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If a galaxy is the accretion disk for a supermassive black hole, how can it be a shape other than disk?

Spam
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2 Answers2

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Because it is not an accretion disk. Accretion disks occur when matter orbits a massive object in such a way that it interacts with itself, losing energy through frictional heating and converging into a hot disk. A galaxy consists of stars orbiting their mutual center of mass, without any appreciable friction or other violent interactions. There may certainly be a black hole at the center, but it is much smaller. A galaxy can be elliptic or irregular, depending on the orbits.

Anders Sandberg
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  • So it's by chance that our galaxy orbit's the super massive in a spiral? – Spam Dec 01 '18 at 01:27
  • Spam, I think you have misunderstood the sizes and masses involved. Supermassive black holes can weigh in at billions of solar masses and have accretion disks a parsec across in some cases. But the milky way has 300 billion stars and even more dark matter, and is tens of thousand of parsecs across. The black hole is dwarfed by the galaxy. – Anders Sandberg Dec 01 '18 at 16:26
  • Oh, I might have misunderstood you. Yes, it is a kind of coincidence that the milky way look like an accretion disk. – Anders Sandberg Dec 01 '18 at 23:24
  • If it's just a coincidence why are there so many spiral galaxies? – Spam Dec 02 '18 at 02:18
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In a galaxies early stage it’s not disc shaped and doesn’t necessarily have a black hole. When two galaxies collide they can disturb each other so much they become sphere shaped and start forming a disc all over again.

Bill Alsept
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