Galaxies, Page 2

Galaxies, Page 2: [Previous Page] [Next Page] [Class Notes] Textbook pages 292 - 304

Understanding our Galaxy's Structure

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. And its rather yellow.. meaning cooler, older stars are there.

The Sun is at the end of one spiral arm, but there is clear evidence of a row of star formation both inside and outside of our position, relative to the center of the galaxy.

Our Galaxy rotates around the center at a rate of 220 km/s. We are about 26,000 light years from the center. It takes about 240 million years for the Sun to go around the Galaxy, so we've been around about 20 times already.

By deriving the speed as a function of different radii we can derive the Rotation Curve of the Galaxy. Knowing the Sun's velocity around the Galaxy we can estimate the mass of the Galaxy out to our radius. It is about 100 billion Solar Mass.

Why do we see Star Formation in the Spiral Arms? Because 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.