Introduction
The History of Black Holes

Albert Einstein's general theory of relativity proved that if gravity could become strong enough, it would rob light of all its energy. For gravity to become this strong, it would have to be an extremely dense object. It would have to have an extremely large mass in a very small space.

In 1916, the German astronomer Karl Schwarzchild (see photo on right) figured out just how small a star would need to become. According to Schwarzchild, a star the size of our Sun (866,000 miles or 1,394,00 kilometers in diameter) would have to shrink to less than four miles (six kilometers) wide. This would be like shrinking the largest mountain to the size of a butterfly.

Photo by Robert Bein, Courtesy AIP Emilio Segrč Visual Archives

If the Sun were the size of a large mountain, when collapsed into a black hole, it would shrink to the size of a small butterfly. And yet, it would still weigh as much as the original mountain!

Can stars become this small? In 1939, the American physicists J. Robert Oppenheimer and Hartland S. Snyder discovered that this is possible only if the star is much larger than our Sun.

For most of their lives, stars remain a constant size because they have a balance of forces: Heat made by burning fuel pushes the star out, and the effect of gravity pulls it in. After billions of years, when the star has used up all of its fuel, it collapses under its own weight.

For most of their lives, stars remain a constant size because they have a balance of forces: Heat made by burning fuel pushes the star out, and the effect of gravity pulls it in.

Oppenheimer and Snyder proved that if the star has more than 3.2 times the mass of the Sun (known as 3.2 solar masses), there is nothing to stop it from collapsing completely.