|8th of July 2015|
|The Evolution of Massive Stars: Closing the Loop in the Galaxies of the Local Group|
|by Phil Massey (Lowell Observatory)|
|Abstract. Massive stars are extremely rare: for every star with a mass 20 times that of the sun, there are over 100,000 solar-type stars in the Milky Way. However, their importance is considerably out of proportion to their scant numbers. Through the actions of their strong stellar winds and eventual disruption as supernovae, they provide most of the mechanical energy input into the interstellar medium. They emit most of the ultraviolet radiation in galaxies, while at the same time powering their far infrared luminosities through the heating of dust. |
Most of the carbon, oxygen, and nitrogen atoms have been created in the cores of these stars, and all of the elements heavier than Fe in the Universe have been formed as a result of the spectacular core-collapse supernova that occur at the end of the lives of these massive stars. They are also the source of the most energetic phenomenon yet found, emitting gamma-ray bursts as they collapse into black holes. In early times they likely played a key role in the re-ionization of the Universe, and their remnants may have formed the seeds of super-massive black holes found in the centers of galaxies today.
Using large telescopes in Chile and southern Arizona, my colleagues and I have been conducting surveys to identify the evolved massive stars in the nearby galaxies of the Local Group, such as the Andromeda Galaxy and the Magellanic Clouds. This work has revealed the first Thorne-Zytkow object (a weird hybrid between a neutron star and a red supergiant), and a new class of Wolf-Rayet stars. In this talk I will also review what else we have learned about the life-cycle of these massive stars from such studies, and what new questions have been raised by these discoveries.