|Annu. Rev. Astron. Astrophys. 1989. 27: Copyright © 1989 by .
All rights reserved Galaxies of the Local Group are all near
enough that rather reliable methods can be used to diagnose what
kinds of stellar populations each contains. There are still many
uncertainties in detail, but experience has shown that the
problem is at least tractable for the local sample, whereas it
becomes extremely difficult and uncertain for more distant
galaxies, where only integrated properties can be measured [see
the excellent discussions in Norman et al. (94)]. There is
usually no difficulty in ascertaining whether or not a galaxy
has any very young stars or uncondensed interstellar material,
though even this question becomes tricky when very small amounts
are being sought. The problem becomes more difficult as older
components are looked for because of the faintness of older
stars and because of the ambiguity in age dating them. (It is
difficult to distinguish a low-mass old star from a low-mass
young star.) Even in the Magell