VY Canis Majoris
Overview
 
VY Canis Majoris (VY CMa) is the largest known star and also one of the most luminous. Located in the constellation Canis Major
Canis Major
Canis Major is one of the 88 modern constellations, and was included in the 2nd-century astronomer Ptolemy's 48 constellations. Its name is Latin for 'greater dog', and is commonly represented as one of the dogs following Orion the hunter...

, it is a red hypergiant
Hypergiant
A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...

, between 1800 and 2100 solar radii, 8.4–9.8 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s in radius, about 3.0 billion km or 1.9 billion miles in diameter, and about 1.5 kiloparsecs
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

 (4,900 light years, or ) distant from Earth.
Encyclopedia
VY Canis Majoris (VY CMa) is the largest known star and also one of the most luminous. Located in the constellation Canis Major
Canis Major
Canis Major is one of the 88 modern constellations, and was included in the 2nd-century astronomer Ptolemy's 48 constellations. Its name is Latin for 'greater dog', and is commonly represented as one of the dogs following Orion the hunter...

, it is a red hypergiant
Hypergiant
A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...

, between 1800 and 2100 solar radii, 8.4–9.8 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s in radius, about 3.0 billion km or 1.9 billion miles in diameter, and about 1.5 kiloparsecs
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

 (4,900 light years, or ) distant from Earth. Unlike most hypergiant star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, which occur in either binary
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...

 or multiple star systems, VY CMa is a single star. It is categorized as a semiregular variable and has an estimated period of 2,000 days. It has an average density of 0.000005 to 0.000010 kg/m3.

Placed 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...

, VY Canis Majoris's surface would extend beyond the orbit of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, although the astrophysicists Philip Massey, Emily Levesque and Bertrand Plez disagree about the star's stated radius, suggesting it is smaller: merely 600 times the size of the Sun, extending past the orbit of Mars.

Nature of VY Canis Majoris

The first known recorded observation of VY Canis Majoris is in the star catalogue of Jérôme Lalande
Jérôme Lalande
Joseph Jérôme Lefrançois de Lalande was a French astronomer and writer.-Biography:Lalande was born at Bourg-en-Bresse...

, on March 7, 1801, which lists VY CMa as a 7th 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...

 star. Further 19th-century studies of its 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...

 demonstrate that the star has been fading since 1850.

Since 1847, VY CMa has been known to be a crimson star. During the 19th century, observers measured at least six discrete components to VY CMa, suggesting the possibility that it is a multiple star
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...

. These discrete components are now known to be bright areas in the surrounding nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

. Visual observations in 1957 and high-resolution imaging in 1998 showed that VY CMa does not have a companion star.

VY CMa is a high-luminosity M star
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...

 with an effective temperature of about 3,000 K, placing it at the upper-right hand corner of the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...

 and suggesting that it is a complexly created star. During its 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...

, it would have been an O star
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...

 with a mass of about 30 to 40 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

.

Measuring the distance

Stellar distances can be calculated by measuring 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"...

es as the Earth orbits around the Sun. However, VY CMa has a tiny parallax with a high margin of error, which makes it unreliable to calculate its distance using this method.

In 1976, Charles J. Lada and Mark J. Reid published the discovery of a bright-rimmed molecular cloud
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 ....

 15 minutes of arc
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....

 east of VY CMa. At the edge of the cloud bordered by the bright rim, an abrupt decrease in the CO emission
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...

 and an increase in brightness of the emission were observed, indicating possible destruction of molecular material and enhanced heating at the cloud-rim interface, respectively. Lada and Reid assumed the distance of the molecular cloud is approximately equal to that of the stars, which are members of open 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...

 NGC 2362
NGC 2362
NGC 2362 is an open cluster in the constellation Canis Major. It was discovered by Giovanni Batista Hodierna before 1654. Its brightest star is Tau Canis Majoris, and therefore it is sometimes called the Tau Canis Majoris Cluster. NGC 2362 has a distance of 1.48 kpc and is a relatively young 4–5...

, that ionize the rim. NGC 2362 has a distance of 1.5 ± 0.5 kiloparsecs
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

 as determined from its color-magnitude diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...

.

VY CMa is projected onto the tip of the rim, suggesting its association with the molecular cloud. In addition to that, the velocity of the molecular cloud is very close to the 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...

 of the star. This further indicates the association of the star with the molecular cloud, and consequently with NGC 2362, which means VY CMa is also at a distance of 1.5 kpc.

Size

University of Minnesota Professor Roberta M. Humphreys estimates the radius of VY CMa at 1,800 to 2,100 solar radii. To illustrate, if Earth's Sun were replaced by VY Canis Majoris, its radius might extend beyond the orbit of Saturn (about 9 AU). Assuming the upper size limit of 2100 solar radii, light would take more than 8.5 hours to travel around the star's circumference, compared to 14.5 seconds for the Sun. It would take Earths to fill the volume of VY Canis Majoris.

If the Earth were to be represented by a sphere one centimeter (0.39") in diameter, the Sun would be represented as a sphere with a diameter of 109 centimeters (3½ ft.), at a distance of 117 meters. At these scales, VY Canis Majoris would have a diameter of approximately 2.3 kilometers (1.43 mi.), assuming the upper limit estimate of its radius.

Luminosity

In 2006, Humphreys used the spectral energy distribution
Spectral Energy Distribution
A spectral energy distribution is a plot of brightness or flux density versus frequency or wavelength of light. It is used in many branches of astronomy to characterize astronomical sources. For example, in radio astronomy, an SED with a negative spectral index around −0.7 would indicate a...

  distance of VY Canis Majoris to calculate its luminosity. Since most of the radiation coming from the star is reprocessed by the dust in the surrounding cloud, she integrated the total fluxes over the entire nebula and showed that VY Canis Majoris has a luminosity of .

Controversy

There are two conflicting opinions of the properties of VY CMa. In one view, the star is a very large and very luminous red hypergiant
Hypergiant
A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...

. In another opinion (such as Massey, Levesque, and Plez's study), the star is a normal red 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...

, with a radius around 600 solar radii. In this case, its surface would extend well past the orbit of Mars in comparison to our solar system.

Like its size, the luminosity of VY CMa is also the subject of debate. Humphreys contests that visual photometry
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

 is not sufficient for stars with enough circumstellar dust to reprocess the visual and red fluxes into the thermal infrared.

VY Canis Majoris also illustrates the conceptual problem of defining the "surface" (and radius) of very large stars. With an average density of 0.000005 to 0.000010 kg/m3, the star is a thousand times less dense than the atmosphere of the Earth (air) at sea level. Since the star does not have a uniform density—as it must have a fusing core—the actual density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of the outer "surface" at the radius and mass stated (mass ÷ volume, where the volume of a sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 is 4/3 * pi * r3) is far less dense than even the outer atmosphere of the Earth. This presents a dilemma for cataloging star radii since, technically, a 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...

 like the Crab Nebula
Crab Nebula
The Crab Nebula  is a supernova remnant and pulsar wind nebula in the constellation of Taurus...

 still has a fusing (neutron) "core" surrounded by an (11 light-year wide) "atmosphere", while statements of our Sun's radius never include its corona (atmosphere of hot gas). This is all the more problematic since our 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 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...

 is hotter and thicker than VY Canis Majoris's "surface". A dividing line at which a star's ejected layers are no longer included in the radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

 of a star has yet to be established.

Life expectancy

The star has been discovered to be very unstable, having thrown off much of its mass into its surrounding nebula. Astronomers, with the help of 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...

, have predicted that VY Canis Majoris will be blown up, as a hypernova
Hypernova
Hypernova , also known as a type 1c Supernova, refers to an incredibly large star that collapses at the end of its lifespan...

, in the next 100,000 years.
Theoretically, a hypernova would cause gamma ray bursts that could damage the contents of the local galaxy, wiping out any cellular life within a number of light years; however, no hypergiant is located close enough to Earth to pose a threat.

See also

  • Orders of magnitude (length)
  • List of largest stars
  • List of most luminous stars
  • WASP-17b
    WASP-17b
    WASP-17b is an exoplanet in the constellation Scorpius that is orbiting the star WASP-17. Its discovery was announced on 11 August 2009. It is the first planet discovered to have a retrograde orbit, meaning it orbits in a direction counter to the rotation of its host star. This discovery changed...

    , the largest known planet
  • R136a1
    R136a1
    R136a1 is a blue hypergiant star and the most massive star known. It is an estimated 265 solar masses. The star is also the most luminous at 8,700,000 times the luminosity of the Sun....

    , the most massive Star

External links

  • Astronomers Map a Hypergiant Star's Massive Outbursts, HubbleSite NewsCenter, 2007-01-08
  • Helton, L. Andrew. Research Topics - VY Canis Majoris, University of Minnesota
    University of Minnesota
    The University of Minnesota, Twin Cities is a public research university located in Minneapolis and St. Paul, Minnesota, United States. It is the oldest and largest part of the University of Minnesota system and has the fourth-largest main campus student body in the United States, with 52,557...

  • "What is the Biggest Star in the Universe?", Universe Today
    Universe Today
    Universe Today is a non-commercial space and astronomy news site, founded in 1999 by Fraser Cain and edited by Nancy Atkinson. The news can then be discussed on the forums. The forum began on 24 July 2003, and was mainly used to discuss the Universe Today news as well as ask space-related...

    , 2008-04-06

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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