# How can a spiral galaxy exist?

A spiral arm orbiting a central mass should be dispersed quite quickly as the outer elements would move more slowly than the inner ones. The Milky Way, is about 59 Galactic Years old, which, one would have thought, would be enough rotations to disperse a spiral structure entirely.

Is there, then, something keeping the spiral arms in existence, and if so what could it be? Or are the spiral galaxies monstrous co-incidences?

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According to wikipedia, the galaxy is about 59 galactic years old. –  seanmonstar Jun 8 '11 at 21:42
@seanmonstar I always though GY meant giga-year, or 1 billion years. –  Carson Myers Jul 12 '11 at 6:08
@seanmonstar, you are quite right; I've edited accordingly. Thank you. –  Brian Hooper Jul 12 '11 at 7:29

The material (gas and stars) in the outer part of a galaxy move with roughly the same velocity as the inner part (for example, see this paper), which means that the inner portions do indeed have a faster angular speed; this is sometimes referred to as the "winding problem."

One important feature of spiral arms is that they are bright more because they have lots of young stars than because they have extra material. Young populations of stars include bright, short-lived, blue stars, which die off over time, leaving the fainter, redder populations. Populations of these young stars are particularly apparent in images like this one. Because of this population of bright, young stars, the density of matter in spiral arms compared to the non-arm disk is not as great as their brightness would suggest.

The usual explanation for spiral arms is that they are the result of density waves rather than moving structures. See this paper for a short review.

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