Star formation and spiral arms

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

· As 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:

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

· 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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