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Angular Momentum Explained: Why Planets, Stars and Galaxies All Spin
A cloud a hundred thousand times larger than the star it will become needs only the faintest turn to end up spinning fast.
Planets spin, stars spin, and galaxies like NGC 4414 turn as whole disks of stars and gas. The question of why has a short answer, conservation of angular momentum, and a longer one about how a universe that may have started with no net spin ends up full of rotating things. The longer answer is built here one step at a time, starting with the conservation laws themselves.
Three quantities that never change
Physics has a small set of conservation laws, and three of them do most of the work in astronomy: energy, linear momentum and angular momentum. Each says the same thing about its own quantity. In a closed system, where nothing enters or leaves, the total amount never changes, however much it moves around inside.
Energy can change form. Rubbing your hands together turns the kinetic energy of the motion into heat through friction, and the palms warm up, but the total is the same before and after. Linear momentum, mass multiplied by velocity, behaves similarly in collisions. When a small billiard ball strikes a heavy bowling ball and both end up with the same share of momentum, the bowling ball barely moves while the billiard ball shoots off fast, because a large mass needs only a small velocity to carry the same momentum.
Angular momentum is the measure of spin. For something moving in a circle it equals mass times speed times distance from the center. Earth on its nearly circular orbit around the Sun is an example: for a fixed amount of angular momentum, a wider orbit would go with a lower speed.
The solar system is not a closed box
The planets' orbits lie close to a single plane, so the solar system as a whole carries a large net angular momentum. If the universe began with zero net spin, how did this piece of it acquire so much?
Two details resolve the puzzle. First, the solar system was never isolated, so the conservation law does not apply to it alone. Second, angular momentum is a vector: it has a direction as well as a size. The solar system's points roughly perpendicular to the plane of the planets' orbits. Other planetary systems point in other directions, and with directions scattered at random, the total over many systems can add up to almost nothing.
Stars and their planets form inside nebulae. Picture a nebula with no overall spin. As separate pockets inside it collapse, they pull on one another through gravity and exert gravitational torques, twists that hand spin from one region to another. Each pocket can end up rotating while the clockwise and counterclockwise contributions still cancel for the nebula as a whole. Neighboring nebulae torque one another in the same way, and the argument repeats at every scale, because gravity works the same everywhere.
Galaxies spin too
NGC 4414 is a disk of stars, gas and dust, and a disk like that is a rotating system. The same reasoning that applies to planetary systems applies to galaxies: if the universe holds no net angular momentum, the spins of all the galaxies must cancel out when added together.
Whether the universe as a whole really carries no net spin is hard to test. It is a reasonable assumption, because no obvious process generates angular momentum from nothing. Then again, no obvious process explains where the total energy of the universe came from either.
The image of NGC 4414 at the top of this page has a history of its own. It was made in 1995 for the Key Project on the Extragalactic Distance Scale, which observed the galaxy on 13 occasions over two months with the Wide Field and Planetary Camera 2 to find Cepheid variable stars and measure the distance to it. The data were later combined into the color picture released by the Hubble Heritage project in 1999. The tilted, flattened disk itself is a sign of rotation, as it is in the nearly edge-on Sombrero Galaxy.
The ice-skater effect
The second half of the story is the ice-skater effect. A skater who starts a spin with arms and legs stretched out turns slowly: much of the body is far from the axis. Pulling the arms and legs in reduces the distance, and to keep angular momentum the same the speed has to rise. The skater spins faster without pushing off against anything.
A collapsing cloud does the same thing on a huge scale. The nebula that will form a planetary system begins roughly 100,000 times the size of the system it becomes. Shrinking by that factor raises the rotation speed by roughly the same factor, so even a barely perceptible turn at the start becomes a fast spin at the end. Hubble has photographed disks of gas and dust around young stars in the Orion Nebula, the kind of disk long suspected to be an early stage of planet formation.
Questions readers ask
Why do planets and stars spin?
They formed from collapsing clouds of gas that had a small amount of rotation. As each cloud shrank, conservation of angular momentum made it turn faster, the way a skater spins up by pulling in their arms. The result is rapidly rotating stars and planets.
Does the universe as a whole rotate?
The usual assumption is that the universe carries no net spin, with the spins of galaxies and smaller systems pointing in random directions and canceling out. The assumption is reasonable, since nothing obvious creates angular momentum, but it is very difficult to measure.
What is a gravitational torque?
It is a twisting effect that one clump of matter exerts on another through gravity. Inside a collapsing nebula, torques between neighboring regions move angular momentum around, so individual regions can spin while the nebula as a whole keeps the same total.
How much faster does a collapsing cloud spin?
Roughly in proportion to how much it shrinks. A nebula some 100,000 times wider than the planetary system it forms speeds up its rotation by a similar factor, so a very slow initial turn becomes a rapid spin.