Understanding Massive Star Evolution

Phil Massey
Lowell Observatory

Massive stars are rare: we expect that for every star with a mass 20
times that of the sun, there are a hundred thousand solar-type stars in
the Galaxy; for every star with a mass 100 times that of the sun,  there
are over a million.  Nevertheless, these scant heavyweights provide most
of the energy input and chemical enrichment of the interstellar medium.
The physics of massive stars is complicated by strong stellar winds
(mass-loss) and convective overshooting, making reliable stellar evolution
models difficult to construct.  I will describe the current state of our
understanding of massive star evolution, emphasizing recent observational
tests using young clusters in the Milky Way and Magellanic Clouds.  These
studies provide the first empirical measurement of the progenitor masses
of various types of evolved massive stars, such as Wolf-Rayet stars,
Luminous Blue Variables, and red supergiants.  The results demonstrate the
significant effect that metallicity has on the evolution of massive stars.