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Why Galaxies Spiral

Week of April 28, 1997

For a hundred years or so now we have looked at pictures of spiral galaxies and wondered at their beauty. It wasn't until the 1920's that we began to understand their nature, that they are vast collections of billions of stars. But it was only a few years ago that we began to understand why they sometimes form spirals.

thumbnail of M51

The nature of gravity is a bit peculiar in such a huge collection of stars. The stars orbit the galaxy center in much the same way the planets go around the Sun, but in this case the stars themselves are contributing to the overall gravitational field of the galaxy. As they orbit in a disk, sometimes there are "waves" of gravity that propagate through that disk. This can cause a galactic traffic jam, making matter pile up at these positions, and stretch out in between the waves. Because stars are relatively small, they are not affected much by these traffic jams. But gas clouds can be huge, trillions of kilometers across, and when they hit the traffic jam they can collide. This can collapse some of the clouds. When clouds collapse, they form stars. The brightest stars will light up the clouds, making them visible.

So when you see a "grand design" spiral, what you are really seeing are the gas clouds being lit by the bright stars that were formed as the clouds collide. Those brightest stars are actually short lived; they rarely live long enough to wander far from the cloud that formed them. So the spiral arms really don't have that many more stars than the space between the arms; it's just that there are more bright stars there. The brighter ones are easier to see!

Incidentally, the picture shown is of a nearby spiral called M51, or the Whirlpool Galaxy (click it for a larger, higher resolution image with caption). The picture was taken with the Hubble Space Telescope as part of the Supernova Intensive Study, or SINS. The star arrowed is one that blew up in 1994. Notice that it sits in a spiral arm; that type of supernova happens only to the biggest and brightest stars, so you'd expect it to be in a spiral arm! Get it?

©2008 Phil Plait. All Rights Reserved.

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