Finding 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 galaxy,
NGC 4622, seen face on.
The very small blue, 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.
And its rather yellow.. meaning cooler, older stars are there.
In order to map out our OWN spiral arms, we must use young star forming regions as our guide in the Galaxy.
It appears we are 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.
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. |