Accretion disk

Big things in space have rings.

These rings are made of gas and dust. The material spins around a big object. This object can be a star. It can even be a black hole.

As the bits of gas spin, they rub together. This rubbing makes them very hot. The heat makes the ring glow with light.

Some rings help make new planets. Other rings sit around very old stars. They can even shoot out long jets of gas.

Space is full of these bright, spinning rings.

Space is full of spinning rings of gas and dust. We call these accretion disks. They form when material orbits a big object. This object might be a star or a black hole.

How does the disk work? The bits of gas and dust rub against each other. This rubbing is called friction. It makes the material very hot. This heat gives off light.

Young stars often have these disks. They are called protoplanetary disks. These disks can help make new planets.

In some cases, the disk is near a black hole. The material spirals inward. It follows a path called a tendex line. As the particles move closer, they go faster. This makes them even hotter. A black hole disk can be hot enough to give off X-rays.

Some disks also make long jets of gas. These jets shoot out from the center. These jets help the system lose angular momentum. This is the force that keeps things spinning. Without this, the material could not fall inward.

An accretion disk is a spinning ring of material in space. This material orbits around a huge central object. Often, that object is a star. Sometimes, the center is a black hole. These disks are found almost everywhere in space. They can be part of active galactic nuclei or gamma ray bursts. They are also found around young stars.

How does this disk work? The material does not just fall straight in. Instead, it follows an inward spiral called a tendex line. This happens because particles rub and bounce against each other. This rubbing creates a turbulent flow. This flow causes frictional heating. As particles rub, they lose angular momentum. Angular momentum is the force that keeps things spinning. Without losing this, the material could not drift inward.

Scientists have studied these disks for a long time. In the 1940s, researchers made the first models. They used basic physical principles to explain them. Later, in 1973, Shakura and Sunyaev proposed a new model. They suggested that turbulence in the gas caused extra viscosity. In 1991, S. A. Balbus and J. F. Hawley found a new mechanism. They discovered magnetorotational instability. This showed how magnetic fields help move material inward.

There are many interesting facts about these disks. Young stars have disks called protoplanetary disks. These disks can grow into new planetary systems.

The temperature of the disk changes what kind of light it gives off. Disks around young stars shine in infrared light. Disks around black holes are much hotter. They can emit X-rays. The process of accretion is very efficient. It can turn 10 percent to 40 percent of an object's mass into energy. This is much more than nuclear fusion.

You can think of an accretion disk like a swirling drain in a bathtub. As water moves toward the center, it spins faster. In space, the gas and dust do something similar. Some disks even create powerful jets of gas. These jets shoot out from the center of the object. They help the system shed angular momentum. This allows the star or black hole to keep growing.

An accretion disk is a massive structure of diffuse material in orbital motion. This material revolves around a central body, which is most often a star. These disks are ubiquitous phenomena throughout the universe. They are found in active galactic nuclei and gamma ray bursts. They also exist as protoplanetary disks around young stars.

These disks are essential to understanding how matter moves in space. They allow objects to grow by pulling in surrounding gas and dust.

The mechanism of an accretion disk relies on the loss of angular momentum. Angular momentum is the property that keeps an orbiting object moving in a circle. As matter enters the disk, it follows a trajectory called a tendex line. This is an inward spiral toward the center. Particles in the disk rub and bounce against each other in a turbulent flow. This creates frictional heating, which radiates energy away. As the particles lose energy, they also lose angular momentum. This loss causes the particles to drift into lower orbits.

As a particle falls into a lower orbit, its velocity actually increases. This happens because a portion of its gravitational potential energy is converted into speed. However, even though the particle is moving faster, it has lost total energy. As the particle orbits closer to the center, the velocity and frictional heating increase. The disk around a black hole becomes hot enough to emit X-rays just outside the event horizon. This process is incredibly efficient at converting mass into energy. Accretion can convert 10 percent to over 40 percent of an object's mass into energy. For comparison, nuclear fusion only converts about 0.7 percent of mass into energy.

Accretion disks can be classified by their central objects and their radiation. Disks surrounding young stars or protostars are called protoplanetary disks. These are the progenitors of planetary systems. They radiate primarily in the infrared part of the spectrum. In contrast, disks around neutron stars or black holes are much hotter. These disks emit radiation in the X-ray part of the spectrum.

Scientists also study the oscillation modes within these disks through a field called diskoseismology.

History shows how our understanding of these disks has evolved. In the 1940s, researchers derived the first models from basic physical principles. These early models required an unknown mechanism to explain how angular momentum moved. In 1973, Shakura and Sunyaev proposed the $\alpha$-disk model. They suggested that turbulence in the gas provided an increased viscosity. This viscosity helped transport angular momentum outward so matter could fall inward.

In 1991, S. A. Balbus and J. F. Hawley discovered magnetorotational instability (MRI). This provided a direct mechanism for how magnetic fields redistribute angular momentum.

Magnetorotational instability explains how magnetic fields drive the disk's behavior. In a magnetized disk, two neighboring fluid elements act like they are connected by a spring. The inner element orbits faster than the outer element. This stretches the magnetic "spring" between them. The tension pulls the inner element back, causing it to slow down. This loss of angular momentum moves the inner element to a lower orbit. Meanwhile, the outer element is pulled forward and moves to a larger radius.

This process can become a runaway effect that drives the entire disk.

Accretion disks are also linked to the creation of powerful astrophysical jets. These jets shoot out from the vicinity of the central object along its rotation axis. They are an efficient way for the star-disk system to shed angular momentum. This allows the system to continue accreting mass without losing too much of it. These jets require large-scale magnetic fields in the inner regions of the disk. The magnetic fields can be carried inward from the interstellar medium. They can also be generated by a magnetic dynamo within the disk itself.