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What is an accretion disk?

31 Aug 2018, 11:05 UTC
What is an accretion disk?
(200 words excerpt, click title or image to see full post)

An artist’s impression of a swirling cloud of dust and particles around a star
Asked by James Davies
Whenever gas nears an object with significant gravity, it moves towards it. If the object – be it a newborn star, a white dwarf or a dreaded black hole – is spinning, then the gas falls into a spinning disk around it, called an accretion disk because the gas is ‘accreting’ onto the object.
Around a newborn star this eventually grows into planets. Around a white dwarf stealing gas from a close companion star, the gas in the accretion disk winds up on the surface, where too much causes a supernova. Around a black hole the gravitational forces are so strong that the gas becomes heated to millions of degrees. In the most extreme cases these discs glow so bright that they can be seen across the universe as a quasar.
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