Beta Pictoris
Encyclopedia
Beta Pictoris is the second brightest star
in the constellation
Pictor
. It is located 63.4 light years from our solar system
, and is 1.75 times as massive and 8.7 times as luminous
as the Sun
. The Beta Pictoris system is very young, only 8–20 million years old, although it is already in the main sequence
stage of its evolution
. Beta Pictoris is the title member of the Beta Pictoris moving group
, an association of young stars which share the same motion through space and have the same age.
Beta Pictoris shows an excess of infrared
emission compared to normal stars of its type, which is caused by large quantities of dust near the star. Detailed observations reveal a large disk of dust and gas orbiting the star, which was the first debris disk
to be imaged around another star. In addition to the presence of several planetesimal
belts and comet
ary activity, there are indications that planet
s have formed within this disk and that the processes of planet formation may still be ongoing. Material from the Beta Pictoris debris disk is thought to be the dominant source of interstellar meteoroid
s in our solar system.
The European Southern Observatory
(ESO) has confirmed the presence of a planet, matching previous predictions, through the use of direct imagery, orbiting in the plane of the debris disk surrounding the star. This planet is currently the closest extrasolar planet to its star ever photographed: the observed separation is roughly the same as the distance between Saturn
and the Sun.
, and is located to the west of the bright star Canopus
. The distance to Beta Pictoris was found by measuring the star's trigonometric parallax. The star has an apparent visual magnitude of 3.861, so is visible to the naked eye
under good conditions, though light pollution
may result in stars dimmer than magnitude 3 being too dim to see. It is the second brightest in its constellation, exceeded only by Alpha Pictoris
, which has an apparent magnitude of 3.30.
The distance to Beta Pictoris and many other stars was measured by the Hipparcos
satellite
. This was done by measuring its trigonometric parallax: the slight displacement in its position observed as the Earth moves around the Sun. Beta Pictoris was found to exhibit a parallax of 51.87 milliarcsecond
s, a value which was later revised to 51.44 milliarcseconds when the data was reanalyzed taking systematic error
s more carefully into account. The distance to Beta Pictoris is therefore 63.4 light years, with an uncertainty
of 0.1 light years.
The Hipparcos satellite also measured the proper motion
of Beta Pictoris: it is traveling eastwards at a rate of 4.65 milliarcseconds per year, and northwards at a rate of 83.10 milliarcseconds per year. Measurements of the Doppler shift of the star's spectrum reveals it is moving away from us at a rate of 20 km/s. Several other stars share the same motion through space as Beta Pictoris and likely formed from the same gas cloud at roughly the same time: these comprise the Beta Pictoris moving group
.
of A6V. The A6 part is the spectral class: the A indicates this is a white star similar to Sirius
and Vega
, as opposed to our Sun
which is a yellow star of spectral type G2V. The 6 means it is intermediate between the hottest A type stars (A0) and the coolest (A9). The Roman numeral V is the luminosity class and indicates that, like the Sun, the star is a main sequence
star. Such stars are powered by the fusion
of hydrogen
in their cores.
The spectrum reveals that Beta Pictoris has an effective temperature
of 8052 K (7,778.9 °C; 14,033.9 °F), which is hotter than our Sun's 5778 K (5,504.9 °C; 9,940.7 °F). Analysis of the spectrum reveals that the star contains a slightly higher ratio of heavy elements, which are termed metals in astronomy to hydrogen than our Sun. This value is expressed as the quantity [M/H], the base-10 logarithm of the ratio of the star's metal fraction to that of the Sun. In the case of Beta Pictoris, the value of [M/H] is 0.05, which means that the star's metal fraction is 12% greater than that of our Sun.
Analysis of the spectrum can also reveal the surface gravity
of the star. This is usually expressed as log g, the base-10 logarithm of the gravitational acceleration
given in CGS units, in this case, cm/s². Beta Pictoris has log g=4.15, implying a surface gravity of 140 m/s², which is about half of the gravitational acceleration at the surface of our Sun (274 m/s²).
As an A-type main sequence star, Beta Pictoris is more luminous than our Sun: combining the apparent magnitude of 3.861 with the distance of 19.44 parsecs gives an absolute magnitude
of 2.42, as compared to our Sun which has an absolute magnitude of 4.83. This corresponds to a visual luminosity 9.2 times greater than that of the Sun. When the entire spectrum of radiation from Beta Pictoris and the Sun is taken into account, Beta Pictoris is found to be 8.7 times more luminous than the Sun.
Many main sequence stars of spectral type A fall into a region of the Hertzsprung–Russell diagram
called the instability strip
, which is occupied by pulsating variable star
s. In 2003, photometric
monitoring of the star revealed variations in brightness of around 1–2 millimagnitudes on frequencies between about 30 and 40 minutes. Radial velocity studies of Beta Pictoris also reveal variability: there are pulsations at two frequencies
, one at 30.4 minutes and one at 36.9 minutes. As a result, the star is classified as a Delta Scuti variable
.
and fitting them to the star's observed properties. This method yields a stellar mass between 1.7 and 1.8 solar mass
es. The star's angular diameter
has been measured using interferometry
with the Very Large Telescope
and was found to be 0.84 milliarcsecond
s. Combining this value with the distance of 63.4 light years gives a radius 1.8 times that of our Sun.
The rotational velocity of Beta Pictoris has been measured to be at least 130 km/s. Since this value is derived by measuring radial velocities
, this is a lower limit on the true rotational velocity: the quantity measured is actually v sin
(i), where i represents the inclination of the star's axis of rotation to the line-of-sight
. If it is assumed that Beta Pictoris is viewed from Earth in its equatorial plane, a reasonable assumption since the circumstellar disk is seen edge-on, the rotation period
can be calculated as approximately 16 hours, which is significantly shorter than that of our Sun (609.12 hours).
Beta Pictoris may have been formed near the Scorpius-Centaurus Association
. The collapse of the gas cloud which resulted in the formation of Beta Pictoris may have been triggered by the shock wave
from a supernova
explosion: the star which went supernova may have been a former companion of HIP 46950, which is now a runaway star. Tracing the path of HIP 46950 backwards suggests that it would have been in the vicinity of the Scorpius-Centaurus Association
about 13 million years ago.
Excess infrared
radiation from Beta Pictoris was detected by the IRAS
spacecraft in 1983. Along with Vega
, Fomalhaut
and Epsilon Eridani
, it was one of the first four stars from which such an excess was detected: these stars are called "Vega-like" after the first such star discovered. Since A-type stars like Beta Pictoris tend to radiate most of their energy at the blue end of the spectrum, this implied the presence of cool matter in orbit around the star, which would radiate at infrared wavelengths and produce the excess. This hypothesis was verified in 1984 when Beta Pictoris became the first star to have its circumstellar disk
imaged optically.
The debris disk around Beta Pictoris is seen edge-on by observers on Earth, and is orientated in a northeast-southwest direction. The disk is asymmetric: in the northeast direction it has been observed out to 1835 astronomical unit
s from the star, while the southwest direction the extent is 1450 AU. The disk is rotating: the part to the northeast of the star is moving away from us, while the part to the southwest of the disc is moving towards us.
Several elliptical rings of material have been observed in the outer regions of the debris disk between 500 and 800 AU: these may have formed as a result of the system being disrupted by a passing star. Astrometric
data from the Hipparcos mission reveal that the red giant
star Beta Columbae
passed within 2 light years of Beta Pictoris about 110,000 years ago, but a larger perturbation would have been caused by Zeta Doradus
, which passed at a distance of 3 light years about 350,000 years ago. However computer simulations favor a lower encounter velocity than either of these two candidates, which suggest that the star responsible for the rings may have been a companion star of Beta Pictoris on an unstable orbit. The simulations suggest a perturbing star with a mass of 0.5 solar mass
es is likely to blame for the structures. Such a star would be a red dwarf
of spectral type M0V.
In 2006, imaging of the system with the Hubble Space Telescope
's Advanced Camera for Surveys
revealed the presence of a secondary dust disk inclined at an angle of about 5° to the main disk and extending at least 130 AU from the star. The secondary disk is asymmetrical: the southwest extension is more curved and less inclined than the northeast. The imaging was not good enough to distinguish between the main and secondary disks within 80 AU of Beta Pictoris, however the northeast extension of the dust disk is predicted to intersect with the main disk at about 30 AU from the star. The secondary disk may be produced by a massive planet in an inclined orbit removing matter from the primary disk and causing it to move in an orbit aligned with the planet.
Studies made with the NASA
Far Ultraviolet Spectroscopic Explorer
have discovered that the disk around Beta Pictoris contains an extreme overabundance of carbon
-rich gas. This helps stabilize the disk against radiation pressure
which would otherwise blow the material away into interstellar space. Currently, there are two suggested explanations for the origin of the carbon overabundance. Beta Pictoris might be in the process of forming exotic carbon-rich planets
, in contrast to the terrestrial planet
s in our solar system, which are rich in oxygen
instead of carbon. Alternatively it may be passing through an unknown phase that might also have occurred early in the development of our solar system: in our solar system there are carbon-rich meteorites known as enstatite chondrites, which may have formed in a carbon-rich environment. It has also been proposed that Jupiter
may have formed around a carbon-rich core.
In 2003, imaging of the inner region of the Beta Pictoris system with the Keck II telescope revealed the presence of several features which are interpreted as being belts or rings of material. Belts at approximately 14, 28, 52 and 82 astronomical unit
s from the star were detected, which alternate in inclination with respect to the main disk.
Observations in 2004 revealed the presence of an inner belt containing silicate
material at a distance of 6.4 AU from the star. Silicate material was also detected at 16 and 30 AU from the star, with a lack of dust between 6.4 and 16 AU providing evidence that a massive planet may be orbiting in this region.
Modeling of the dust disk at 100 AU from the star suggests the dust in this region may have been produced by a series of collisions initiated by the destruction of planetesimal
s with radii of about 180 kilometers. After the initial collision, the debris undergoes further collisions in a process called a collisional cascade. Similar processes have been inferred in the debris disks around Fomalhaut
and AU Microscopii
.
of Beta Pictoris shows strong short-term variability that was first noticed in the red-shifted part of various absorption lines, which was interpreted as being caused by material falling onto the star. The source of this material was suggested to be small comet
-like objects on orbits which take them close to the star where they begin to evaporate, termed the "falling evaporating bodies" model. Transient blue-shifted absorption events were also detected, though less frequently: these may represent a second group of objects on a different set of orbits. Detailed modeling indicates the falling evaporating bodies are unlikely to be mainly icy like comets, but instead are probably composed of a mixed dust and ice core with a crust of refractory
material. These objects may have been perturbed onto their star-grazing orbits by the gravitational influence of a planet in a mildly eccentric orbit around Beta Pictoris at a distance of roughly 10 AU from the star. Falling evaporating bodies may also be responsible for the presence of gas located high above the plane of the main debris disk.
On November 21, 2008, it was announced that infrared observations made in 2003 with the Very Large Telescope
had revealed a candidate planetary companion to the star.
In the autumn of 2009 the planet was successfully observed on the other side of the parent star, confirming the existence of the planet itself and earlier observations. It is believed that in 15 years it will be possible to record the whole orbit of the planet.
The radial velocity method used to discover the majority of currently-known extrasolar planets is not well suited to studying A-type stars like Beta Pictoris, and its very young age makes the noise even worse. Current limits derived from this method are enough to rule out hot Jupiter
-type planets more massive than 2 Jupiter mass
es at a distance of less than 0.05 AU from the star. For planets orbiting at 1 AU, planets with less than 9 Jupiter masses would have evaded detection. Therefore to find planets in the Beta Pictoris system, astronomers look for the effects that the planet has on the circumstellar environment.
Multiple lines of evidence suggested the existence of a massive planet orbiting in the region around 10 AU from the star: the dust-free gap between the planetesimal belts at 6.4 AU and 16 AU suggest this region is being cleared out; a planet at this distance would explain the origin of the falling evaporating bodies, and the warps and inclined rings in the inner disk suggest a massive planet on an inclined orbit is disrupting the disk.
The observed planet by itself cannot explain the structure of the planetesimal belts at 30 AU and 52 AU from the star. These belts might be associated with smaller planets at 25 and 44 AU, with around 0.5 and 0.1 Jupiter masses respectively. Such a system of planets, if it exists, would be close to a 1:3:7 orbital resonance
. It may also be that the rings in the outer disc at 500–800 AU are indirectly caused by the influence of these planets.
The object was observed at an angular distance of 411 milliarcsecond
s from Beta Pictoris, which corresponds to a distance in the plane of the sky of 8 AU. For comparison, the orbital radii of the planets Jupiter and Saturn
are 5.2 AU and 9.5 AU respectively. The separation in the radial direction, that is unknown, so this is a lower limit on the true separation. Estimates of its mass depend on theoretical models of planetary evolution, and predict the object has about 8 Jupiter masses and is still cooling, with a temperature ranging from 1400–1600 K. These figures come with the caveat that the models have not yet been tested against real data in the likely ranges of mass and age for the planet.
It is possible that a transit of this planet was observed in November 1981. If this is confirmed, the semimajor axis may be narrowed down to 7.6–8.7 AU and its orbital period to 15.9–19.5 yrs. The inferred radius of the transiting object is 2–4 Jupiter radii, which is larger than predicted by theoretical models. This may indicate that, as may also be the case for the planet Fomalhaut b
, it is surrounded by a large ring system or a moon-forming disc.
revealed the presence of a stream of particles coming from the direction of Beta Pictoris, which appears to be the dominant source of interstellar meteoroids in our solar system. The particles in the Beta Pictoris dust stream are relatively large, with radii exceeding 20 micrometer
s, and their velocities suggest that they must have left the Beta Pictoris system at roughly 25 km/s. These particles may have been ejected from the Beta Pictoris debris disk as a result of the migration of gas giant planets within the disk and may be an indication that the Beta Pictoris system is forming an Oort cloud
. Numerical modeling of dust ejection indicates radiation pressure may also be responsible and suggests that planets further than about 1 AU from the star cannot directly cause the dust stream.
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...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Pictor
Pictor
Pictor is a small and faint constellation in the southern sky , located between the brilliant star Canopus and the Large Magellanic Cloud. Its name is Latin for painter, but it is in fact an abbreviation of its original name Equuleus Pictoris, the 'painter's easel', and it is normally represented...
. It is located 63.4 light years from 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...
, and is 1.75 times as massive and 8.7 times as luminous
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...
as 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...
. The Beta Pictoris system is very young, only 8–20 million years old, although it is already in 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...
stage of its 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...
. Beta Pictoris is the title member of the Beta Pictoris moving group
Beta Pictoris Moving Group
The Beta Pictoris Moving Group is a young moving group of stars located relatively near Earth. A moving group, in astronomy, is a group of stars that share a common motion through space as well as a common origin...
, an association of young stars which share the same motion through space and have the same age.
Beta Pictoris shows an excess of 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...
emission compared to normal stars of its type, which is caused by large quantities of dust near the star. Detailed observations reveal a large disk of dust and gas orbiting the star, which was the first debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...
to be imaged around another star. In addition to the presence of several planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
belts and comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
ary activity, there are indications that planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s have formed within this disk and that the processes of planet formation may still be ongoing. Material from the Beta Pictoris debris disk is thought to be the dominant source of interstellar meteoroid
Meteoroid
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...
s in our solar system.
The European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
(ESO) has confirmed the presence of a planet, matching previous predictions, through the use of direct imagery, orbiting in the plane of the debris disk surrounding the star. This planet is currently the closest extrasolar planet to its star ever photographed: the observed separation is roughly the same as the distance between 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,...
and the Sun.
Location and visibility
Beta Pictoris is a star in the southern constellation of Pictor, the EaselEasel
An easel is an upright support used for displaying and/or fixing something resting upon it.-Etymology:The word is an old Germanic synonym for donkey...
, and is located to the west of the bright star Canopus
Canopus
Canopus |Alpha]] Carinae) is the brightest star in the southern constellation of Carina and Argo Navis, and the second brightest star in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.53.Canopus is a supergiant of spectral...
. The distance to Beta Pictoris was found by measuring the star's trigonometric parallax. The star has an apparent visual magnitude of 3.861, so is visible to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
under good conditions, though light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
may result in stars dimmer than magnitude 3 being too dim to see. It is the second brightest in its constellation, exceeded only by Alpha Pictoris
Alpha Pictoris
Alpha Pictoris is the brightest star in the constellation Pictor with an apparent magnitude of 3.30. It is located about 99 light years from the Sun...
, which has an apparent magnitude of 3.30.
The distance to Beta Pictoris and many other stars was measured by 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...
satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
. This was done by measuring its trigonometric parallax: the slight displacement in its position observed as the Earth moves around the Sun. Beta Pictoris was found to exhibit a parallax of 51.87 milliarcsecond
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....
s, a value which was later revised to 51.44 milliarcseconds when the data was reanalyzed taking systematic error
Systematic error
Systematic errors are biases in measurement which lead to the situation where the mean of many separate measurements differs significantly from the actual value of the measured attribute. All measurements are prone to systematic errors, often of several different types...
s more carefully into account. The distance to Beta Pictoris is therefore 63.4 light years, with an uncertainty
Measurement uncertainty
In metrology, measurement uncertainty is a non-negative parameter characterizing the dispersion of the values attributed to a measured quantity. The uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity. All measurements are subject to uncertainty and a measured...
of 0.1 light years.
The Hipparcos satellite also measured the 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...
of Beta Pictoris: it is traveling eastwards at a rate of 4.65 milliarcseconds per year, and northwards at a rate of 83.10 milliarcseconds per year. Measurements of the Doppler shift of the star's spectrum reveals it is moving away from us at a rate of 20 km/s. Several other stars share the same motion through space as Beta Pictoris and likely formed from the same gas cloud at roughly the same time: these comprise the Beta Pictoris moving group
Beta Pictoris Moving Group
The Beta Pictoris Moving Group is a young moving group of stars located relatively near Earth. A moving group, in astronomy, is a group of stars that share a common motion through space as well as a common origin...
.
Spectrum, luminosity and variability
According to measurements made as part of the Nearby Stars Project, Beta Pictoris has a spectral typeStellar 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...
of A6V. The A6 part is the spectral class: the A indicates this is a white star similar to 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...
and Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
, as opposed to 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...
which is a yellow star of spectral type G2V. The 6 means it is intermediate between the hottest A type stars (A0) and the coolest (A9). The Roman numeral V is the luminosity class and indicates that, like the Sun, the star is a 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...
star. Such stars are powered by the fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
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...
in their cores.
The spectrum reveals that Beta Pictoris has an 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...
of 8052 K (7,778.9 °C; 14,033.9 °F), which is hotter than our Sun's 5778 K (5,504.9 °C; 9,940.7 °F). Analysis of the spectrum reveals that the star contains a slightly higher ratio of heavy elements, which are termed metals in astronomy to hydrogen than our Sun. This value is expressed as the quantity [M/H], the base-10 logarithm of the ratio of the star's metal fraction to that of the Sun. In the case of Beta Pictoris, the value of [M/H] is 0.05, which means that the star's metal fraction is 12% greater than that of our Sun.
Analysis of the spectrum can also reveal the surface gravity
Surface gravity
The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...
of the star. This is usually expressed as log g, the base-10 logarithm of the gravitational acceleration
Gravitational acceleration
In physics, gravitational acceleration is the acceleration on an object caused by gravity. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass....
given in CGS units, in this case, cm/s². Beta Pictoris has log g=4.15, implying a surface gravity of 140 m/s², which is about half of the gravitational acceleration at the surface of our Sun (274 m/s²).
As an A-type main sequence star, Beta Pictoris is more luminous than our Sun: combining the apparent magnitude of 3.861 with the distance of 19.44 parsecs gives 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...
of 2.42, as compared to our Sun which has an absolute magnitude of 4.83. This corresponds to a visual luminosity 9.2 times greater than that of the Sun. When the entire spectrum of radiation from Beta Pictoris and the Sun is taken into account, Beta Pictoris is found to be 8.7 times more luminous than the Sun.
Many main sequence stars of spectral type A fall into a region 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...
called the instability strip
Instability strip
The Instability strip is a nearly vertical region in the Hertzsprung–Russell diagram which is occupied by pulsating variable stars .The instability strip intersects the main sequence in the region of A...
, which is occupied by pulsating variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s. In 2003, photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
monitoring of the star revealed variations in brightness of around 1–2 millimagnitudes on frequencies between about 30 and 40 minutes. Radial velocity studies of Beta Pictoris also reveal variability: there are pulsations at two frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, one at 30.4 minutes and one at 36.9 minutes. As a result, the star is classified as a Delta Scuti variable
Delta Scuti variable
A Delta Scuti variable is a variable star which exhibits variations in its luminosity due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the...
.
Mass, radius and rotation
The mass of Beta Pictoris has been determined by using models of stellar evolutionStellar 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 fitting them to the star's observed properties. This method yields a stellar mass between 1.7 and 1.8 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. 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...
has been measured using interferometry
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,...
with the 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...
and was found to be 0.84 milliarcsecond
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....
s. Combining this value with the distance of 63.4 light years gives a radius 1.8 times that of our Sun.
The rotational velocity of Beta Pictoris has been measured to be at least 130 km/s. Since this value is derived by measuring radial velocities
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...
, this is a lower limit on the true rotational velocity: the quantity measured is actually v sin
Sine
In mathematics, the sine function is a function of an angle. In a right triangle, sine gives the ratio of the length of the side opposite to an angle to the length of the hypotenuse.Sine is usually listed first amongst the trigonometric functions....
(i), where i represents the inclination of the star's axis of rotation to the line-of-sight
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
. If it is assumed that Beta Pictoris is viewed from Earth in its equatorial plane, a reasonable assumption since the circumstellar disk is seen edge-on, the rotation period
Rotation period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...
can be calculated as approximately 16 hours, which is significantly shorter than that of our Sun (609.12 hours).
Age and formation
The presence of significant amounts of dust around the star implies a young age of the system and led to debate about whether it had joined the main sequence or was still a pre–main sequence star However when the star's distance was measured by Hipparcos it was revealed that Beta Pictoris was located further away than previously thought and hence was more luminous than originally believed. Once the Hipparcos results were taken into account, it was found that Beta Pictoris was located close to the zero age main sequence and was not a pre–main sequence star after all. Analysis of Beta Pictoris and other stars within the Beta Pictoris moving group suggests that they are around 12 million years old. Allowing for uncertainties, the age may range between 8 and 20 million years.Beta Pictoris may have been formed near the Scorpius-Centaurus Association
Scorpius-Centaurus Association
The Scorpius-Centaurus Association is the nearestOB Association to the Sun. This stellar association is composed of three subgroups The Scorpius-Centaurus Association (sometimes called Sco-Cen or Sco OB2) is the nearestOB Association to the Sun. This stellar association is composed of three...
. The collapse of the gas cloud which resulted in the formation of Beta Pictoris may have been triggered by the 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...
from 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...
explosion: the star which went supernova may have been a former companion of HIP 46950, which is now a runaway star. Tracing the path of HIP 46950 backwards suggests that it would have been in the vicinity of the Scorpius-Centaurus Association
Scorpius-Centaurus Association
The Scorpius-Centaurus Association is the nearestOB Association to the Sun. This stellar association is composed of three subgroups The Scorpius-Centaurus Association (sometimes called Sco-Cen or Sco OB2) is the nearestOB Association to the Sun. This stellar association is composed of three...
about 13 million years ago.
Debris disks
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...
radiation from Beta Pictoris was detected by the IRAS
IRAS
The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....
spacecraft in 1983. Along with Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
, Fomalhaut
Fomalhaut
Fomalhaut is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. Fomalhaut can be seen low in the southern sky in the northern hemisphere in fall and early winter evenings. Near latitude 50˚N, it sets around the time Sirius rises, and does not...
and Epsilon Eridani
Epsilon Eridani
Epsilon Eridani is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years , it has an apparent magnitude of 3.73...
, it was one of the first four stars from which such an excess was detected: these stars are called "Vega-like" after the first such star discovered. Since A-type stars like Beta Pictoris tend to radiate most of their energy at the blue end of the spectrum, this implied the presence of cool matter in orbit around the star, which would radiate at infrared wavelengths and produce the excess. This hypothesis was verified in 1984 when Beta Pictoris became the first star to have its circumstellar disk
Circumstellar disk
A circumstellar disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form...
imaged optically.
The debris disk around Beta Pictoris is seen edge-on by observers on Earth, and is orientated in a northeast-southwest direction. The disk is asymmetric: in the northeast direction it has been observed out to 1835 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s from the star, while the southwest direction the extent is 1450 AU. The disk is rotating: the part to the northeast of the star is moving away from us, while the part to the southwest of the disc is moving towards us.
Several elliptical rings of material have been observed in the outer regions of the debris disk between 500 and 800 AU: these may have formed as a result of the system being disrupted by a passing star. Astrometric
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
data from the Hipparcos mission reveal that the red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
star Beta Columbae
Beta Columbae
Beta Columbae is a K-type giant star and is the second brightest star in the constellation of Columba. It has the traditional name Wezn , from the Arabic وزن "weight".In Chinese, , meaning Son, refers to an asterism consisting of β Columbae and λ Columbae...
passed within 2 light years of Beta Pictoris about 110,000 years ago, but a larger perturbation would have been caused by Zeta Doradus
Zeta Doradus
Zeta Doradus is a solitary, yellow-white, main sequence star located thirty-eight light-years away, in the constellation Dorado.This star will be in constellation Pictor around 6400 AD.- External links :* *...
, which passed at a distance of 3 light years about 350,000 years ago. However computer simulations favor a lower encounter velocity than either of these two candidates, which suggest that the star responsible for the rings may have been a companion star of Beta Pictoris on an unstable orbit. The simulations suggest a perturbing star with a mass of 0.5 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 is likely to blame for the structures. Such a star would be a red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
of spectral type M0V.
In 2006, imaging of the system with 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 Advanced Camera for Surveys
Advanced Camera for Surveys
The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...
revealed the presence of a secondary dust disk inclined at an angle of about 5° to the main disk and extending at least 130 AU from the star. The secondary disk is asymmetrical: the southwest extension is more curved and less inclined than the northeast. The imaging was not good enough to distinguish between the main and secondary disks within 80 AU of Beta Pictoris, however the northeast extension of the dust disk is predicted to intersect with the main disk at about 30 AU from the star. The secondary disk may be produced by a massive planet in an inclined orbit removing matter from the primary disk and causing it to move in an orbit aligned with the planet.
Studies made with the NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
Far Ultraviolet Spectroscopic Explorer
Far Ultraviolet Spectroscopic Explorer
The Far Ultraviolet Spectroscopic Explorer is a space-based telescope operated by the Johns Hopkins University Applied Physics Laboratory. FUSE was launched on a Delta II rocket on June 24, 1999, as a part of NASA's Origins program...
have discovered that the disk around Beta Pictoris contains an extreme overabundance of 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...
-rich gas. This helps stabilize the disk against radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
which would otherwise blow the material away into interstellar space. Currently, there are two suggested explanations for the origin of the carbon overabundance. Beta Pictoris might be in the process of forming exotic carbon-rich planets
Carbon planet
A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form if protoplanetary discs are carbon-rich and oxygen-poor. According to planetary science, it would develop differently from Earth, Mars and Venus, planets...
, in contrast to the terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
s in our solar system, which are rich in 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...
instead of carbon. Alternatively it may be passing through an unknown phase that might also have occurred early in the development of our solar system: in our solar system there are carbon-rich meteorites known as enstatite chondrites, which may have formed in a carbon-rich environment. It has also been proposed that 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,...
may have formed around a carbon-rich core.
Planetesimal belts
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s from the star were detected, which alternate in inclination with respect to the main disk.
Observations in 2004 revealed the presence of an inner belt containing silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
material at a distance of 6.4 AU from the star. Silicate material was also detected at 16 and 30 AU from the star, with a lack of dust between 6.4 and 16 AU providing evidence that a massive planet may be orbiting in this region.
Modeling of the dust disk at 100 AU from the star suggests the dust in this region may have been produced by a series of collisions initiated by the destruction of planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
s with radii of about 180 kilometers. After the initial collision, the debris undergoes further collisions in a process called a collisional cascade. Similar processes have been inferred in the debris disks around Fomalhaut
Fomalhaut
Fomalhaut is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. Fomalhaut can be seen low in the southern sky in the northern hemisphere in fall and early winter evenings. Near latitude 50˚N, it sets around the time Sirius rises, and does not...
and AU Microscopii
AU Microscopii
AU Microscopii is a red dwarf star located 10 parsecs away – about 8 times as far as our closest star after the Sun. AU Mic is a young star, only 12 million years old, less than 1% of the age of the Sun. It has only half the mass of the Sun and is only one-tenth as bright...
.
Falling evaporating bodies
The spectrumSpectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of Beta Pictoris shows strong short-term variability that was first noticed in the red-shifted part of various absorption lines, which was interpreted as being caused by material falling onto the star. The source of this material was suggested to be small comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
-like objects on orbits which take them close to the star where they begin to evaporate, termed the "falling evaporating bodies" model. Transient blue-shifted absorption events were also detected, though less frequently: these may represent a second group of objects on a different set of orbits. Detailed modeling indicates the falling evaporating bodies are unlikely to be mainly icy like comets, but instead are probably composed of a mixed dust and ice core with a crust of refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
material. These objects may have been perturbed onto their star-grazing orbits by the gravitational influence of a planet in a mildly eccentric orbit around Beta Pictoris at a distance of roughly 10 AU from the star. Falling evaporating bodies may also be responsible for the presence of gas located high above the plane of the main debris disk.
Planetary system
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...
had revealed a candidate planetary companion to the star.
In the autumn of 2009 the planet was successfully observed on the other side of the parent star, confirming the existence of the planet itself and earlier observations. It is believed that in 15 years it will be possible to record the whole orbit of the planet.
The radial velocity method used to discover the majority of currently-known extrasolar planets is not well suited to studying A-type stars like Beta Pictoris, and its very young age makes the noise even worse. Current limits derived from this method are enough to rule out hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
-type planets more massive than 2 Jupiter mass
Jupiter mass
Jupiter mass , is the unit of mass equal to the total mass of the planet Jupiter . Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs....
es at a distance of less than 0.05 AU from the star. For planets orbiting at 1 AU, planets with less than 9 Jupiter masses would have evaded detection. Therefore to find planets in the Beta Pictoris system, astronomers look for the effects that the planet has on the circumstellar environment.
Multiple lines of evidence suggested the existence of a massive planet orbiting in the region around 10 AU from the star: the dust-free gap between the planetesimal belts at 6.4 AU and 16 AU suggest this region is being cleared out; a planet at this distance would explain the origin of the falling evaporating bodies, and the warps and inclined rings in the inner disk suggest a massive planet on an inclined orbit is disrupting the disk.
The observed planet by itself cannot explain the structure of the planetesimal belts at 30 AU and 52 AU from the star. These belts might be associated with smaller planets at 25 and 44 AU, with around 0.5 and 0.1 Jupiter masses respectively. Such a system of planets, if it exists, would be close to a 1:3:7 orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
. It may also be that the rings in the outer disc at 500–800 AU are indirectly caused by the influence of these planets.
The object was observed at an angular distance of 411 milliarcsecond
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....
s from Beta Pictoris, which corresponds to a distance in the plane of the sky of 8 AU. For comparison, the orbital radii of the planets Jupiter and 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,...
are 5.2 AU and 9.5 AU respectively. The separation in the radial direction, that is unknown, so this is a lower limit on the true separation. Estimates of its mass depend on theoretical models of planetary evolution, and predict the object has about 8 Jupiter masses and is still cooling, with a temperature ranging from 1400–1600 K. These figures come with the caveat that the models have not yet been tested against real data in the likely ranges of mass and age for the planet.
It is possible that a transit of this planet was observed in November 1981. If this is confirmed, the semimajor axis may be narrowed down to 7.6–8.7 AU and its orbital period to 15.9–19.5 yrs. The inferred radius of the transiting object is 2–4 Jupiter radii, which is larger than predicted by theoretical models. This may indicate that, as may also be the case for the planet Fomalhaut b
Fomalhaut b
Fomalhaut b is an extrasolar planet approximately 25 light-years away in the constellation of Piscis Austrinus. The planet was discovered orbiting the A-type main sequence star Fomalhaut in 2008 in photos taken by the Hubble Space Telescope...
, it is surrounded by a large ring system or a moon-forming disc.
Dust stream
In 2000, observations made with the Advanced Meteor Orbit Radar facility in New ZealandUniversity of Canterbury
The University of Canterbury , New Zealand's second-oldest university, operates its main campus in the suburb of Ilam in the city of Christchurch, New Zealand...
revealed the presence of a stream of particles coming from the direction of Beta Pictoris, which appears to be the dominant source of interstellar meteoroids in our solar system. The particles in the Beta Pictoris dust stream are relatively large, with radii exceeding 20 micrometer
Micrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...
s, and their velocities suggest that they must have left the Beta Pictoris system at roughly 25 km/s. These particles may have been ejected from the Beta Pictoris debris disk as a result of the migration of gas giant planets within the disk and may be an indication that the Beta Pictoris system is forming an Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
. Numerical modeling of dust ejection indicates radiation pressure may also be responsible and suggests that planets further than about 1 AU from the star cannot directly cause the dust stream.