Black+Hole+Period+4

http://cosmology.berkeley.edu/Education/BHfaq.html#q1 http://hubblesite.org/explore_astronomy/black_holes/ http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html http://hubblesite.org/explore_astronomy/black_holes/ http://www.usatoday.com/tech/science/space/2008-01-15-black-holes-spin_N.htm