What is a black hole?

A black hole is an invisible region of space that is thought to have such intense gravity that not even light can escape. Scientists believe that a black hole is created when a giant star collapses in upon itself as it dies. A star lives as long as it can burn fuel. The burning of fuel acts as a counterforce against gravity; without that counterforce, a star's gravity would cause it to collapse in on itself. So when that fuel runs out, gravity takes over and crushes the star. If the star is large enough and has a strong enough force of gravity, it will become a black hole when it collapses. Scientists have discovered evidence of several black holes in our galaxy, and they believe there may be millions more that have not been identified.