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The Question

(Submitted November 25, 2001)

From what I understand, stellar black holes are formed by the death of stars, and supermassive black holes by the collapse of large gas clouds or joining of multiple black holes but what causes the formation of mid-mass black holes (of the order of mass of 500 - 1,000's of suns)?

The Answer

Thanks for your question. There are indeed generally two categories of black holes --those formed by massive stars, perhaps less than 100-200 times the mass of the Sun, and supermassive black holes (as big as a million Suns) found at the center of most galaxies. There is actually new evidence for what you call medium sized black holes which according to one theory, may have formed from the merging of smaller black holes over time or simply from the slow build up of accreting nearby material. There is good article about this recent medium sized black hole (about 500 times the mass of our Sun) at:

These recent observations made the news because such medium mass black holes were not expected. There are several theoretical ideas, but we do not have a detailed theory yet.

Hope this helps,
Georgia & Koji
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