Q& A: Black Holes
In your story of Mid-sized black holes the comment was made,
"The black hole found in M82 packs the mass of at least 500 suns
into a region about the size of the Moon." How is this possible
since I thought black holes had infinite density and zero
The region described refers to the diameter of the event horizon
of the black hole, which is proportional to the mass of the
black hole, and equal to 3000 km for a non-rotating black hole.
for more information.