Accretion disk

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.