Kinds of Black Holes
Non-Rotating Black Holes

In 1967 the Canadian scientist Werner Israel showed that, according to general relativity, non-rotating black holes must be simple. They are perfectly spherical, their size depends only on their mass, and only two black holes with the same mass are identical.

Can Black Holes
Be Perfectly Spherical?

At first, many people, including Israel himself, argued that since black holes had to be perfectly spherical, a black hole could only form from the collapse of an object that is perfectly spherical. Any real star in our universe--which could never be perfectly spherical--could therefore only collapse to become a naked singularity.

There was, however, a different interpretation of Israel's result, which was proposed by Roger Penrose and John Wheeler in particular. They argued that the rapid movements involved in a star's collapse would mean that the gravitational waves it gave off would make it even more spherical, and by the time it had settled down to a stationary state, it would be precisely spherical.

According to this idea, any non-rotating star, however complicated its shape and internal structure, would end up after gravitational collapse as a perfectly spherical black hole, whose size would depend only on its mass. John Wheeler summed this up in the phrase "a black hole has no hair." Further calculations supported this view, and it soon became to be adopted generally.

Any non-rotating star will end up after gravitational collapse as a perfectly spherical black hole, whose size will depend only on its mass.