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 disc around it, called an accretion disc 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 disc 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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