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The Wikipedia article says this, but for those who just read the comments, the formation of supermassive black holes (SMBHs) is very much an open problem! There are two main theories here:

1. You have a collection of black holes that formed from stars in a very dense cluster at the nucleus of the Galaxy early in its life that all merged to form an intermediate mass black hole, which then grew into a SMBH.

2. An intermediate mass black hole formed directly from the direct collapse of a large gas cloud and then grew into a SMBH.

But even the better known case of the formation of stellar mass black holes is still not well understood!




> 2. An intermediate mass black hole formed directly from the direct collapse of a large gas cloud and then grew into a SMBH.

Based on my limited understanding of cosmology, wouldn't the gas first form a star, and not form a SMBH until the star's nuclear reaction became too weak?


Not if the gas cloud is large enough. Small gas clouds (white dwarfs, neutron 'star's) produce enough presure from particle degeneracy to support themselves against gravity, medium clouds (stars) need active heating (from fusion) to keep gas particles moving fast enough to remain far enough apart to avoid forming a black hole, and for sufficiently large clouds even having the gas particles moving at the speed of light wouldn't be enough to avoid forming a black hole.


Put enough mass in the right radius and fusion can't do jack about it. Over a large enough area, IIRC it doesn't actually take a very high density, so it never has to reach a point where it can fuse before the event horizon forms.


I recall someone calculating that a sphere of gas with a radius equal to Neptune's orbit, with the same density of air at sea level would mass 4 million solar masses. That means if we filled thw Solar system with air, it would form a SMBH.




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