Observational characterization of intermediate mass stars

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Rich ensembles of stars known as open clusters are beautiful sights in our Galaxy, and the common age, distance, and original chemical composition of their member stars make them ideal calibrators of intrinsic properties (luminosity, temperature, and evolutionary status) for associated objects. Especially important are Cepheid variables, pulsating supergiant stars used as distance indicators to nearby galaxies because the duration of their characteristic brightness cycles relates directly to intrinsic brightness. An additional characteristic described by rate of period change and pulsation amplitude correlates more precisely with their evolutionary status, and may resolve questions about pulsation mode as well. Another common constituent of young clusters are pulsating M supergiants, standard candles in their own right much like Cepheids, but also the last stage of evolution for stars 20-25 times more massive than the Sun prior to their explosion as Type II supernovae. Like Cepheids, the common thread linking such objects is that they represent advanced stages of stellar evolution, where existing observational data can be used to test predictions from increasingly-sophisticated stellar evolutionary models with reality. In the case of M supergiants, it may be possible to detect an observational signature that provides advanced warning of their forthcoming demise.