Stellar Populations and Star Formation Histories

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 can use these integrated colors to tell us about the history of galaxies. For example, 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.

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

luminosity of HII regions
far infrared luminosity (young stars heat dust, which radiate infrared)

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

Question: What determines the star forming properties and history of different spiral galaxies?