(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)?
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
for "Ask an Astrophysicist"