Gamma-ray bursts are evidence

In the 1930s, Subrahmanyan Chandrasekhar looked at what happens to a star when it has used up all its nuclear fuel. The end result depends on the star’s mass. If the star is really big then its dense core – which may itself be three or more times the mass of the Sun – collapses all the way down to a black hole. The final core collapse happens quickly, in a matter of seconds, and it releases a tremendous amount of energy in the form of a gammaray burst. This burst can radiate as much energy into space as an ordinary star emits in its entire lifetime. Telescopes on Earth have detected many of these bursts, some of which come from galaxies billions of light years away, so we can actually see black holes being born.