Massive stars in galaxies

Abstract

The study of the evolution of massive stars in different galactic environments has become one of the most rapidly growing fields of stellar research. In this paper I will review the observations relevant to understanding the evolution of massive stars in galaxies of different types via a comparison of their H-R diagrams and the characteristics of their most-massive stars. The upper H-R diagrams of several Sc-type spirals and Magellanic-type irregular galaxies reveal very similar massive-star populations. Most of the observed differences can be explained as statistical effects in which the smaller galaxies have fewer of the most-massive stars. However, preliminary results for M 31 and M 81, both Sb-type spirals, suggest that either the star-formation rate or the IMF may depend on the morphological type of the galaxy.The most-luminous normal stars of different spectral types define an upper luminosity boundary in the H-R diagram set by the limit to the stability of these hypergiant stars. This stability limit is most likely due to radiation pressure (i.e., the Eddington limit) in the hot stars, while turbulence may also play a role especially in the cooler hypergiants. The luminous blue variables are a group of highly unstable, hot stars including P Cyg, S Dor, and r\ Car. They are very massive stars which, as they evolve away from the main sequence, have reached the stability limit of their photospheres.Two outstanding questions remain to be answered by future observations-the roles of chemical composition and morphological type on the stellar content and evolution of the stellar populations in different galaxies.

Αστρικά Συστήματα Στα Νέφη Του Μαγγελάνου

“…Η μελέτη αστέρων μεγάλης μάζας στην κύρια ακολουθία περιορίζεται στον Γαλαξία μας σε ένα σχετικά μικρό χώρο σε μία περιοχή ακτίνας περίπου 3 kpc γύρω από τον ήλιο (Humphreys, 1987). Στη περιοχή αυτή περιορίζεται επίσης η μελέτη των Γαλαξιακών αστρικών ομάδων.…”