Spiral arms

This is the Whirlpool Galaxy, 25 million light years away, seen from straight above. Two bright arms wind out from its middle. Many galaxies have arms like these, and so does our own. They look like the most solid thing about a galaxy. They are the least.

Here is the puzzle. A galaxy does not turn like a wheel. Stars near the middle get round much sooner than stars further out. If an arm were a fixed string of stars, the inner end would keep lapping the outer end, and within a few turns the arm would be wound up like a watch spring. Galaxies have turned dozens of times. Their arms should be gone.

So an arm cannot be a fixed set of stars. It is a pattern, a place in the disc where stars and gas are crowded closer together for a while. It works like a jam on a motorway. The jam stays put, or creeps along slowly, while the cars in it keep changing. Each star drifts in, slows, and comes out the other side.

The crowding alone is not what makes the arms stand out. What does is the gas. Squeezed as it enters an arm, the gas falls together and makes new stars. The pink patches in this picture are clouds where that is going on, and the blue is clusters of young hot stars. The brightest of those burn out before they have time to leave, so the arm is always lit and always by new stars.

Something has to set the pattern going. In the Whirlpool it is the small yellow galaxy at the end of one arm, which has been drifting past for hundreds of millions of years and stirring the disc with its gravity. In other galaxies a bar does the job. This one, NGC 1300, has a straight bar of stars across its middle and an arm starting from each end. About two spirals in three have a bar.

Our own galaxy has a bar too, and two main arms with several lesser ones. Nobody has seen them from outside, and working out the pattern from inside is slow, hard work. This galaxy, NGC 6744, is thought to look much as ours would. The Sun sits near the inner edge of a short spur between two arms, on a road it has been round about twenty times.