Page 14, The Milky Way and Galaxies: [Last Page] [Next Page] [Lecture Notes] [Class Home Page]

Our Galaxy's Spiral Arms

What is more difficult to imagine is how our galaxy looks from the ``top''. We believe it looks something like this picture of M81. The very small white, lumpy regions along the Spiral Arms are young clusters of Massive stars. Massive stars are short lived, and thus live close to the dust and gas they just formed from. We also see the BULGE at the center of the Galaxy. The light from the Bulge is very smooth, not lumpy like the Spiral Arms. Why do we see Star Formation in the Spiral Arms?







Recall, our galaxy rotates differentially. Stars orbiting further out travel at the same speed, but have a larger circumference to cover. Soon, they begin to lag behind stars on inner orbits.

As the stars inside wind up faster, a density wave develops. In this density wave, lots of gas and material builds up and Giant Molecular Clouds form. This, in turn, creates sites of star formation.