What is an Accretion Disk?

An accretion disk might remind you of an old-school desktop PC program disk. ("To play 'Oregon Trail,' begin by inserting your accretion disk.") However, accretion disks are far more thrilling; they exist in some of the most remarkable places in the cosmos. You could encounter one in a binary star system or even around a black hole. But how can you identify one? The key indicator of an accretion disk is its characteristic thick, fuzzy ring that encircles a celestial body like a star or black hole.

This celestial ring is made of materials that form an accretion disk: gas, dust, and matter. In the case of black holes, the accretion disk forms when any nearby gas or matter is drawn in by the black hole’s gravitational pull, eventually falling into it.

Wait a minute: it doesn't just fall straight in. Due to a phenomenon known as the conservation of angular momentum, caused by the velocity of a falling object, the matter spirals inward. As it nears the center, the spiraling speed increases, causing it to break apart into streams of atoms. Similar to how water spirals down a drain, the matter circles around the hole. The atoms flatten, resembling a twirling pizza in the sky, creating the characteristic fuzzy doughnut shape of the accretion disk. Eventually, the matter loses its angular momentum and falls into the low point [source: Astronomy Cast].

Why should black holes have all the fun of grabbing matter? Stars can form accretion disks too. Imagine two stars in a binary system. These stars don't simply sit next to each other; the smaller star orbits the larger one. The massive star pulls gases or matter from the smaller star, eventually consuming it, but not before the matter is pulled into orbit around the bigger star, forming an accretion disk [source: Ciardullo].

Accretion disks are key to spotting stars and even black holes. The friction between gases and matter in the disks generates extreme heat, allowing us to detect the X-rays emitted by the superheated gases. These disks also assist scientists in determining a black hole's mass. As the disk moves closer to the black hole, it accelerates and gains energy. This also results in radiation emission, which enables astronomers to calculate the velocity of the matter. From this, they can estimate the black hole's mass [source: Robbins et al.].