Accretion disk
An accretion disk is a flat, disk-shaped structure composed of gas and dust that orbits in a spiral around a compact object— black hole, neutron star, or white dwarf. As it falls toward thecentral object, the material heats up to extreme temperatures and emits enormous amounts of radiation. It is one of the brightest structures in the universe.
How does an accretion disk form?
When matter falls toward a compact object, it does not fall directly onto it. Because of its angular momentum (a property related to its rotation), it spirals inward and forms a flattened disk around the object. The different layers of the disk orbit at different speeds—the inner layers faster than the outer ones. This difference in speed creates viscous friction, which heats the gas to temperatures that can reach millions of degrees.
Intense radiation
Falling matter releases a colossal amount of gravitational energy. For a black hole, the efficiency of converting mass into energy can reach 10 to 40%—far higher than the 0.7% of nuclear fusion. This is why the accretion disks around supermassive black holes—which power quasars—are the brightest objects in theuniverse, visible billions of light-years away.
The first image of an accretion disk
In 2019, the Event Horizon Telescope project captured the first high-resolution image of an accretion disk: the one surrounding the black hole M87*. The visible luminous ring corresponds to gas orbiting extremely close to the event horizon. The central shadow is the event horizon itself, blocking all light coming from behind.
Did you know?
The relativistic jets observed in some active systems—columns of matter ejected perpendicular to the disk at speeds close to the speed of light— are thought to be produced by the intense magnetic fields generated in the accretion disk. The exact mechanism remains an active area of research.
Frequently Asked Questions
Do all black holes have an accretion disk?
No. An accretion disk forms only if the black hole has a source of matter to accumulate. An isolated black hole in a vacuum does not have one. Sagittarius A*, the black hole at the center of the Milky Way, has a very faint disk—it is relatively inactive.
What is the temperature of an accretion disk?
It depends on the central object and the accretion rate. Around a white dwarf, a few tens of thousands of degrees. Around a stellar black hole, it’s a few million degrees. Around a supermassive black hole, the inner regions can reach several hundred million degrees.
What is the difference between an accretion disk and a protoplanetary disk?
A protoplanetary disk surrounds a young star and is the birthplace of planets—it is cold and dense with dust. An accretion disk surrounds a compact object and is the site of intense energetic accretion. Both are disks, but their natures and roles are very different.