Do Black Holes Spin and Why?

The title almost sounds like something from a science fiction movie, but it is a legitimate question. The simple answer is that yes, as far as we know, all black holes spin and must spin. The “why” requires a little more explanation.

First, an explanation is in order to define what a black hole is.

When a star the size of our sun dies, it first flares up into a red giant star, then collapses in on itself, becoming a white dwarf. A million earths would fit inside the sun, but when it becomes a white dwarf, it will become about the size of our moon, though it will be extremely dense, weighing almost what the sun weighs now. Gradually, it will cool and become a black dwarf. Don’t worry, that probably won’t happen for about another 5 billion years.

When a star that is 2-3 times as massive as the sun dies, it first explodes in a supernova, throwing off the outer layers, then condenses. Electrons, with a negative electrical charge, cancel out protons, with a positive charge, producing neutrons, which have no electrical charge. Without an electrical charge to repel other particles, the star becomes a densely-packed neutron star. About 90% of an atom is nothing but empty space, but since neutrons have no charge, they can be packed together, without space between the particles.

A star that is 3 times as massive as the sun would become a neutron star that was less than 6 miles in diameter but still possessing the same gravitational pull and mass. Here is where we diverge a little.

Every star we’ve ever looked at rotates, just as the earth and all of the planets do. Our own sun rotates, too.

As an end-of-life star contracts, the rotation speeds up. Appropriately, this is visualized as what happens when an ice skater spins on the ice and tucks in their arms and legs. They spin faster without exerting any more effort.

Neutron stars emit energy from their poles. If the spinning is such that the pole sweeps the earth, we detect the streams of energy as pulses. These are called pulsars and the spin rate starts out very fast. A supernova explosion in the year 1039 resulted in a neutron star in the Crab Nebula that we see today, emitting pulses at a rate of about 30 per second. That means that the neutron star is rotating 30 times every second.

In a star is 10 times more massive than the sun, even the mass of the neutrons isn’t enough to stop it from collapsing farther. The star, 10 times larger than our sun and able to hold 10 million earths, would become far smaller than the period at the end of this sentence. Again, it would still have the gravitational pull of a star 10 times bigger than our sun. Since it is condensed into an area so tiny, eventually even smaller than an atom, it becomes a black hole. It seems to wink out of existence, but the gravitation is so great that not even light can escape. 

We can’t see a black hole directly, but there is a huge amount of proof that they exist. Two are located in the center of our own galaxy.

Getting back to the original question, since they form from a spinning star, black holes must spin, probably so fast that we don’t have the means to measure it. It would probably be on the order of 1,000,000,000,000,000 times per second. That is one quadrillionth of a second, per revolution.