Why do black holes spin

Indeed, even though stars eventually run out of nuclear fuel, they maintain their spin after death. When stars are in the midst of their normal life cycles, their hot lower layers exert enough pressure to support the tremendous weight of their upper layers. But once stars run out of fuel and their fusion reactions end, they can no longer hold up this weight and collapse onto themselves. In the supernova explosions that precede the formation of black holes, some of the mass of the star is blown off, carrying away part of the total angular momentum of the star.

The remaining matter falls towards the center of the star, spinning faster and faster as it goes. Just as a skater who brings his arms closer to his sides speeds up, so, too, a collapsing star winds itself up and spins faster when it is contracting. This acceleration enables the universe to conserve its total angular momentum; as matter falls in closer to the spin axis, it must increase in speed.

Sher 25 arrow is a massive blue star, surrounded by an hourglass nebula it ejected 6, years ago. Most probably, it will explode as a supernova, paving the way for the formation of a black hole. By the time this matter has fallen past the point of no return, called the event horizon, it has concentrated the angular momentum of the black hole into a very small volume, which greatly distorts the surrounding space-time.

The angular momentum persists even after the matter that caused it has cut itself off from our universe, collapsing beyond the event horizon. Presently, a team of researchers at Stanford University is designing the Gravity Probe satellite to measure the distortion of space-time due to the angular momentum of our own spinning Earth. Although the space-time distortion near Earth is exceedingly weak compared to that around a black hole, the same physics are at work.

Measuring this distortion would offer further support for Einstein's Theory of General Relativity. Newsletter Get smart. Sign up for our email newsletter. US state, Canadian province, or country. Tonight's Sky — Select location. Tonight's Sky — Enter coordinates. UTC Offset:. Picture of the Day Image Galleries. Watch : Mining the Moon for rocket fuel.

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Groups Why Join? Astronomy Day. The Complete Star Atlas. Robert Walty Stephens City, Virginia. Tracking such a spot recently allowed astronomers to measure the spin of a black hole. When astronomers say they have estimated the spin of a black hole, what they have measured is its angular momentum. This is the total angular momentum of all the material that ever crossed the event horizon and got trapped inside. It is this angular momentum that whirls space-time around a black hole, which is the effect we call spin.

Imagine yourself on a spaceship traveling into a black hole. X-ray light from the disk is given a boost of energy from the rotation, and by measuring that boost, we can determine the spin. Another way is to take a direct image of the black hole, as we did with the one at the center of M The ring of light we see is brighter on the side rotating toward us.

But a new paper in Astrophysical Journal Letters argues that there is another way to measure spin. Their method uses a property known as frame dragging. When a mass rotates, it twists space around it slightly. Hundreds of stars orbit the black hole at the center of our galaxy.

About forty of them, known as S-stars, have orbits with a close approach to the black hole. Over time their orbits are shifted by the frame-dragging effect. If we can measure these shifts, we can measure the spin—the greater the spin, the greater the orbit shift.