There is actually a lot of uncertainty about how supermassive black holes form and it is an active area of research. Starting with a stellar-mass black hole and just growing through accretion doesn't really work well because it's hard to have enough matter, within a small enough volume, for the black hole to grow to billions of solar masses in size.
Yep. There are stellar-mass black holes that form after supernovae. These have roughly 5-80 times the mass of our sun. Then there are super-massive black holes, which typically exist in the centers of galaxies. These have between 100,000-50,000,000,000 times the mass of our sun.
Interestingly, there is a gap in the distribution of masses of observed black holes. Intermediate-mass black holes (between 80 and 100,000 solar masses) have proven much harder to find, which suggests that supermassive black holes may not develop from stellar mass black holes. It may also mean they are just harder to identify, for some reason.
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u/strib666 Apr 10 '19
There is actually a lot of uncertainty about how supermassive black holes form and it is an active area of research. Starting with a stellar-mass black hole and just growing through accretion doesn't really work well because it's hard to have enough matter, within a small enough volume, for the black hole to grow to billions of solar masses in size.