Q& A: Black Holes

Q:
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 volume.

A:
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. See http://chandra.harvard.edu/xray_sources/bh_midmass.html for more information.