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The Question

(Submitted November 06, 2000)

Could you tell me the basics of accretion discs?

The Answer

Accretion disks arise when material (usually gas) is being transferred from one celestial object to another. "accretion" means collecting of additional material. Two major places where astronomers see accretion disks are in binary star systems (two stars orbiting each other) and active galactic nuclei.

I will discuss an accretion disk in a binary star system, but the basic ideas are the same for all cases. If one star in a binary system is a compact object such as a very dense white dwarf star and the other star is a normal star like the Sun, the white dwarf can pull gas off the normal star and accrete it onto itself. Since the stars are revolving around each other and since angular momentum must be conserved, this gas cannot fall directly onto the white dwarf, but instead spirals in to the white dwarf much like water spirals down a bathtub drain. Thus material flowing from the normal star to the white dwarf piles up in a dense spinning accretion disk orbiting the white dwarf. The gas in the disk becomes very hot due to friction and being tugged on by the white dwarf and eventually loses angular momentum and falls onto the white dwarf. Since this hot gas is being accelerated it radiates energy, usually in x rays which astronomers detect and use to identify and study accretion disks.

You can find some more basic information at: http://imagine.gsfc.nasa.gov/docs/features/exhibit/asca_agndisk.html and more at http://antwrp.gsfc.nasa.gov/apod/ap991219.html (part of Astronomy Picture of the Day).

Cheers,

Hans Krimm for "Ask an Astrophysicist"

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