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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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