Circumstellar disc

A star can have a ring around it.

This ring is made of gas and dust. It can also have rocks in it.

Young stars have these big rings. The ring holds the stuff that makes planets.

Tiny bits of dust stick together. They grow into bigger rocks.

These rocks can grow into planets. This is how a new star system forms.

Sometimes the rings change over time. They can get thin or lose their gas.

Space is full of these amazing rings. They help us see how worlds begin.

A star can have a ring of material around it. This is called a circumstellar disc.

These discs are shaped like a ring or a pancake. They are made of gas and dust. They also hold rocks and bits of ice.

Young stars often have big discs. These are called protoplanetary discs.

In these discs, tiny grains of dust stick together. They grow into larger rocks called planetesimals. These rocks can then grow into full planets. This is how new solar systems form.

As time passes, the discs change. Some become transition discs. These have less gas and dust than the young ones. Other discs become debris discs. These are very thin and have almost no gas. They are made of dust from rocks hitting each other.

Sometimes, a star is part of a pair. This is called a binary system. These stars can also have discs. A disc might orbit just one star. It might even orbit both stars at once. This is called a circumbinary disc.

A circumstellar disc is a ring of material that orbits a star. These discs can look like a torus, a pancake, or a simple ring. They are made of gas, dust, and small rocks called planetesimals.

Around very young stars, these discs act as reservoirs. They hold the material that might one day become planets. Around older stars, the discs show that planets have already formed. Even around white dwarfs, these discs can show that planetary material survived.

How does a disc form around a young star? It starts within a giant molecular cloud. Gravity causes a pocket of matter to collapse. As it falls inward, the material has angular momentum. This movement causes the gas to form a rotating disc around the new star.

This rotating disc is called a protoplanetary disc. It is made mostly of gas like hydrogen. This gas continues to feed the central star. The main phase of this growth lasts a few million years.

Inside these discs, a slow change happens over time. Tiny grains of rock and ice begin to stick together. These grains grow into larger objects called planetesimals. If the disc has enough mass, these objects grow even faster. This can lead to the creation of planetary embryos.

A Sun-like star usually takes about 100 million years to form. Eventually, the disc changes into a transition disc. These have less gas and dust. Finally, they may become debris discs. These are very thin and have almost no gas left.

Sometimes, a star is part of a binary system with two stars. These systems can have different kinds of discs. A circumprimary disc orbits the more massive star. A circumsecondary disc orbits the less massive star.

There is also a circumbinary disc. This type of disc orbits around both stars at once. These discs often have an inner cavity. This empty space is caused by spiral density waves. One example of a circumbinary disc is around the star system GG Tauri.

We can see these discs in our own space too. Our Solar System has a reservoir of small bodies called the asteroid belt. It sits between the orbits of Mars and Jupiter. Beyond Neptune, we find the Edgeworth-Kuiper belt. There is also a scattered disc and the Oort cloud.

In our system, collisions between asteroids create interplanetary dust. This is similar to the exozodiacal dust found around other stars. These patterns help us understand how every solar system works.

A circumstellar disc is a rotating structure of matter that orbits a star. These discs can take many shapes, such as a torus, a ring, or a pancake. They are composed of various materials, including gas, dust, planetesimals, asteroids, or fragments from collisions.

These structures are vital to understanding how solar systems develop. Around very young stars, they serve as reservoirs of material for planet formation. Around mature stars, they prove that planetesimal formation has already occurred. Even around white dwarfs, these discs show that planetary material survived the star's entire evolution.

The formation process begins within a giant molecular cloud. According to the nebular hypothesis, a young star, or protostar, forms when a pocket of matter collapses due to gravity. As this material falls inward, it possesses angular momentum, which is a property of rotating objects. This momentum causes the infalling gas to flatten into a rotating protoplanetary disc around the new star.

This disc is primarily composed of gas, such as hydrogen, and can contain a few percent of the central star's mass. The main accretion phase, where the disc feeds the star, typically lasts a few million years. During this time, accretion rates usually range between 10^−7 and 10^−9 solar masses per year.

As the disc evolves, it moves through several distinct stages. First, the protoplanetary disc stage features massive amounts of primordial gas and dust. During this phase, small dust grains made of rock and ice begin to coagulate into larger planetesimals.

If the disc is massive enough, runaway accretion occurs, creating planetary embryos. Next comes the transition disc stage, where there is a significant reduction in gas and dust. These discs often show large inner holes. Finally, the debris disc stage occurs. These are tenuous discs with very little gas, where the dust is considered "second generation" because it comes from collisions rather than primordial material.

In binary star systems, the presence of two stars creates more complex disc types. A circumprimary disc orbits the more massive star of the pair. A circumsecondary disc orbits the less massive star, though this requires a high level of angular momentum in the infalling gas.

A circumbinary disc orbits both stars simultaneously. These typically form later and have an inner radius much larger than the distance between the two stars. An example of this is the GG Tauri system. These circumbinary discs often feature an inner cavity. This empty space is created by spiral density waves located at Lindblad resonances. The size of this cavity is proportional to how far apart the two stars are.

Accretion in these systems is not always a steady process. In circumbinary discs, the rate of material falling into the inner cavity varies. For non-eccentric binaries, this variability follows the Keplerian orbital period of the inner gas. For eccentric binaries, the variability matches the binary's own orbital period. This happens because each star component "scoops" matter from the disc at the apocenter of its orbit.

Over much longer timescales, eccentric binaries also show variability due to the apsidal precession of the inner edge of the cavity. This can affect how planets form or migrate within the disc.

Discs can also become misaligned with the plane of the binary system. While most discs are axisymmetric to the binary plane, certain forces can cause a warp or tilt. These forces include the Bardeen-Petterson effect, radiation pressure, or a misaligned dipole magnetic field.

Evidence of these tilted discs is seen in systems like Her X-1 and SS 433. In these cases, the tilted disc causes a periodic blockage of X-ray emissions. This blockage can last between 50 and 200 days, which is much slower than the actual binary orbit. Such misalignments can even cause a disc to tear into multiple separate, precessing discs.

We can observe the remnants of these processes within our own Solar System. Our system contains several reservoirs of small bodies, such as the asteroid belt between Mars and Jupiter. Beyond Neptune, we find the Edgeworth-Kuiper belt and the scattered disc.

Collisions between these bodies create interplanetary dust. This is very similar to the exozodiacal dust found around other stars. By studying these different stages and types, scientists can piece together the history of how planetary systems are built and how they eventually dissipate.