Abstract
Several decades of observations of the most massive and most luminous stars have revealed a complex upper HR Diagram, shaped by mass loss, and inhabited by a variety of evolved stars exhibiting the consequences of their mass loss histories. This introductory review presents a brief historical overview of the HR Diagram for massive stars, highlighting some of the primary discoveries and results from their observation in nearby galaxies. The sections in this volume include reviews of our current understanding of different groups of evolved massive stars, all losing mass and in different stages of their evolution: the Luminous Blue Variables (LBVs), B[e] supergiants, the warm hypergiants, Wolf–Rayet stars, and the population of OB stars and supergiants in the Magellanic Clouds.
Highlights
IntroductionThe reviews and papers in this Special Issue focus on the properties of the most luminous stars in nearby galaxies, those galaxies in which the brightest individual stars are resolved and can be observed
The reviews and papers in this Special Issue focus on the properties of the most luminous stars in nearby galaxies, those galaxies in which the brightest individual stars are resolved and can be observed.The most luminous stars are the most massive and because of their intrinsic brightness and relatively short lifetimes they provide our first probes of the progress of stellar evolution in different environments.The study of massive stars in other galaxies offers many advantages
An empirical comparison of these HR Diagrams, for the massive stars in our region of the Milky Way and for the LMC, revealed comparable populations of massive stars based on the distribution of their spectral types and luminosities across the HR Diagrams
Summary
The reviews and papers in this Special Issue focus on the properties of the most luminous stars in nearby galaxies, those galaxies in which the brightest individual stars are resolved and can be observed. Studies of stellar populations in nearby galaxies have the advantage that all the stars are at approximately the same distance, a distance that is relatively well known, especially in comparison with the uncertain distances of individual stars in our own galaxy. Internal extinction within the galaxy can be variable and must still be corrected In this introductory review, I present a brief historical overview with emphasis on some of the main developments in the study of massive stars in nearby galaxies. The study of individual stars and their placement on the HR Diagram requires several types of data: accurate multicolor photometry for spectral energy distributions (SEDs) and the measurement of interstellar extinction; infrared observations of circumstellar dust and mass loss; and, most important, spectroscopy, for classification, temperatures, luminosity indicators, emission lines, and evidence for Galaxies 2019, 7, 75; doi:10.3390/galaxies7030075 www.mdpi.com/journal/galaxies. Other Local Group galaxies are included as well as results for stars in a few nearby spiral galaxies
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