loses material to a binary companion or into interstellar space.-Causes:A number of...

 involving huge plumes of gas being expelled from its surface. There is even evidence of stellar companions
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...

 orbiting within this gaseous envelope, possibly contributing to the star's eccentric behavior.

Astronomers believe Betelgeuse is only 10 million years old, but has evolved rapidly because of its high mass. It is thought to be a runaway star from the Orion OB1 Association
Orion OB1 Association
The Orion OB1 stellar association is a contingent group of several dozen hot giant stars of spectral types O and B. Associated are thousands of lower-mass stars, and a number of protostars. It is part of the larger Orion Molecular Cloud Complex...

, which also includes the late type O and B stars in Orion's belt—Alnitak, Alnilam and Mintaka. Currently in a late stage of stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

, Betelgeuse is expected to explode as a type II supernova
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...

, possibly within the next million years.

Observational history

Betelgeuse and its red coloration have been noted since antiquity
Classical antiquity
Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

; the classical astronomer Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 described its color as ὑπόκιρρος (hypókirros), a term which was later described by a translator of Ulugh Beg
Ulugh Beg
Ulugh Bek was a Timurid ruler as well as an astronomer, mathematician and sultan. His commonly-known name is not truly a personal name, but rather a moniker, which can be loosely translated as "Great Ruler" or "Patriarch Ruler" and was the Turkic equivalent of Timur's Perso-Arabic title Amīr-e...

's Zij-i Sultani as rubedo, Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 for "ruddiness". In contrast, the historical record of Chinese astronomers during the first century BC mention Betelgeuse as having a yellow color. Prior to the modern systems of stellar classification
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

, Angelo Secchi
Angelo Secchi
-External links:...

 had created his own system of spectral analysis
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 with Antares
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...

 and Betelgeuse as the prototypes for his Class III (orange to red) stars.

With the history of astronomy
History of astronomy
Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, and astrological practices of pre-history: vestiges of these are still found in astrology, a discipline long interwoven with public and governmental astronomy, and not...

 intimately associated with mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

 and astrology
Astrology
Astrology consists of a number of belief systems which hold that there is a relationship between astronomical phenomena and events in the human world...

 prior to the scientific revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

, the red star, like the planet Mars that derives its name from a Roman war god
Mars (mythology)
Mars was the Roman god of war and also an agricultural guardian, a combination characteristic of early Rome. He was second in importance only to Jupiter, and he was the most prominent of the military gods worshipped by the Roman legions...

, has been closely associated with the martial archetype
Archetype
An archetype is a universally understood symbol or term or pattern of behavior, a prototype upon which others are copied, patterned, or emulated...

 of conquest for millennia, and by extension the motif of death and rebirth. In South African mythology, Betelgeuse was a lion watching in a predatory manner the three zebras represented by Orion's belt.

Herschel's discovery

The variation in Betelgeuse's brightness was first described in 1836 by Sir John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

, when he published his observations in Outlines of Astronomy; he noted an increase in activity from 1836–1840, followed by a subsequent reduction. In 1849, he noted a shorter cycle of variability which peaked in 1852. Later observers recorded unusually high maxima with an interval of several years, but only small variations from 1957 to 1967. The records of the American Association of Variable Star Observers
American Association of Variable Star Observers
Since its founding in 1911, the American Association of Variable Star Observers has coordinated, collected, evaluated, analyzed, published, and archived variable star observations made largely by amateur astronomers and makes the records available to professional astronomers, researchers, and...

 (AAVSO) show a maximum apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 (brightness) of 0.2 in the years 1933 and 1942, with a minimum fainter than magnitude 1.2 in both 1927 and 1941. This variability in brightness may explain why Johann Bayer
Johann Bayer
Johann Bayer was a German lawyer and uranographer . He was born in Rain, Bavaria, in 1572. He began his study of philosophy in Ingolstadt in 1592, and moved later to Augsburg to begin work as a lawyer. He grew interested in astronomy during his time in Augsburg...

, with the publication of his Uranometria
Uranometria
Uranometria is the short title of a star atlas produced by Johann Bayer.It was published in Augsburg, Germany, in 1603 by Christophorus Mangus under the full title Uranometria : omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa. This translates to...

in 1603, designated the star alpha as it may have rivalled the usually brighter Rigel (beta).

In 1920, Albert Michelson
Albert Abraham Michelson
Albert Abraham Michelson was an American physicist known for his work on the measurement of the speed of light and especially for the Michelson-Morley experiment. In 1907 he received the Nobel Prize in Physics...

 and Francis Pease mounted a 6 metre (20 ft) interferometer
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

 on the front of the 2.5 metre (100 inch) telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 at Mount Wilson Observatory
Mount Wilson Observatory
The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,715 foot peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles...

. Helped by John Anderson
John August Anderson
John August Anderson was an American astronomer. He was born in Rollag, a small community in Clay County, Minnesota to the south of Hawley....

, the trio measured the angular diameter of α Orionis at 0.047", a figure which resulted in a diameter of 3.84 × 108 km (240 million miles or 2.58 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

) based on the then-current parallax
Stellar parallax
Stellar parallax is the effect of parallax on distant stars in astronomy. It is parallax on an interstellar scale, and it can be used to determine the distance of Earth to another star directly with accurate astrometry...

 value of 0.018". However there was known uncertainty owing to limb darkening
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...

 and measurement errors—a central theme which would be the focus of scientific inquiry for almost a century. Beginning with this first angular
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

 measurement at visible wavelengths
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

, researchers have since conducted multiple investigations ranging from the ultraviolet to the mid infrared
Infrared astronomy
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers...

 with controvertible results.

The 1950s and '60s saw important scientific developments, the two Stratoscope
Stratoscope
The Stratoscopes were two balloon-borne astronomical telescopes which flew from the 1950s to the 1970s and observed in the optical and infrared regions of the spectrum. Both were controlled remotely from the ground....

 projects and the 1958 publication of Structure and Evolution of the Stars, principally the work of Martin Schwarzschild
Martin Schwarzschild
Martin Schwarzschild was a German American astronomer. He was the son of famed astrophysicist Karl Schwarzschild and the nephew of the Swiss astrophysicist Robert Emden.-Biography:...

 and his close colleague at Princeton University
Princeton University
Princeton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....

, Richard Härm. The book taught a generation of astrophysicists how to use nascent computer technology to create stellar models while the Stratoscope projects, by taking instrumented balloons above the Earth's turbulence
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

, produced some of the finest images of solar granules
Granule (solar physics)
Granules on the photosphere of the Sun are caused by convection currents of plasma within the Sun's convective zone. The grainy appearance of the solar photosphere is produced by the tops of these convective cells and is called granulation.The rising part of the granules is located in the center...

 and sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....

s ever seen, thus confirming the existence of convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 in the solar atmosphere. Both developments would prove to have a significant impact on our understanding of the structure of red supergiants like Betelgeuse.

Aperture masking

The 1970s saw several notable advances in interferometry
Astronomical interferometer
An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution by means of interferometry....

 from the Berkeley Space Sciences Laboratory
Space Sciences Laboratory
The Space Sciences Laboratory is an Organized Research Unit of the University of California, Berkeley. It is located in the Berkeley Hills above the university campus...

 working in the infrared and Antoine Labeyrie
Antoine Émile Henry Labeyrie
Antoine Émile Henry Labeyrie is a French astronomer and holds since 1991 the "Observational Astrophysics" chair at the Collège de France....

 in the visible, when researchers began to combine images from multiple telescopes and later invented "fringe-tracking" technology. But it was not until the late 1980s and early 1990s, when Betelgeuse became a regular target for aperture masking interferometry
Aperture masking interferometry
Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...

 that significant breakthroughs occurred in visible-light and infrared imaging
Infrared photography
In infrared photography, the film or image sensor used is sensitive to infrared light. The part of the spectrum used is referred to as near-infrared to distinguish it from far-infrared, which is the domain of thermal imaging. Wavelengths used for photography range from about 700 nm to about...

. Pioneered by John E. Baldwin
John E. Baldwin
John Evan Baldwin FRS has worked at the Cavendish Astrophysics Group since 1954. He played a pivotal role in the development of interferometry in Radio Astronomy, and later astronomical optical interferometry and lucky imaging...

 and other colleagues of the Cavendish Astrophysics Group
Cavendish Astrophysics Group
The Cavendish Astrophysics Group is based at the Cavendish Laboratory at the University of Cambridge. The group operates all of the telescopes at the Mullard Radio Astronomy Observatory except for the 32m MERLIN telescope, which is operated by Jodrell Bank.The group is the second largest of three...

, the new technique contributed some of the most accurate measurements of Betelgeuse to date while revealing a number of bright spots on the star's photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

. These were the first optical and infrared images of a stellar disk
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

 other than the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, first from ground-based interferometers and later from higher-resolution observations of the COAST telescope
Cambridge Optical Aperture Synthesis Telescope
COAST, the Cambridge Optical Aperture Synthesis Telescope, is a multi-element optical astronomical interferometer with baselines of up to 100 metres, which uses aperture synthesis to observe stars with angular resolution as high as one thousandth of one arcsecond COAST, the Cambridge Optical...

, with the "bright patches" or "hotspots" potentially corroborating a theory put forth by Schwarzschild decades earlier of massive convection
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...

 cells dominating the stellar surface.

In 1995, the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

's Faint Object Camera
Faint Object Camera
The Faint Object Camera was a camera installed on the Hubble Space Telescope from launch in 1990 until 2002. It was replaced by the Advanced Camera for Surveys.The camera was built by Dornier GmbH and was funded by the European Space Agency...

 captured an ultraviolet image of comparable resolution—the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star. The image was taken at ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 wavelengths since ground-based instruments cannot produce images in the ultraviolet with the same precision as Hubble. Like earlier images, this ultraviolet image also contained a bright patch, indicating a hotter region of about 2,000K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

, in this case on the southwestern portion of the star's surface. Subsequent ultraviolet spectra taken with the Goddard High Resolution Spectrograph
Goddard High Resolution Spectrograph
The Goddard High Resolution Spectrograph was a spectrograph installed on the Hubble Space Telescope. It was replaced by the Near Infrared Camera and Multi-Object Spectrometer in 1997.-GHRS facts:...

 suggested that the hot spot was one of Betelgeuse's poles of rotation. This would give the rotational axis an inclination of about 20° to the direction of Earth, and a position angle
Position angle
Position angle, usually abbreviated PA, is a measurement derived from observing visual binary stars. It is defined as the angular offset in degrees of the secondary star to the primary, relative to the north celestial pole....

 from celestial North
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...

 of about 55°.

Recent studies

The first decade of the 21st century has witnessed major advances on multiple fronts, the most central of which have been the imaging of the star's photosphere at different wavelengths
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 and the study of α Ori's complex circumstellar shells. At the dawn of the millennium, Betelgeuse was measured in the mid-infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 using the Infrared Spatial Interferometer
Infrared Spatial Interferometer
The Infrared Spatial Interferometer is an astronomical interferometer array of three 65 inch telescopes operating in the mid-infrared. The telescopes are fully mobile and their current site on Mount Wilson allows for placements as far as 70 m apart, giving the resolution of a telescope of that...

 (ISI) producing a limb darkened
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...

 estimate of 55.2 ± 0.5 milliarcseconds
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

 (mas)—a figure entirely consistent with Michelson's findings eighty years earlier. At the time of its publication, the estimated parallax from the Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...

 mission was 7.63 ± 1.64 mas, yielding an estimated radius for Betelgeuse of 3.6 AU. However, numerous interferometric studies in the near-infrared have appeared since from the Paranal Observatory
Paranal Observatory
Paranal Observatory is an astronomical observatory located on Cerro Paranal at 2,635 m altitude and operated by the European Southern Observatory. The Very Large Telescope is the largest telescope on Paranal, actually composed of four separate 8.2 m telescopes...

 in Chile arguing for much tighter diameters. Nevertheless, on June 9, 2009, Nobel
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

 Laureate Charles Townes
Charles Hard Townes
Charles Hard Townes is an American Nobel Prize-winning physicist and educator. Townes is known for his work on the theory and application of the maser, on which he got the fundamental patent, and other work in quantum electronics connected with both maser and laser devices. He shared the Nobel...

 announced that the star had shrunk 15% since 1993 at an increasing rate. He presented evidence that UC Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...

's ISI
Infrared Spatial Interferometer
The Infrared Spatial Interferometer is an astronomical interferometer array of three 65 inch telescopes operating in the mid-infrared. The telescopes are fully mobile and their current site on Mount Wilson allows for placements as far as 70 m apart, giving the resolution of a telescope of that...

 atop Mt. Wilson Observatory had observed 15 consecutive years of stellar contraction. Despite the apparent diminution of Betelgeuse's size, Townes and his colleague, Edward Wishnow, pointed out that the star's visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers
American Association of Variable Star Observers
Since its founding in 1911, the American Association of Variable Star Observers has coordinated, collected, evaluated, analyzed, published, and archived variable star observations made largely by amateur astronomers and makes the records available to professional astronomers, researchers, and...

 (AAVSO), had shown no significant dimming over that same time frame. This finding of a diminishing radius coupled with a relatively constant flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...

 puts into question some of the fundamental theories of stellar structure.
Enveloping this whole discussion have been numerous inquiries into the abstruse dynamics of Betelgeuse's extended atmosphere. For decades astronomers have understood that red giants dominate mass return to the Galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 creating opaque outer shells, yet the actual mechanics of such stellar mass loss
Stellar mass loss
Stellar mass loss is a phenomenon observed in some massive stars. It occurs when a triggering event causes the ejection of a large portion of the star's mass. Stellar mass loss can also occur when a star gradually loses material to a binary companion or into interstellar space.-Causes:A number of...

 have remained a mystery. With recent advances in interferometric methodologies, astronomers may be close to resolving this conundrum. In July 2009, images released by the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...

, taken by the ground-based Very Large Telescope
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...

 Interferometer (VLTI), showed vast plumes of gas being ejected into the surrounding atmosphere with distances approximating 30 AU. Comparable to the distance between the Sun and Neptune, this mass ejection is but one of multiple dynamics occurring in the surrounding atmosphere. Astronomers have identified at least 6 different shells surrounding Betelgeuse. As the century unfolds, solving the mystery of mass loss in the late stages of a star's evolution may reveal those factors which precipitate the explosive deaths of these stellar giants.

Visibility

Betelgeuse is easy to spot in the night sky, as it appears in proximity to the famous belt of Orion
Orion's Belt
The term Orion's Belt or the Belt of Orion may refer to:* Orion's Belt, an asterism consisting of three bright stars in a row in the constellation Orion* Orion's Belt, a 1985 film* Orion's Belt, a browser game...

 and has a distinctive orange-red color to the naked eye. In the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

, beginning in January of each year, it can be seen rising in the east just after sunset. By mid-March, the star is due south in the evening sky and visible to virtually every inhabited region of the globe, with only a few obscure research stations in Antarctica at latitudes south of 82° unable to see it. In large cities in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

 (e.g., Sydney
Sydney
Sydney is the most populous city in Australia and the state capital of New South Wales. Sydney is located on Australia's south-east coast of the Tasman Sea. As of June 2010, the greater metropolitan area had an approximate population of 4.6 million people...

, Buenos Aires
Buenos Aires
Buenos Aires is the capital and largest city of Argentina, and the second-largest metropolitan area in South America, after São Paulo. It is located on the western shore of the estuary of the Río de la Plata, on the southeastern coast of the South American continent...

, and Cape Town
Cape Town
Cape Town is the second-most populous city in South Africa, and the provincial capital and primate city of the Western Cape. As the seat of the National Parliament, it is also the legislative capital of the country. It forms part of the City of Cape Town metropolitan municipality...

) the star rises almost 49° above the horizon. Once May arrives, the red giant can be glimpsed but briefly on the western horizon just after the Sun sets.

The apparent magnitude of α Ori is listed in SIMBAD
SIMBAD
SIMBAD is an astronomical database of objects beyond the Solar System...

 at 0.42, making it on average the ninth brightest star in the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

—just ahead of Achernar
Achernar
Achernar , sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the ninth-brightest star in the night sky. Of the top ten apparent brightest stars —Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse—Achernar is the hottest...

. Because Betelgeuse is a variable star whose brightness ranges between 0.2 and 1.2, there are periods when it will surpass Procyon
Procyon
Procyon is the brightest star in the constellation Canis Minor. To the naked eye, it appears to be a single star, the seventh brightest in the night sky with a visual apparent magnitude of 0.34...

 to become the eighth brightest star. As Rigel, with a nominal apparent magnitude of 0.12, has been reported to fluctuate slightly in brightness, by 0.03 to 0.3 magnitudes, it is also possible for Betelgeuse to occasionally outshine Rigel and become the seventh brightest star. At its faintest, it will fall behind Deneb
Deneb
Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...

 as the 19th brightest star and compete with Mimosa
Beta Crucis
Mimosa or Becrux is the second brightest star in the constellation Crux and is one of the brightest stars in the night sky....

 for the 20th position.

Betelgeuse has a color index
Color index
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...

 (B–V) of 1.85—a figure which points to the advanced "redness" of this celestial object. The photosphere has an extended atmosphere
Stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. It is divided into several regions of distinct character:...

 which displays strong lines of emission
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

 rather than absorption
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

, a phenomenon which occurs when a star is surrounded by a thick gaseous envelope. This extended gaseous atmosphere has been observed moving both away from and towards Betelgeuse, depending apparently on radial velocity fluctuations in the photosphere. Only about 13% of the star's radiant energy
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...

 is emitted in the form of visible light, with most of its radiation occurring in the infrared. If our eyes were sensitive to radiation at all wavelengths, Betelgeuse would appear as the brightest star in the sky. Betelgeuse is the brightest near-IR source in the sky with a J band
J band
J band can refer to two different regions of the electromagnetic spectrum, in the radio and near-infrared.-Radio:The J band is the range of radio frequencies from 10 GHz to 20 GHz in the electromagnetic spectrum. This is equal to wave lengths between...

 magnitude
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...

 of -2.99.

Parallax

Since the first successful parallax measurement was conducted in 1838 by Friedrich Bessel
Friedrich Bessel
-References:* John Frederick William Herschel, A brief notice of the life, researches, and discoveries of Friedrich Wilhelm Bessel, London: Barclay, 1847 -External links:...

, astronomers have been puzzled by Betelgeuse's distance, the uncertainty of which has made reliable estimates of many stellar parameters difficult. An accurate distance and angular diameter will reveal a star's radius and effective temperature
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...

, leading to a clear understanding of bolometric
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...

 luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

; luminosity combined with an understanding of isotopic abundance
Natural abundance
In chemistry, natural abundance refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass of these isotopes is the atomic weight listed for the element in the periodic table...

s provides an estimate of the stellar age
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

 and mass. In 1920, when the first interferometric studies were performed on the star's diameter, the assumed parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

 was 0.0180 arcseconds. That equated to a distance of 56 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s (pc) or roughly 180 light years (ly) and produced not only an inaccurate radius for the star, but every other stellar characteristic. Since then, there has been an ongoing inquiry as to the actual distance of this mysterious star with proposed distances as high as 400 parsecs or about 1,300 light years.

Before the publication of the Hipparcos Catalogue (1997), there were two respected publications with up-to-date parallax data on Betelgeuse. The first was the Yale University Observatory (1991) with a published parallax of π = 9.8 ± 4.7 mas
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

, yielding a distance of roughly 102 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

 or 330 ly. The second was the Hipparcos Input Catalogue (1993) with a trigonometric parallax of π = 5 ± 4 mas
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

, a distance of 200 pc or 650 ly—almost twice the Yale estimate. With such uncertainty, researchers were adopting a wide range of distance estimates, a phenomenon which fueled much debate—not only in terms of the star's distance, but also its impact on other stellar parameters.
The long-awaited results from the Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...

 mission were finally released in 1997. Instead of resolving the issue, a new parallax figure was published of π = 7.63 ± 1.64 mas
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

, which equated to a distance of 131 pc or roughly 430 ly. Because stars like Betelgeuse vary in brightness, they raise specific problems in quantifying their distance. As a result, the large cosmic error
Cosmic noise
Cosmic noise and galactic radio noise is random noise that originates outside the Earth's atmosphere. It can be detected and heard on radio receivers.- Elaboration :Cosmic noise characteristics are similar to those of thermal noise...

 in the Hipparcos solution could well be of stellar origin, relating possibly to movements of the photocenter, of order 3.4 mas
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

, in the Hipparcos photometric Hp band.

Recent advances in radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

 appear to have prevailed in this debate. New high spatial resolution, multi-wavelength radio positions of Betelgeuse conducted by Graham Harper and colleagues using NRAO
National Radio Astronomy Observatory
The National Radio Astronomy Observatory is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy...

's Very Large Array
Very Large Array
The Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...

 (VLA) have produced a more precise estimate, which combined with the recent Hipparcos data furnished a new astrometric solution: π = 5.07 ± 1.10 mas
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

, a tighter error factor
Margin of error
The margin of error is a statistic expressing the amount of random sampling error in a survey's results. The larger the margin of error, the less faith one should have that the poll's reported results are close to the "true" figures; that is, the figures for the whole population...

 yielding a distance of 197 ± 45 pc or 643 ± 146 ly.

The next computational
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

 breakthrough will likely come from the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

's upcoming Gaia mission when it undertakes a detailed analysis of physical properties for each star observed, revealing luminosity, temperature, gravity and composition. Gaia will achieve this by repeatedly measuring the positions of all objects down to magnitude 20, and those brighter than magnitude 15, to an accuracy of 24 microarcseconds—akin to measuring the diameter of a human hair from 1000 km away. On-board detection equipment will ensure that variable stars like Betelgeuse will all be detected to this faint limit, thus addressing most of the limitations of the earlier Hipparcos mission. The nearest stars, in fact, will have their distances measured to within an unprecedented 0.001% error factor. Even stars near the Galactic centre
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

, some 30,000 light-years away, will have their distances measured to within a factor of 20%.

Variability

As a pulsating variable
Semiregular variable star
Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light...

star with sub-classification "SRC", researchers have offered different hypotheses to explain α Ori's volatile choreography
Choreography
Choreography is the art of designing sequences of movements in which motion, form, or both are specified. Choreography may also refer to the design itself, which is sometimes expressed by means of dance notation. The word choreography literally means "dance-writing" from the Greek words "χορεία" ...

—a phenomenon which causes an absolute magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...

 oscillation from −5.27 and −6.27. Our current understanding of stellar structure suggests that the outer layers of this supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...

 gradually expand and contract, causing the surface area (photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

) to alternately increase and decrease, and the temperature to rise and fall—thus eliciting the measured cadence in the star's brightness between its dimmest magnitude of 1.2, seen as early as 1927, and its brightest of 0.2, seen in 1933 and 1942. A red supergiant like Betelgeuse will pulsate this way because its stellar atmosphere is inherently unstable. As the star contracts, it absorbs more and more of the energy
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...

 that passes through it, causing the atmosphere to heat up and expand. Conversely, as the star expands, its atmosphere becomes less dense allowing the energy to escape and the atmosphere to cool, thus initiating a new contraction phase. Calculating the star's pulsations and modeling its periodicity have been difficult, as it appears there are several cycles interlaced. As discussed in papers by Stebbins and Sanford in the 1930s, there are short-term variations of around 150 to 300 days that modulate a regular cyclic variation with a period of roughly 5.7 years.

In fact, the supergiant consistently displays irregular photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

, polarimetric and spectroscopic variations, which points to complex activity on the star's surface and in its extended atmosphere
Stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. It is divided into several regions of distinct character:...

. In marked contrast to most giant stars that are typically long period variable
Long period variable
A long period variable is a type of variable star in which variations in brightness occur over long timescales of months or years. Long period variables are giant stars and brighter, from spectral class F and redwards, but most are red giants and AGB giants, meaning spectral class M, S or C...

s with reasonably regular periods, red giants are generally semiregular
Semiregular variable star
Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light...

 or irregular
Irregular variable
An irregular variable is a type of variable star in which variations in brightness show no regular periodicity. There are two main sub-types of irregular variable: eruptive and pulsating.Eruptive irregular variables are divided into three categories:...

 with pulsating characteristics. In a landmark paper published in 1975, Martin Schwarzschild
Martin Schwarzschild
Martin Schwarzschild was a German American astronomer. He was the son of famed astrophysicist Karl Schwarzschild and the nephew of the Swiss astrophysicist Robert Emden.-Biography:...

 attributed these brightess fluctuations to the changing granulation pattern
Granule (solar physics)
Granules on the photosphere of the Sun are caused by convection currents of plasma within the Sun's convective zone. The grainy appearance of the solar photosphere is produced by the tops of these convective cells and is called granulation.The rising part of the granules is located in the center...

 formed by a few giant convection cell
Convection cell
A convection cell is a phenomenon of fluid dynamics that occurs in situations where there are density differences within a body of liquid or gas. The convection usually requires a gravitational field but in microgravity experiments, thermal convection has been observed without gravitational effects...

s covering the surface of these stars. For the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, these convection cells, otherwise known as solar granules
Granule (solar physics)
Granules on the photosphere of the Sun are caused by convection currents of plasma within the Sun's convective zone. The grainy appearance of the solar photosphere is produced by the tops of these convective cells and is called granulation.The rising part of the granules is located in the center...

, represent the foremost mode of heat transfer—hence those convective elements
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 which dominate the brightness variations in the solar photosphere. The typical diameter for a solar granule is about 2,000 km (yielding a surface area roughly the size of India), with an average depth of 700 km. With a surface of roughly 6 trillion km2, there are about 2 million of such granules lying on the Sun's photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

, which because of their number produce a relatively constant flux. Beneath these granules, it is conjectured that there are 5 to 10 thousand supergranules
Supergranulation
Supergranulation is a particular pattern on the Sun's surface. It was discovered in the 1950s by A.B.Hart using Doppler velocity measurements showing horizontal flows on the photosphere ....

, the average diameter of which is 30,000 km with a depth of about 10,000 km. By contrast, Schwardschild argues that stars like Betelgeuse may have only a dozen monster granules with diameters of 180 million km or more dominating the surface of the star with depths of about 60 million km, which, because of the very low temperatures and extremely low density found in red giant envelopes
Circumstellar envelope
Circumstellar envelope is the part of the star,having roughly spherical shape and not gravitationally bound to the star core.Usually circumstellar envelopes are formed from the dense stellar wind or presentbefore formation of the star...

, result in convective inefficiency. Consequently, if only a third of these convective cells are visible to us at any one time, the time variations in their observable light may well be reflected in the brightness variations of the integrated light of the star.

Schwarzschild's hypothesis of gigantic convection cells dominating the surface of red giants and supergiants seems to have stuck with the astronomical community. When the Hubble Space Telescope captured its first direct image of Betelgeuse in 1995 revealing a mysterious hot spot, astronomers attributed it to convection. Two years later, astronomers observed intricate asymmetries in the brightness distribution of the star revealing at least three bright spots, the magnitude of which was "consistent with convective surface hotspots". Then in the year 2000, another team of astronomers led by Alex Lobel of the Harvard–Smithsonian Center for Astrophysics (CfA) noted that Betelgeuse exhibits raging storms of hot and cold gas in its turbulent atmosphere. The team surmised that huge areas of the star's photosphere vigorously bulge out in different directions at times, ejecting long plumes of warm gas into the cold dust envelope. Another explanation that was also given was the occurrence of shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

s caused by warm gas traversing cooler regions of the star. The team investigated the atmosphere of Betelgeuse over a period of five years between 1998 and 2003 with the Space Telescope Imaging Spectrograph
Space Telescope Imaging Spectrograph
The Space Telescope Imaging Spectrograph is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. It operated continuously from 1997 until a power supply failure in 2004. After repairs, it began operating again in 2009...

 aboard Hubble. They found that the bubbling action of the chromosphere tosses gas out one side of the star, while it falls inward at the other side, similar to the slow-motion churning of a lava lamp
Lava lamp
A lava lamp is a decorative novelty item that contains blobs of colored wax inside a glass vessel filled with clear liquid; the wax rises and falls as its density changes due to heating from a incandescent light bulb underneath the vessel. The appearance of the wax is suggestive of pāhoehoe lava,...

.

Angular size

A third challenge that has confronted astronomers has been measuring the star's angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

. On December 13, 1920, Betelgeuse became the first star outside the Solar System to ever have its diameter measured. Although interferometry was still in its infancy, the experiment proved a success and Betelgeuse was found to have a uniform disk of 0.047 arcseconds. The astronomers' insights on limb darkening were noteworthy; in addition to a measurement error of 10%, the team concluded that the stellar disk was likely 17% larger due to the diminishing intensity of light around the edges—hence an angular diameter of about 0.055". Since then, there have been other studies conducted, which have produced angles that range from 0.042 to 0.069 arcseconds. Combining that data with historical distance estimates of 180 to 815 ly yields a projected diameter of the stellar disk of anywhere from 2.4 to 17.8 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

, hence a radius of 1.2 to 8.9 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 respectively. Using the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 as a yardstick
Yardstick
A yardstick is a straightedge used to physically measure lengths of up to a yard high. Yardsticks are flat wooden boards with markings at regular intervals.-Construction:...

, the orbit of Mars is about 1.5 AU, Ceres in the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

 2.7 AU, Jupiter 5.5 AU—consequently a photosphere which, depending on Betelgeuse's actual distance from Earth, could well extend beyond the Jovian orbit but not quite as far as Saturn at 9.5 AU.

The precise diameter has been hard to define for several reasons:
  1. The rhythmic expansion and contraction of the photosphere, as theory suggests, means the diameter is never constant;
  2. There is no definable "edge" to the star as limb darkening
    Limb darkening
    Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...

     causes the optical emissions to vary in color and decrease the farther one extends out from the center;
  3. Betelgeuse is surrounded by a circumstellar envelope composed of matter being ejected from the star—matter which both absorbs and emits light—making it difficult to define the edge of the photosphere;
  4. Measurements can be taken at varying wavelengths within the electromagnetic spectrum
    Electromagnetic spectrum
    The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

    , with each wavelength revealing something different. Studies have shown that angular diameters are considerably larger at visible wavelengths, decrease to a minimum in the near-infrared, only to increase again in the mid-infrared. The difference in reported diameters can be as much as 30–35%, yet because each wavelength measures something different, comparing one finding with another is problematic;
  5. Atmospheric twinkling
    Scintillation (astronomy)
    Scintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through a medium, most commonly the atmosphere ....

     limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.


To overcome these challenges, researchers have employed various solutions. The concept of astronomical interferometry was first conceived by Hippolyte Fizeau
Hippolyte Fizeau
Armand Hippolyte Louis Fizeau was a French physicist.-Biography:Fizeau was born in Paris. His earliest work was concerned with improvements in photographic processes. Following suggestions by François Arago, Léon Foucault and Fizeau collaborated in a series of investigations on the interference of...

 in 1868. He proposed the observation of stars through two apertures to obtain interferences that would furnish information on the star's spatial intensity distribution. Since then, the science of interferometry has evolved considerably where multiple-aperture interferometers are now used consisting of numerous images superimposed on each other. These "speckled" images are then synthesized using Fourier analysis—a method used for a wide array of astronomical objects including the study of binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

s, quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s, asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s and galactic nuclei
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...

. The emergence of adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 since 1990 has revolutionized high angular resolution astronomy, while space observatories
Space observatory
A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects...

 like Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...

, Hubble
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 and Spitzer
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 have produced other significant breakthroughs. Recently another instrument, the Astronomical Multi-BEam Recombiner (AMBER), is providing new insights. As part of the VLTI, AMBER is capable of combining the beams of three telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s simultaneously, allowing researchers to achieve milliarcsecond spatial resolution. Also by combining three baselines instead of two, which is customary with conventional interferometry, AMBER enables astronomers to compute the closure phase
Closure phase
The closure phase is an observable quantity in imaging astronomical interferometry, which allowed the use of interferometry with very long baselines. It forms the basis of the self-calibration approach to interferometric imaging...

—an important element in astronomical imaging.

The current debate revolves around which wavelength—the visible
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

, near-infrared (NIR) or mid-infrared (MIR)—produces the most accurate angular measurement. The most widely adopted solution, it appears, is the one performed with the ISI
Infrared Spatial Interferometer
The Infrared Spatial Interferometer is an astronomical interferometer array of three 65 inch telescopes operating in the mid-infrared. The telescopes are fully mobile and their current site on Mount Wilson allows for placements as far as 70 m apart, giving the resolution of a telescope of that...

 in the mid-infrared by astronomers from the Space Sciences Laboratory
Space Sciences Laboratory
The Space Sciences Laboratory is an Organized Research Unit of the University of California, Berkeley. It is located in the Berkeley Hills above the university campus...

 at U.C. Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...

. In the epoch
Epoch (astronomy)
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are subject to perturbations and vary with time...

 year 2000, the group, under the leadership of John Weiner, published a paper showing Betelgeuse with a uniform disk of 54.7 ± 0.3 mas, ignoring any possible contribution from hotspots, which are less noticeable in the mid-infrared. The paper also included a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas—a figure which equates to a radius of roughly 5.5 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 (1,180 R), assuming a distance of 197.0 ± 45 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

. Nevertheless, given the angular error factor of ± 0.5 mas combined with a parallax error of ± 45 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

 found in Harper's numbers, the photosphere's radius could actually be as small as 4.2 AU or as large as 6.9 AU.

Across the Atlantic, another team of astronomers led by Guy Perrin of the Observatoire de Paris
Paris Observatory
The Paris Observatory is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world...

 produced a document in 2004 arguing that the near-infrared figure of 43.33 ± 0.04 mas was a more accurate photospheric measurement. "A consistent scenario to explain the observations of this star from the visible
to the mid-infrared can be set-up", Perrin reports. "The star is seen through a thick, warm extended atmosphere that scatters light at short wavelengths thus slightly increasing its diameter. The scatter becomes negligible above 1.3 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

. The upper atmosphere being almost transparent in K and L—the diameter is minimum at these wavelengths where the classical photosphere can be directly seen. In the mid-infrared, the thermal emission of the warm atmosphere increases the apparent diameter of the star." The argument has yet to receive widespread support among astronomers.

More recent studies done in the near-infrared with the IOTA
Infrared Optical Telescope Array
The Infrared Optical Telescope Array began with an agreement in 1988 among five Institutions, the Smithsonian Astrophysical Observatory, Harvard University, the University of Massachusetts, the University of Wyoming, and MIT/Lincoln Laboratory, to build a two-telescope stellar interferometer for...

 and VLTI
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...

 have brought strong support to Perrin's analysis yielding diameters that range from 42.57 to 44.28 mas with minimal error factors less than than 0.04 mas. Central to this discussion, however, is a second paper published by the Berkeley team in 2009, this time led by Charles Townes, reporting that the radius of Betelgeuse had actually shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate. Unlike most papers heretofore published, this study encompassed a 15-year horizon at one specific wavelength. Earlier studies have typically lasted one to two years by comparison and have explored multiple wavelengths, often yielding vastly different results. The diminution in angular separation equates to a range of values between 56.0 ± 0.1 mas seen in 1993 to 47.0 ± 0.1 mas seen in 2008—a contraction of almost 0.9 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 in 15 years or roughly 1,000 km per hour. What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggests that if a cycle does exist, it is probably a few decades long. Other possible explanations are photospheric protrusions due to convection or a star that is not spherical but rather asymmetric causing but the appearance of expansion and contraction as the star rotates on its axis. Consequently, until a full cycle of data has been gathered, we will not know whether the 1993 figure of 56.0 mas represents a maximum extension of the star or its mean, or whether the 2008 figure of 47.0 in fact represents a minimum. It will probably take another 15 years or longer (2025) before we know with any certainty, meaning that the Jovian
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 orbit of 5.5 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 will probably serve as the star's "average" radius for some time.

Once considered as having the largest angular diameter of any star in the sky after the Sun, in 1997 Betelgeuse lost that distinction when a group of astronomers measured R Doradus
R Doradus
R Doradus is the name of a red giant Mira variable star in the far-southern constellation Dorado, although visually it appears more closely associated with the constellation Reticulum. Its distance from Earth is 204 ± 9 light-years...

 with a diameter of 57.0 ± 0.5 mas. Betelgeuse is now considered to be in third place, although R Doradus, being much closer to Earth at about 200 ly, has an actual diameter roughly one-third that of Betelgeuse.

Properties

Betelgeuse's spectral class
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 is listed as M2Iab in the SIMBAD
SIMBAD
SIMBAD is an astronomical database of objects beyond the Solar System...

 astronomical database, signifying that it is a red Class M star. The "ab" suffix is derived from the Yerkes spectral classification system, and indicates that it is an intermediate luminous supergiant, less bright than other supergiants like Deneb
Deneb
Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...

. However, given some of the recent findings, this classification may be outdated, as there is evidence Betelgeuse is actually much more luminous than Deneb and other stars in its class.

Assuming average radius of 5.5 AU and a distance of 197 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

, theoretical calculations would yield a luminosity figure in excess of 180,000 Suns (L) at maximum. When the star contracts as it appears to have since 1993, its luminosity would diminish to about 130,000L. Either way, that amount of electromagnetic energy dwarfs Deneb's output of about 50,000L. But with most of the star's radiant energy occurring in the infrared and huge amounts being absorbed by circumstellar matter, the human eye simply cannot perceive the star's intrinsic brightness.

Given the many uncertainties surrounding Betelgeuse, no consensus has yet emerged regarding the star's mass. Estimates range from 5 to 30 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

es (M) with most investigators showing a preference for a relatively large mass ranging from 10 to 20M. One model reports a mass at the lower end of the scale at 14M, although a mass ranging from 18 to 20 is more commonplace.

Typical of red supergiants, Betelgeuse is a cool star with surface temperatures in the last decade reported between 3,500 to 3,600K. It is also a slow rotator, with the most recent velocity recorded at 5 km/s. Depending on its photospheric radius, it could take the star anywhere from 25 to 32 years to turn on its axis—extremely slow when compared with a fast rotator like Pleione
Pleione (star)
Pleione is a binary star in the Pleiades star cluster , located roughly 390 light years away in the constellation of Taurus. Pleione was not given a Bayer designation, but did receive a Flamsteed number—hence its designation 28 Tauri...

 in the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...

 star cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...

, which turns on its axis once every 11.8 hours.

In 2002, astronomers using sophisticated computer simulations began to speculate that Betelgeuse might exhibit magnetic activity in its extended atmosphere, a factor where even moderately strong fields could have a meaningful influence over the star's dust, wind and mass-loss properties. A series of spectropolarimetric observations obtained in 2010 with the Bernard Lyot Telescope
Bernard Lyot Telescope
The Bernard Lyot Telescope is a 2 m Cassegrain telescope operating in the visible domain, since 1980. It is located at 2877 m elevation on the Pic du Midi in the French Pyrenees...

 at Pic du Midi Observatory revealed the presence of a weak magnetic field at the surface of Betelgeuse, suggesting that the giant convective motions of supergiant stars are able to trigger the onset of a small-scale dynamo
Dynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...

.

Space motion

The kinematics
Kinematics
Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces that cause the motion....

 of Betelgeuse are not easily explained. The age of a Class M supergiant with an initial mass of 20M is roughly 10 million years. Given its current space motion, a projection back in time would take Betelgeuse around 290 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s farther from the galactic plane
Galactic plane
The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles...

 where there is no star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...

 region
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....

—an implausible scenario. Although the radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

 and proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

 for the 25 Ori
25 Orionis
25 Orionis, less commonly known by its Bayer designation Psi1 Orionis is a fifth-magnitude star in the constellation Orion...

 subassociation
Orion OB1 Association
The Orion OB1 stellar association is a contingent group of several dozen hot giant stars of spectral types O and B. Associated are thousands of lower-mass stars, and a number of protostars. It is part of the larger Orion Molecular Cloud Complex...

 have yet to be measured, α Ori's projected pathway across the heavens does not appear to intersect with it either. Also, formation close to the far younger Orion Nebula Cluster (ONC, also known as Ori OB1d) is doubtful. Very Long Baseline Array
Very Long Baseline Array
The Very Long Baseline Array is a system of ten radio telescopes controlled remotely from the Array Operations Center in Socorro, New Mexico by the National Radio Astronomy Observatory. The array works together as the world's largest dedicated, full-time astronomical instrument using the...

 astrometry yields a distance to the ONC between 389 and 414 parsecs. Consequently, it is likely that Betelgeuse has not always had its current motion through space and has changed course at one time or another, possibly the result of a nearby stellar explosion
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

.

The most likely star-formation scenario for Betelgeuse is that it is a runaway star from the Orion OB1 Association
Orion OB1 Association
The Orion OB1 stellar association is a contingent group of several dozen hot giant stars of spectral types O and B. Associated are thousands of lower-mass stars, and a number of protostars. It is part of the larger Orion Molecular Cloud Complex...

. Originally a member of a high-mass multiple system within Ori OB1a, Betelgeuse was probably formed about 10–12 million years ago from the molecular clouds observed in Orion, but has evolved rapidly due to its unusually high mass.

Like many of the young stars in Orion where masses greater than 10 solar can be found in abundance, Betelgeuse will use its fuel quickly and not live very long. On the Hertzsprung-Russell diagram, Betelgeuse has moved off the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 and has swelled and cooled to become a red supergiant
Red supergiant
Red supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...

. Although young, Betelgeuse has probably exhausted the hydrogen in its core—unlike its OB cousins born about the same time—causing it to contract under the force of gravity into a hotter and denser state. As a result, it has begun to fuse helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 into carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 producing enough radiation to unfurl its outer envelopes of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and helium. Its extreme luminosity is being generated by a mass so large that the star will eventually fuse higher elements through neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...

, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

, sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

, and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 all the way to iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, at which point it will probably collapse and explode as a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

.

Density

As an early M-type supergiant, Betelgeuse is one of the largest, most luminous and yet one of the most ethereal stars known. A radius of 5.5 AU is roughly 1,180 times the radius of the Sun—a sphere so huge that it could contain over 2 quadrillion Earths (2.15 × 1015) or more than 1.6 billion (1.65 × 109) Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

s. That is the equivalent of Betelgeuse being a giant football stadium like Wembley Stadium
Wembley Stadium
The original Wembley Stadium, officially known as the Empire Stadium, was a football stadium in Wembley, a suburb of north-west London, standing on the site now occupied by the new Wembley Stadium that opened in 2007...

 in London with the Earth a tiny pearl
Pearl
A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other...

, 1 millimeter in diameter, orbiting a Sun the size of a mango
Mango
The mango is a fleshy stone fruit belonging to the genus Mangifera, consisting of numerous tropical fruiting trees in the flowering plant family Anacardiaceae. The mango is native to India from where it spread all over the world. It is also the most cultivated fruit of the tropical world. While...

. Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...

, by contrast, with a radius of 1.71R, would be roughly the size of a soccer ball. Moreover, recent observations of Betelgeuse exhibiting a 15% contraction in angular diameter would equate to a shortening of the star's radius from about 5.5 to 4.6 AU, assuming that the photosphere is a perfect sphere. A reduction of this magnitude would correspond to a diminution in photospheric volume of approximately 41% or 680 million Suns.
Not only is the photosphere enormous, but the star is surrounded by a vast and complex circumstellar environment where light could take over three years just to escape. In the outer reaches of the photosphere, the density is extremely low. In volume, Betelgeuse exceeds the Sun by a factor of about 1.6 billion. Yet the actual mass of the star is believed to be no more than 18 to 19 Suns (M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

), with certain mass loss estimates projected at one to two Suns since birth. Consequently, the average density of this stellar mystery is less than twelve parts-per-billion
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...

 (1.119 × 10−8) that of the Sun. If we compare such star matter to the density of ordinary air at sea level, the ratio is roughly 1.286 × 10−5, a density so ethereal, one would have to soar above the noctilucent cloud
Noctilucent cloud
Night clouds or Noctilucent clouds are tenuous cloud-like phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night...

s in the Earth's mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

 to experience it. Such star matter is so tenuous, in fact, that Betelgeuse has often been called a "red-hot vacuum".

Circumstellar dynamics

In the late phase of stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

, massive stars like Betelgeuse exhibit high rates of mass loss
Stellar mass loss
Stellar mass loss is a phenomenon observed in some massive stars. It occurs when a triggering event causes the ejection of a large portion of the star's mass. Stellar mass loss can also occur when a star gradually loses material to a binary companion or into interstellar space.-Causes:A number of...

, possibly as much as 1M every 10,000 years, resulting in a complex circumstellar environment
Circumstellar envelope
Circumstellar envelope is the part of the star,having roughly spherical shape and not gravitationally bound to the star core.Usually circumstellar envelopes are formed from the dense stellar wind or presentbefore formation of the star...

 that is constantly in flux. All stars exhibit mass loss. Rates vary from about 10−14 to 10−4 yr -1 depending on spectral type, luminosity class, rotation rate, companion proximity, and evolutionary stage. Exactly how this mass loss occurs, however, has been a mystery confronting astronomers for decades. When Schwarzschild
Martin Schwarzschild
Martin Schwarzschild was a German American astronomer. He was the son of famed astrophysicist Karl Schwarzschild and the nephew of the Swiss astrophysicist Robert Emden.-Biography:...

 first proposed his theory of monster convection cells, he argued it was the likely cause in red supergiants. Prior attempts to explain mass loss in terms of solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 theory had proven unsuccessful as they led to a contradiction with observations involving circumstellar shells. Other theories that have been advanced include magnetic activity, global pulsations and shock structures as well as stellar rotation.

As a result of work done by Pierre Kervella and his team at the Paris observatory in 2009, astronomers may be close to solving this mystery. What Kervella noticed was a large plume of gas extending outward at least six times the stellar radius indicating that the star is not shedding matter evenly in all directions. The plume's presence, in fact, implies that the spherical symmetry of its photosphere, often observed in the infrared, is not preserved in its close environment. Asymmetries on the stellar disk had been reported many times at different wavelengths. However, due to the refined capabilities of the NACO adaptive optics on the VLT
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...

, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, Kervella noted, were large-scale convection cells or polar mass loss, possibly due to rotation. Probing deeper still with the AMBER instrument on ESO's
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...

 Very Large Telescope Interferometer, Keiichi Ohnaka from the Max Planck Institute
Max Planck Society
The Max Planck Society for the Advancement of Science is a formally independent non-governmental and non-profit association of German research institutes publicly funded by the federal and the 16 state governments of Germany....

 in Bonn
Bonn
Bonn is the 19th largest city in Germany. Located in the Cologne/Bonn Region, about 25 kilometres south of Cologne on the river Rhine in the State of North Rhine-Westphalia, it was the capital of West Germany from 1949 to 1990 and the official seat of government of united Germany from 1990 to 1999....

 observed that the gas in the supergiant's extended atmosphere is vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella.

Evidence of circumstellar shells surrounding M supergiants was first proposed by Walter Adams
Walter Sydney Adams
Walter Sydney Adams was an American astronomer.-Life and work:He was born in Antioch, Syria to missionary parents, and was brought to the U.S. in 1885 He graduated from Dartmouth College in 1898, then continued his education in Germany...

 and Elizabeth MacCormack in 1935 when they observed anomalies in the spectral signature of such stars and concluded that the likely cause was an expanding gaseous envelope. The first indication of vastly extended envelopes occurred in 1955 with the work of Armin Deutsch
Armin Joseph Deutsch
Armin Joseph Deutsch , was an American astronomer and a science fiction writer.He received a degree in astronomy from the University of Chicago in 1946. He is noted for the concept of Doppler tomography, which he presented at a symposium at Mount Wilson Observatory in 1958...

 who noticed when studying the Rasalgethi
Alpha Herculis
Alpha Herculis is a multiple star system in the constellation Hercules. It has the traditional name Rasalgethi or Ras Algethi , and the Flamsteed designation 64 Herculis...

 system that spectroscopic peculiarities were mysteriously occurring in the G star companion, α2 Her
Alpha Herculis
Alpha Herculis is a multiple star system in the constellation Hercules. It has the traditional name Rasalgethi or Ras Algethi , and the Flamsteed designation 64 Herculis...

. This led him to conclude that the whole system had to be enveloped by a circumstellar shell composed of matter being ejected by the main star, M supergiant α1 Her, and extending to at least 170 stellar radii. In the mid 1970s, Andrew Bernat undertook a detailed analysis of four circumstellar shells, Betelgeuse, Antares
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...

, Rasalgethi
Alpha Herculis
Alpha Herculis is a multiple star system in the constellation Hercules. It has the traditional name Rasalgethi or Ras Algethi , and the Flamsteed designation 64 Herculis...

 and Mu Cephei
Mu Cephei
Mu Cephei , also known as Herschel's Garnet Star, is a red supergiant star in the constellation Cepheus. It is one of the largest and most luminous stars known in the Milky Way...

, concluding that red stars dominate mass return to the Galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

.

In addition to the photosphere, researchers have now identified six other components of Betelgeuse's complex atmosphere. Extending outward, we find a compact molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 (CO). There is also evidence of corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...

l plasma in the star's extended atmosphere, a phenomenon that heretofore was not believed to exist in late stage stars off the main sequence. Some of these elements are known to be asymmetric while others overlap.

At about .45 stellar radii (~2−3 AU) above the photosphere, the closest membrane appears to be the molecular layer known as the MOLsphere. Studies show it to be composed of water-vapor and carbon monoxide with an effective temperature of about 1500 ± 500K. Water-vapor had been originally detected in the supergiant's spectrum back in the 1960s with the two Stratoscope
Stratoscope
The Stratoscopes were two balloon-borne astronomical telescopes which flew from the 1950s to the 1970s and observed in the optical and infrared regions of the spectrum. Both were controlled remotely from the ground....

 projects but had been ignored for decades. Recent studies suggest that the MOLsphere may also contain SiO
Silicon monoxide
Silicon monoxide is the chemical compound with the formula SiO. In the vapour phase it is a diatomic molecule. It has been detected in stellar objects and it has been described as the most common oxide of silicon in the universe....

 and Al2O3
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...

—molecules which could explain the formation of dust particles.

Between two and seven stellar radii (~10−40 AU), astronomers have identified another region known as an asymmetric gaseous envelope composed of elemental abundances C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, N
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 and O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

. The radio-telescope images taken in 1998 confirm that Betelgeuse has a dense atmosphere with a "remarkably complex structure". Observations show the atmosphere to be boiling with a temperature of 3,450 ± 850K—similar to the temperature recorded on the star's surface but much lower than surrounding gas in the same region. The VLA images also showed this lower-temperature gas progressively decreasing in temperature as it extends outward—the existence of which, although unexpected, turns out to be the most abundant constituent of Betelgeuse's atmosphere. "This alters our basic understanding of red-supergiant star atmospheres", explained Jeremy Lim, the team's leader. "Instead of the star's atmosphere expanding uniformly because of gas heated to very high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere." This is the same region in which Kervella's 2009 finding of a bright plume, possibly containing C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

N
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 and extending at least six photospheric radii in the southwest direction of the star, is believed to exist.

The chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....

, as mentioned earlier, was resolved in 1996 at about 2.2 times the optical disk (~10 AU) at ultraviolet wavelengths and is reported to have a temperature no higher than 5,500K. The image was taken with the Faint Object Camera on-board the Hubble Space Telescope and also revealed a bright area in the southwest quadrant of the disk. However in 2004 observations with the STIS
Space Telescope Imaging Spectrograph
The Space Telescope Imaging Spectrograph is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. It operated continuously from 1997 until a power supply failure in 2004. After repairs, it began operating again in 2009...

, Hubble's high-precision spectrometer, pointed to the existence of warm chromospheric plasma at least one arcsecond away from the star. At a distance of 197 pc, the size of the chromosphere could be 200 AU or seven times the Neptunian
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

 orbit. The CfA team led by Alex Lobel concluded that the spatially resolved STIS spectra directly demonstrate the co-existence of warm chromospheric plasma with cool gas in Betelgeuse's circumstellar dust envelope.

The first attestation of a dust shell surrounding Betelgeuse was put forth by Sutton et al. when the team noted in 1977 that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using heterodyne interferometry, the team concluded that α Ori emits the majority of its excess outside 12 stellar radii or roughly the distance of the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 at 50 to 60 AU, depending on the assumed stellar radius. Since then, there have been numerous studies done of this dust envelope at varying wavelengths yielding decidedly different results. More recent studies have estimated the inner radius of the dust shell anywhere from 0.5 to 1.0 arcseconds, hence 25 to 50 stellar radii or 100 to 200 AU. What these studies point out is that the dust environment surrounding Betelgeuse is anything but static. In 1994, Danchi et al. reported that Betelgeuse undergoes sporadic dust production involving decades of activity followed by inactivity. A few years later, a group of astronomers led by Chris Skinner noticed significant changes in the dust shell's morphology in just one year, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hot spots. The 1984 report of a giant asymmetric dust shell located 1 pc (206,265 AU) from the star has not been corroborated in recent studies, although another report published the same year said that three dust shells were found extending four light years from one side of the decaying star, suggesting that, like a snake, Betelgeuse sheds its outer layers as it journeys across the heavens.

Although the exact size of the two outer CO
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 shells remains elusive, preliminary estimates suggest that one shell extends from about 1.5 to 4.0 arcseconds with the other expanding as far as 7.0 arcseconds. Using the Jovian orbit of 5.5 AU as the "average" radius for this gargantuan star, (~0.055" diameter), the inner shell would extend from roughly 50 to 150 stellar radii (~300 to 800 AU) with the outer shell extending out as far as 250 stellar radii (~1400 AU). With the heliopause estimated at about 100 AU, the size of this outer shell is almost fourteen times the size of our Solar System.

Approaching supernova

The future fate of Betelgeuse depends on its mass—a critical factor which is not well understood. Since most investigators concede a mass greater than 10M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

, the most likely scenario is that the supergiant will continue to burn and fuse elements until its core is iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, at which point Betelgeuse will explode as a type II supernova
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...

. During this event the core will collapse, leaving behind a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...

 remnant some 20 km in diameter.

Betelgeuse is already old for its size class and is expected to explode relatively soon compared to its age. Solving the riddle of mass-loss will be the key to knowing when a supernova may occur, an event expected anytime in the next million years. Supporting this hypothesis are a number of unusual features that have been observed in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 of the Orion Molecular Cloud Complex
Orion Molecular Cloud Complex
The Orion Molecular Cloud Complex refers to a large group of bright nebula, dark clouds, and young stars located in the constellation of Orion. The cloud itself is between 1,500 and 1,600 light-years away and is hundreds of light-years across...

, which suggest that there have been multiple supernova explosions in the recent past. Betelgeuse's suspected birthplace in the Orion OB1 Association
Orion OB1 Association
The Orion OB1 stellar association is a contingent group of several dozen hot giant stars of spectral types O and B. Associated are thousands of lower-mass stars, and a number of protostars. It is part of the larger Orion Molecular Cloud Complex...

 is the probable location for such supernovae. Since the oldest subgroup in the association has an approximate age of 12 million years, the more massive stars likely had sufficient time to evolve to this stage. Also, because runaway stars are believed to be caused by supernova explosions, there is strong evidence that OB stars μ Columbae
Mu Columbae
Mu Columbae is a star in the constellation of Columba. It is one of the few O-class stars that is visible to the unaided eye. The star is known to lie approximately 1,300 light years from our solar system .This is a relatively fast rotating star that completes a full revolution approximately every...

, AE Aurigae
AE Aurigae
AE Aurigae is a runaway star in the constellation Auriga; it lights the Flaming Star Nebula.AE Aurigae is a blue O-type main sequence dwarf with a mean apparent magnitude of +5.99. It is classified as an Orion type variable star and its brightness varies irregularly between magnitudes +5.78 and...

 and 53 Arietis
53 Arietis
53 Arietis is a variable star of Beta Cephei type, alternatively denoted UW Arietis, in the constellation Aries. Its mean apparent magnitude is 6.13 and its spectral type is O or early B. It is a runaway star, thought to have been produced as a consequence of one of a pair of binary stars colliding...

 all originated with such an explosion in Ori OB1 2.2, 2.7 and 4.9 million years ago.

At its current distance from Earth, such a supernova explosion would be the brightest recorded, outshining the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 in the night sky and becoming easily visible in broad daylight. Professor J. Craig Wheeler of The University of Texas at Austin predicts the supernova will emit 1053 erg
Erg
An erg is the unit of energy and mechanical work in the centimetre-gram-second system of units, symbol "erg". Its name is derived from the Greek ergon, meaning "work"....

s of neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...

s, which will pass through the star's hydrogen envelope in around an hour, then reach the solar system several centuries later. Since its rotational axis is not pointed toward the Earth, Betelgeuse's supernova is unlikely to send a gamma ray burst
Gamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...

 in the direction of Earth large enough to damage ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s. The flash of ultraviolet radiation from the explosion will likely be weaker than the ultraviolet output of the Sun. The supernova could brighten to an apparent magnitude of −12 over a two-week period, then remain at that intensity for 2 to 3 months before rapidly dimming. The year following the explosion, radioactive decay of cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

 to iron will dominate emission from the supernova remnant
Supernova remnant
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...

, and the resulting gamma rays will be blocked by the expanding envelope of hydrogen. If the neutron star remnant becomes a pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

, then it could produce gamma rays for thousands of years.

Star system

In 1985, Margarita Karovska, in conjunction with other astrophysicists at the CfA, announced the discovery of two close companions orbiting Betelgeuse. Analysis of polarization data from 1968 through 1983 indicated a close companion with a periodic orbit of about 2.1 years. The team realized that the observed polarization could be caused by a systemic asymmetry created by the close companion orbiting the supergiant inside its extended dust envelope. Using speckle interferometry, the team concluded that the closer of the two companions was located at 0.06 ± 0.01" (~9 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

) from the main star with a position angle
Position angle
Position angle, usually abbreviated PA, is a measurement derived from observing visual binary stars. It is defined as the angular offset in degrees of the secondary star to the primary, relative to the north celestial pole....

 (PA) of 273 degrees, an orbit that would potentially place it within the star's chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....

. The more distant companion was estimated at 0.51 ± 0.01" (~77 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

) with a PA of 278 degrees. The magnitude differences with respect to the primary, measured at 656.3 (
H-alpha
H-alpha is a specific red visible spectral line created by hydrogen with a wavelength of 656.28 nm, which occurs when a hydrogen electron falls from its third to second lowest energy level...

) and 656.8 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

 (red continuum), were 3.4 and 3.0 for the close component and 4.6 and 4.3 for the distant component.
In the years that followed, astronomers conducted a number of studies in the hope of obtaining additional confirmation. In 1987, Andrea Dupree, one of Karovska's colleagues at the CfA, observed: "Periastron of the recently discovered close optical companion to Alpha Ori is predicted to be 1986.7; detection of atmospheric disturbances similar to those found subsequent to the last periastron (~ 1984.6) would give strong support to the presence of a companion." However, in the years that followed no such confirmation was ever published. Rather, in 1990, David F. Buscher, John E. Baldwin
John E. Baldwin
John Evan Baldwin FRS has worked at the Cavendish Astrophysics Group since 1954. He played a pivotal role in the development of interferometry in Radio Astronomy, and later astronomical optical interferometry and lucky imaging...

 and a team of collaborators from the Cavendish Astrophysics Group
Cavendish Astrophysics Group
The Cavendish Astrophysics Group is based at the Cavendish Laboratory at the University of Cambridge. The group operates all of the telescopes at the Mullard Radio Astronomy Observatory except for the 32m MERLIN telescope, which is operated by Jodrell Bank.The group is the second largest of three...

 made a number of high-resolution images of the supergiant at wavelengths of 633, 700, and 710 nm using the nonredundant masking method
Aperture masking interferometry
Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...

. At all these wavelengths, they remarked, there was unambiguous evidence for an asymmetric feature on the surface of the star, which contributed 10−15 percent of the star's total observed flux. Their conclusion was that such a phenomenon could be caused by a close companion passing in front of the stellar disk, differential photospheric brightening due to the effects of stellar rotation or the more likely scenario of "large-scale convection in the stellar atmosphere" as suggested by Schwarzschild.

The Cavendish colleagues published another paper in 1992, this time under the helm of Richard. W. Wilson, noting that the brightness features on the surface of Betelgeuse appear to be "too bright to be associated with a passage of the suggested companions in front of the red giant." They also noticed that these features were fainter at 710 nanometers compared to 700 by a factor of 1.8, indicating that such features would have to reside within the molecular atmosphere of the star.

That same year, Karovska published a new paper reconfirming her team's exegesis, but also noting that there was a meaningful correlation between the calculated position angles of the orbiting companion and the reported asymmetries, suggesting a possible connection between the two. She observed that any such correlation could be caused one of three factors: a companion star, tidal distortion of the supergiant's atmosphere, or by a photospheric bright spot. She concludes: "To determine the nature of the companion (which presently remains a puzzle), it is crucial to obtain further speckle observations using large aperture telescopes, coordinated with other ground-based observations and the observations from space."

Since then, researchers have turned their attention to analyzing the intricate dynamics of the star's extended atmosphere and little else has been published on the possibility of orbiting companions, although as Xavier Haubois and his team reiterate in 2009, the possibility of a close companion contributing to the overall flux has never been fully ruled out. Dommanget's double star catalog
Catalog of Components of Double and Multiple Stars
The Catalog of Components of Double and Multiple Stars, or CCDM, is an astrometric star catalogue of double and multiple stars. It was made by Jean Dommanget and Omer Nys at the Royal Observatory of Belgium in order to provide an input catalogue of stars for the Hipparcos mission...

 (CCDM) lists at least four adjacent stars, all within three arcminutes of this stellar giant, yet aside from apparent magnitudes and position angles, little else is known. As the decade unfolds and new technologies are brought to unraveling the star's enigmatic past, we will likely see conclusive evidence, one way or another, of any potential star system. Given the planned capabilities of the upcoming Gaia mission, a confirmation could occur any time after the mission's scheduled launch in December 2012.

Spelling

Betelgeuse has been known variously as Betelgeux, and Beteigeuze in German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

 (according to Bode
Johann Elert Bode
Johann Elert Bode was a German astronomer known for his reformulation and popularization of the Titius-Bode law. Bode determined the orbit of Uranus and suggested the planet's name.-Biography:...

). Betelgeux and Betelgeuze were used until the early 20th century, when the spelling Betelgeuse became universal.

Pronunciation

There is no consensus for the correct pronunciation of the name, and pronunciations for the star are as varied as its spellings:
Oxford English Dictionary
Oxford English Dictionary
The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

 and Royal Astronomical Society of Canada
Royal Astronomical Society of Canada
The Royal Astronomical Society of Canada is a national, non-profit, charitable organization devoted to the advancement of astronomy and related sciences. At present, there are 29 local branches of the Society, called centres, located in towns and cities across the country from St. John's,...

 Oxford English Dictionary
Oxford English Dictionary
The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

 (The Friendly Stars) (Canadian Oxford Dictionary, Webster's Collegiate Dictionary)

Etymology

The last part of the name, "-elgeuse", comes from the Arabic
Arabic language
Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...

  , a historical Arabic name of the constellation Orion, a feminine name in old Arabian legend, and of uncertain meaning.
Because , the root
Root (linguistics)
The root word is the primary lexical unit of a word, and of a word family , which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents....

 of , means "middle", roughly means "the Central One". Later, was also designated as the scientific Arabic name for Orion and for Gemini
Gemini (constellation)
Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations today. Its name is Latin for "twins", and it is associated with the twins Castor and Pollux in Greek mythology...

. The current Arabic name for Orion is ("the Giant"), although the use of in the name of the star has continued.

There is some uncertainty surrounding the first element of the name, rendered as "Bet-". However, "abet" or is the Arabic word for "armpit", which is where the star is located in the constellation. Betelgeuse is often mistranslated as "armpit of the central one".

In his 1899 work Star-Names and Their Meanings
Star Names: Their Lore and Meaning
Star Names: Their Lore and Meaning is a book by Richard Hinckley Allen, published in 1899.It discusses the names of stars and constellations and their origin.-Author:...

, American amateur naturalist Richard Hinckley Allen stated the derivation was from the , which he claimed degenerated into a number of forms including Bed Elgueze, Beit Algueze, Bet El-gueze, Beteigeuze and more, to the (then) current forms Betelgeuse, Betelguese, Betelgueze and Betelgeux. The star was named Beldengeuze in the Alfonsine Tables, and Italian Jesuit priest and astronomer Giovanni Battista Riccioli
Giovanni Battista Riccioli
Giovanni Battista Riccioli was an Italian astronomer and a Catholic priest in the Jesuit order...

 had called it Bectelgeuze or Bedalgeuze.

Paul Kunitzsch, Professor of Arabic Studies at the University of Munich, refuted Allen's derivation and instead proposed that the full name is a corruption of the Arabic meaning "the Hand of al-Jauzā', i.e., Orion.
European mistransliteration into medieval Latin
Medieval Latin
Medieval Latin was the form of Latin used in the Middle Ages, primarily as a medium of scholarly exchange and as the liturgical language of the medieval Roman Catholic Church, but also as a language of science, literature, law, and administration. Despite the clerical origin of many of its authors,...

 led to the first character y (, with two dots underneath) being misread as a b (, with only one dot underneath).
During the Renaissance
Renaissance
The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...

, the star's name was written as ("house of Orion") or , incorrectly thought to mean "armpit of Orion" (a true translation of "armpit" would be , transliterated as ). This led to the modern rendering as Betelgeuse.
Other writers have since accepted Kunitzsch's explanation. The 17th century English translator Edmund Chilmead
Edmund Chilmead
Edmund Chilmead was an English writer and translator, who produced both scholarly works and hack writing. He is also known as a musician.He studied at Magdalen College, Oxford, where he graduated M.A. in 1631...

 gave it the name Ied Algeuze ("Orion's Hand"), from Christmannus
Jakob Christmann
Jakob Christmann was a German Orientalist who also studied problems of astronomy.- Life :...

.

Other Arabic names recorded include ("the Right Hand"), ("the Arm"), and ("the Shoulder"), all appended to "of the giant", as . In Persian
Persian language
Persian is an Iranian language within the Indo-Iranian branch of the Indo-European languages. It is primarily spoken in Iran, Afghanistan, Tajikistan and countries which historically came under Persian influence...

, however, the name is , derived from the Arabic , "armpit of Orion".

Allen linked Orion's association with stormy weather to that of this deity of storms. was its Sanskrit
Sanskrit
Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 name, as part of a Hindu understanding of the constellation as a running antelope or stag. Other terms included the Persian
Persian language
Persian is an Iranian language within the Indo-Iranian branch of the Indo-European languages. It is primarily spoken in Iran, Afghanistan, Tajikistan and countries which historically came under Persian influence...

  "the Arm" via Brown, and Coptic
Coptic language
Coptic or Coptic Egyptian is the current stage of the Egyptian language, a northern Afro-Asiatic language spoken in Egypt until at least the 17th century. Egyptian began to be written using the Greek alphabet in the 1st century...

  "an Armlet".

A Sanskrit name for Betelgeuse was ãrdrã "the moist one", eponymous of the Ardra
Ardra (nakshatra)
Ardra is the name of a nakshatra in Hindu astrology, the 4th or 6th depending on numbering scheme used.The Sanskrit name ãrdrã translates to "the moist one".It is associated with the star Betelgeuse ....

 lunar mansion
Nakshatra
Nakshatra is the term for lunar mansion in Hindu astrology. A nakshatra is one of 27 sectors along the ecliptic...

 in Hindu astrology.
In traditional Chinese astronomy
Chinese astronomy
Astronomy in China has a very long history, with historians considering that "they [the Chinese] were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs."...

, Betelgeuse was known as (, the Fourth (Star of the constellation) of Three (Stars)) as the constellation of was at first a name only for the three stars in the girdle of Orion. Four stars were later added to this constellation, but the earlier name stuck. In Japan, this star was called Heike-boshi (suggested by the red butterfly flag of the Heike clan), , "the Star of the Heike clan" or Kin-waki, , "the Gold (Star) beside (Mitsu-boshi
Orion's Belt
The term Orion's Belt or the Belt of Orion may refer to:* Orion's Belt, an asterism consisting of three bright stars in a row in the constellation Orion* Orion's Belt, a 1985 film* Orion's Belt, a browser game...

)."

In popular culture

The star's unusual name inspired the 1988 film Beetlejuice
Beetlejuice
Beetlejuice is a 1988 American comedy horror film directed by Tim Burton, produced by The Geffen Film Company and distributed by Warner Bros...

, and script writer Michael McDowell
Michael McDowell (author)
Michael McEachern McDowell was an American novelist and screenwriter. He received a B.A. and an M.A. from Harvard College and a Ph.D in English from Brandeis University in 1978...

 was impressed at how many people made the connection. He added they had received a suggestion the sequel be named Sanduleak-69 202 after the former star of SN 1987A
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...

. In August Derleth
August Derleth
August William Derleth was an American writer and anthologist. Though best remembered as the first publisher of the writings of H. P...

's short story "The Dweller in the Darkness" set in H. P. Lovecraft
H. P. Lovecraft
Howard Phillips Lovecraft --often credited as H.P. Lovecraft — was an American author of horror, fantasy and science fiction, especially the subgenre known as weird fiction....

's Cthulhu Mythos
Cthulhu Mythos
The Cthulhu Mythos is a shared fictional universe, based on the work of American horror writer H. P. Lovecraft.The term was first coined by August Derleth, a contemporary correspondent of Lovecraft, who used the name of the creature Cthulhu - a central figure in Lovecraft literature and the focus...

, Betelgeuse is the home of the 'benign' Elder Gods. There has been much debate over the identity of the red star Borgil mentioned in Lord of the Rings, with Aldebaran
Aldebaran
Aldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...

, Betelgeuse and even the planet Mars touted as candidates. Professor Kristine Larsen has concluded the evidence points to it being Aldebaran as it precedes Menelvagor (Orion). Astronomy writer Robert Burnham, Jr.
Robert Burnham, Jr.
Robert Burnham, Jr. was an American astronomer. He is best known for writing the classic three-volume Burnham's Celestial Handbook.-Early work:...

 proposed the term padparadaschah which denotes a rare orange sapphire in India, for the star. In the popular science fiction series "The Hitchhiker's Guide to the Galaxy
The Hitchhiker's Guide to the Galaxy
The Hitchhiker's Guide to the Galaxy is a science fiction comedy series created by Douglas Adams. Originally a radio comedy broadcast on BBC Radio 4 in 1978, it was later adapted to other formats, and over several years it gradually became an international multi-media phenomenon...

" by Douglas Adams
Douglas Adams
Douglas Noel Adams was an English writer and dramatist. He is best known as the author of The Hitchhiker's Guide to the Galaxy, which started life in 1978 as a BBC radio comedy before developing into a "trilogy" of five books that sold over 15 million copies in his lifetime, a television...

, Ford Prefect
Ford Prefect (character)
Ford Prefect is a fictional character in The Hitchhiker's Guide to the Galaxy by the British author Douglas Adams. He is the only character other than the protagonist, Arthur Dent, to appear throughout the entire Hitchhiker's saga.-Name:Although Ford had taken great care to blend into Earth...

 was from "a small planet somewhere in the vicinity of Betelgeuse."

There have been two American navy ships named after the star, both World War II vessels, the launched in 1939 and launched in 1944. In 1979, a French supertanker named Betelgeuse
Betelgeuse incident
The Betelgeuse incident, also known as the Betelgeuse or Whiddy Island disaster, occurred on 8 January 1979, at around 1:00 a.m., when the oil tanker Betelgeuse exploded in West Cork, Ireland, at the offshore jetty of the Whiddy Island Oil Terminal, due to the failure of the ship's structure...

was moored off Whiddy Island
Whiddy Island
Whiddy Island is an island near the head of Bantry Bay, Ireland. It is approximately long and wide. The topography comprises gently-rolling glacial till, with relatively fertile soil...

 discharging oil when it exploded, killing 50 people in one of the worst disasters in Ireland's history.

See also


External links

  1. Mars and Orion Over Monument Valley Stunning skyscape showing the relative brightness of Betelgeuse and Rigel.
  2. The Spotty Surface of Betelgeuse A reconstructed image showing two hotspots, possibly convection cells.
  3. Simulated Supergiant Star Freytag's "Star in a Box" illustrating the nature of Betelgeuse's "monster granules".
  4. Why Stars Twinkle Image of Betelgeuse showing the effect of atmospheric twinkling in a microscope.
  5. Canaries Sky The glowing nebula
    Nebula
    A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

    s surrounding Betelgeuse.
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