Black Holes
Most black holes are the result of a stellar explosion called a supernova. As a very massive star explodes, its core is crushed to enormous density—in fact, the star's density becomes so large that its gravity increases to the point that not even light can escape from the dead star. This is what makes a black hole black.
Not all stars will end their lives as black holes. As star must have a mass at least at least 25 times larger than our Sun to end in a supernova explosion powerful enough to create a black hole. Fewer than 1 out of 1000 stars in our Milky Way are massive enough to become a black hole.
Image at right: A growing black hole, called a quasar (courtesy of NASA/JPL-Caltech).
DID YOU KNOW?
Still confused about density? Try an experiment from HowStuffWorks.com.
