Star formation and spiral arms

Key points: What creates spiral arms; nature of density waves

Are the spiral arms in galaxies just the result of stars closer to the center orbiting faster than those further out?

NO, if this were the case, the arms would become more tightly wound with time. Since it takes the Sun only about 200 million years to orbit the Galactic Center, the spiral pattern would have wound up tight and disappeared long ago.

Instead, we explain them in terms of preferential formation of massive, hot stars.

Two differing models may explain the arms; actually we think both things are happening together:

1) Density Wave theory

2) Sequential star formation

Spiral Density Waves

Stars revolve around the Center independent of any spiral arms, but a disk galaxy is only marginally stable and will tend to "clump up" into large-scale arm-like density enhancements that the stars and gas move through. We all have experienced density waves -- density waves of cars are what cause traffic jams! (from William Beaty, http://amasci.com/amateur/traffic/trafexp.html, smashup added by G. Rieke)

Stars moving along their orbits in galaxies can create density enhancements a lot like a traffic jam. For example, if the orbits are elliptical, they can align to produce enhancements that look like spiral arms. Effects such as those in the figure are created continuously in the disks of spiral galaxies, because the disks are only marginally stable.  (from Wikepedia Free Encyclopedia, http://en.wikipedia.org/wiki/Spiral_galaxy).

As an illustration, here is a calculation of the behavior of an isolated galaxy (from F. Bournaud and F. Combes, http://www.obspm.fr/actual/nouvelle/aug02/accretion.en.shtml). The old stars are in red, the young stars in blue, the gas is in yellow. After the development of a spiral, a bar is formed by disk instability in the first 500 million years. This bar soon will weaken, then disappear, but other bars will replace it, for example the second strong bar at about 7 billion years, at the end of the animation.

The effects are enhanced substantially if the disk is disturbed from outside, for example by collision with a smaller galaxy (collisions with large galaxies are usually too disruptive for the disk to survive). For example, a collision of a large galaxy with a disk with a small galaxy is simulated to the left. Compare it with M51 to the right. (simulation from Chris Mihos, http://burro.astr.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html)

Gas clouds move into these density enhancements, they are slowed by the local gravitational field, and as a result they are compressed to trigger the formation of new stars which in turn can light up the HII regions

Young, bright stars show where the density wave just passed

We show the idea schematically as a green molecular cloud passes through a red spiral arm. The cloud is compressed and starts to form massive blue stars, which quickly evolve into supernova explosions (yellow). (animation by G. Rieke)

Sequential Star Formation

In this model:

1.) star formation is triggered by a nearby supernova explosion, whose expanding shock moves through the galaxy and compresses interstellar clouds that it intercepts

2.) differential rotation causes this propagating star formation to assume a spiral shape

"Children of the Sun," by Roger Francois, The Electric Gallery, http://www.egallery.com/francois.html		Starburst candy
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