Massive star evolution: The formation of massive stars in clusters and the evolution of the atmospheres of supergiants

This dissertation presents the results of three distinct projects regarding the evolution of massive stars. Mid-infrared observations of the central 400 pc of the starburst galaxy M82 disclose the existence of multiple star forming region each with sizes and inferred masses comparable to globular clusters. Hundreds to thousands of massive stars have formed in each of the identified clusters. At least 20% of the star formation in M82 is found to occur in super star clusters. Radio observations of the nearby supergiant VY CMa are fit to an isothermal outer atmosphere with a radio photosphere. An extrapolation of the model can account for the observed total mass-loss rate of the star. Additionally, mid-infrared observations of VY CMa suggests that warm dust is extended in the anisotropically around the star, in a direction consistent with the asymmetric near-infrared reflection nebula. A mid-IR imaging survey of 18 red supergiants is presented. Beyond the asymmetries found in VY CMa, the supergiants NML Cyg, TV Gem, and alpha Ori are tentatively reported to have an anisotropic distribution of dust. The remaining supergiants in the survey show no asymmetry or are unresolved. The resolved sizes and upper limits to the sizes of the sources are used to find a best fit to the dust emissivity index for this sample of oxygen-rich supergiants.