C0 - recap
galaxies
- systems of stars bound by gravity
- galactic spectra are collections of stellar spectra
- since age, colour and brightness are correlated, for galaxies, the colours/spectra are a balance of these factors
- also consider how many stars are expected for a given mass, and how early the galaxy form stars (star formation rate)
- these depend on the initial mass function (IMF), which relates the number of stars formed for a given mass,
, to the mass as:
- hence, smaller stars are more numerous
- also consider how recent the last episode of star formation was
- massive stars have shorter lifetimes