What causes objects such as stars and black holes to spin?

Every object in the universe--from a basic star to an exotic black hole--spins, and the origin of that spin can be traced back to the very beginning of time. Within instants after the Big Bang, the primordial fireball of energy expanded at an incredible speed, then later cooled and solidified into all the matter in the universe. Had this fireball been uniform in all directions, everything we see today would be completely homogeneous: There would be a perfectly uniform distribution throughout space of primordial hydrogen and helium, and cosmic microwave background radiation (CBR).

But this fireball was not perfectly uniform, as satellite observations of the CBR show. Some regions were denser than others, and some expanded more rapidly. The denser regions in the universe began to collapse under their own gravitational force, forming the clumps of matter that became giant cosmic structures. An example is the Great Attractor, a huge wall of galaxy clusters that stretches hundreds of millions of light-years across and is located nearly half a billion light-years away from us.

The spin of such cosmic objects is described by a conserved quantity called angular momentum, which accounts for both the speed of the rotating mass and its distance from the spin axis. The largest clumps of matter in the universe had an initial angular momentum--and these clumps broke up into ever smaller clumps, forming smaller clusters of galaxies, groups of galaxies, individual galaxies, solar systems within galaxies and ultimately, individual stars and planets. The smaller clumps each got their own share of the original total angular momentum, continuing down in scale so that everything today has some spin. This process is much like the continuing breakup of turbulent water, spinning off ever smaller individual vortices, each with its own characteristic angular momentum.

For example, in our own solar system, both the planets and our sun spin in the same direction because they were formed from the same primordial cloud of gas and dust. (One of the exceptions to this general trend is Uranus, which may have been knocked over onto its side by some titanic collision in its distant past.) The planets orbit the sun in the same direction in which the sun spins. So, too, moons formed at the same time as their respective planets orbit those planets in the same direction as the planet's spin.

Black holes formed after the Big Bang as stars evolved and died. And because the stars that created the black holes were originally spinning, so were their progeny. 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.

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.