Stellar Populations in Spiral Galaxies


How can we study what kinds of stars are in galaxies? We can only resolve individual stars in the very closest of galaxies (Andromeda and a few others). We generally study stellar populations of galaxies using integrated colors or spectra.

We find that disks of spiral galaxies are generally bluer in their outer regions -- this is called a color gradient. What does this tell us about the stellar populations in galaxies?

We also find the bulges of spirals are redder than the disks. What does this tell us?

We can measure the global star formation rate in disk galaxies by using:

and we find that for normal spirals, the star formation rate is a few Msun/year. We can also study the amount of interstellar gas spirals have; it's usually about a few billion Msun. So how long can disk galaxies keep making stars?

The colors and gas contents of spirals show trends along the Hubble sequence: Sa galaxies are redder, and have a relatively low gas:star ratio. Sc galaxies are bluer, and have a high gas:star ratio.


Typically this is explained by different star formation histories: