Circumstellar disk
Encyclopedia
A circumstellar disk is a torus
, pancake or ring-shaped accumulation of matter
composed of gas
, dust
, planetesimal
s, asteroid
s or collision fragments in orbit
around a star
. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place and around white dwarf
s, they indicate that planetary material survived the whole of stellar evolution. Such a disk can manifest itself in various ways.
, which results in the formation of a gaseous protoplanetary disk
around the young, rotating star. The former is a rotating circumstellar disk of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, mainly in the form of gas which is itself mainly hydrogen. The accretion disk phase lasts a few to 10 million years. Accretion rates are typically 10−7 to 10−9 solar masses per year but can vary.
The disk gradually cools in what is known as the T tauri star stage. Within this disk, the formation of small dust grains made of rocks and ices can occur, and these can coagulate into planetesimal
s. If the disk is sufficiently massive, the runaway accretions begin, resulting in the appearance of planetary embryos. The formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...
, pancake or ring-shaped accumulation of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
composed of gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
, dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
, 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, 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 or collision fragments in orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
around a 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...
. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place and around white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
s, they indicate that planetary material survived the whole of stellar evolution. Such a disk can manifest itself in various ways.
Young star
According to the currently accepted model of star formation, sometimes referred to as the nebular hypothesis, a star is formed by the gravitational collapse of a pocket of matter within a giant molecular cloud. The infalling material possesses some amount of angular momentumAngular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
, which results in the formation of a gaseous protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
around the young, rotating star. The former is a rotating circumstellar disk of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, mainly in the form of gas which is itself mainly hydrogen. The accretion disk phase lasts a few to 10 million years. Accretion rates are typically 10−7 to 10−9 solar masses per year but can vary.
The disk gradually cools in what is known as the T tauri star stage. Within this disk, the formation of small dust grains made of rocks and ices can occur, and these can coagulate into 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. If the disk is sufficiently massive, the runaway accretions begin, resulting in the appearance of planetary embryos. The formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.
Belt or cloud
- Asteroid beltAsteroid beltThe 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...
is a reservoir of small bodies in our Solar System located between the orbit of Mars and Jupiter. It is a source of interplanetary dust. - Edgeworth-Kuiper beltKuiper beltThe 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...
- Öpik-Oort cloudOort cloudThe 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...
/ Hills Cloud, only the inner Oort cloud has a toroid-like shape. The outer Oort cloud is more spherical in shape.
Binary system
- Circumprimary disk, is where a disk orbits the primary (i.e. more massive) star of the binary star system.
- Circumsecondary disk is one around the secondary (i.e. less massive) star of the binary star system.
- Circumbinary disk, is where a disk orbits both the primary and the secondary of the binary system.
Dust
- Debris diskDebris diskA 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...
consists of planetesimals along with fine dust and small amounts of gas generated through their collisions and evaporation. The original gas and small dust particles have been dispersed or accumulated into planets. - Zodiacal cloud or interplanetary dust is the material in the Solar System created by collisions of asteroids and evaporation of comet seen to observers on Earth as a band of scattered light along the ecliptic before sunrise or after sunset.
- Exozodiacal dustExozodiacal dustExozodiacal dust is the exoplanetary analog of zodiacal dust, the 1-100 micrometre-sized grains of silicate dust that fill the plane of the solar system, especially interior to the asteroid belt. As for the zodiacal dust, these grains are probably produced by outgasing comets as well as by...
is dust around another star than the Sun in a location analogous to that of the Zodiacal Light in our own Solar System.
External links
- Image Gallery of Dust disks (from Paul KalasPaul KalasPaul Kalas is a Greek American astronomer known for his discoveries of debris disks around stars. Kalas led a team of scientists to obtain the first visible-light images of an extrasolar planet with orbital motion around the star Fomalhaut, at a distance of 25 light years from Earth...
, "Circumstellar Disk Learning Site)"