What's An Accretion Disk?

By: Kate Kershner  | 

Accretion disks are one way that we can spot stars and even black holes.
Accretion disks are one way that we can spot stars and even black holes.
© Mark Garlick/Science Photo Library/Corbis

An accretion disk sounds like something you might have installed on an old-school desktop PC in order to load a program. ("To play 'Oregon Trail,' start by inserting your accretion disk.") But accretion disks are way cooler; we find them in some of the most interesting places in the universe. You could spot an accretion disk in a binary star system, or around a black hole, for instance. But how would you even know what you were looking at? The biggest "tell" of an accretion disk is that it surrounds a celestial object (like a star or black hole) with a thick, fuzzy halo.

That celestial ring is the stuff that makes up an accretion disk: gas, dust, matter. In the case of black holes, an accretion disk is formed when any gas or matter that comes near it is snatched into the grasp of the hole. The matter then tumbles down into it.

Advertisement

But hold on a second: It doesn't just fall straight in. Instead, because of a process called conservation of angular momentum, which results from the velocity acting upon a falling object, the matter spirals as it goes in. The spiraling matter gets faster and faster as it gets closer, breaking apart into atom streams. Like water draining in a bathtub, the matter trails around and around the hole. Its atoms flatten out like a twirling pizza pie in the sky — creating the telltale fuzzy doughnut of the accretion disk. Eventually, the matter loses angular momentum and falls into the low point [source: Astronomy Cast].

But why should black holes have all the matter-grabbing fun? Stars also create accretion disks. Imagine two stars in a binary star system. These stars don't just hang out next to each other; the smaller one orbits the bigger one. The big star pulls any gases or matter from the little star into it, eventually gobbling them up — but not before the gas or matter is pulled into orbit around the more massive neighbor, creating (you got it!) an accretion disk [source: Ciardullo].

Accretion disks are one way that we can spot stars and even black holes. Friction between gases and matter makes the accretion disks extremely hot; we can see the X-rays that the super hot gases of the accretion disk give off. Accretion disks can even help scientists determine the mass of a black hole. When the disk gets closer to the black hole, it speeds up and gains energy. It also gives off radiation, which lets astronomers determine how fast the matter is moving. From there, they can extrapolate the mass of the black hole [source: Robbins et al.].

Advertisement

Originally Published: Aug 19, 2015

Accretion Disk FAQ

How hot is an accretion disk?
According to the Max Planck Institute for Astrophysics, friction between gases and matter make accretion disks on the edge of black holes extremely hot. Scientists there predict the temperatures can reach up to 10 million degrees.
How do accretion disks help scientists?
Accretion disks can help scientists determine the mass of a black hole. When the disk gets closer to the black hole, it speeds up and gains energy. It also gives off radiation, which lets astronomers determine how fast the matter is moving. From there, astronomers can extrapolate the mass of the black hole.
What are accretion disks made out of?
An accretion disk surrounds a celestial object (like a star or black hole) with a thick, fuzzy halo. That celestial ring is the stuff that makes up an accretion disk: gas, dust, matter.
Why are accretion disks flat?
The matter spirals as it goes in a black hole. The spiraling matter gets faster and faster as it gets closer, breaking apart into atom streams. Like water draining in a bathtub, the matter trails around and around the hole. Its atoms flatten out creating the telltale fuzzy doughnut of the accretion disk. Eventually, the matter loses angular momentum and falls into the low point.
Where do accretion disks appear in the solar system?
Black holes and binary star systems.

Lots More Information

Related Articles

  • Astronomy Cast. "Accretion Disks." July 11, 2013. (Sept. 11, 2014) http://www.astronomycast.com/2013/07/ep-306-accretion-discs/
  • Ciardullo, Robin. "Binary Star Evolution." Penn State University. (Sept. 11, 2014) http://www2.astro.psu.edu/users/rbc/a1/lec16n.html
  • Encyclopædia Britannica. "Accretion Disks." 2014. (Sept. 11, 2014) http://www.britannica.com/EBchecked/topic/3072/accretion-disk
  • Krimm, Hans. "Ask an Astrophysicist." NASA. Nov. 6, 2000. (Sept. 11, 2014) http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/001106a.html
  • Masetti, Maggie. "Can You Hear a Black Hole?" NASA. Oct. 29, 2013. (Sept. 11, 2014) http://asd.gsfc.nasa.gov/blueshift/index.php/2013/10/29/maggies-blog-can-you-hear-a-black-hole/
  • Robbins, Stuart et al. "Black Holes." Journey Through The Galaxy. Jan. 11, 2006. (Sept. 11, 2014) http://burro.astr.cwru.edu/stu/stars_blackhole.html
  • Wanjek, Christopher. "Ring Around the Black Hole." NASA. Feb. 21, 2011. (Sept. 11, 2014) http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=265

Advertisement

Loading...