THE MILKY WAY GALAXY II

There are two populations of stars.

Population I - younger, higher metallicity stars found in the disk
Population II - older, lower metallicity stars found in the halo and bulge

Pop I stars are second and third generation stars that formed from the remains of the dead first generation of stars. The first generation stars are Pop II. The location of these stars in the galaxy provide a clear picture of the story of star formation in our galaxy (and presumably, other galaxies).

Spiral arms in our Galaxy were found by using 21-cm radiation from hydrogen atoms in gas clouds. The hydrogen atom is like a small magnet and when the electron flips its spin, it is like two magnets flipping and it releases energy in the form of light with a wavelength of 21-cm.

We mapped out most of the galaxy this way, except for the other side. We even have found evidence that there may be a bar in our galaxy.

Another tracer of spiral arms are "O" and "B" stars. This is because the gas clouds in spiral arms are where stars form. "O" and "B" stars are young (they never get really old) and are bright (easy to see). As stars age, they migrate out of the spiral arms.

Star formation in spiral arms is described by the density wave theory. Another mechanism for star formation is called self-sustaining star formation. This latter mechanism is used to explain the presence of branches and spurs in spiral structures.