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Massive Star Evolution

Stellar evolution is the study of how stars change with age.  A star like the sun will burn hydrogen in its core for about about ten billion years before expanding into a red giant (engulfing the Earth) and eventually evolving to a white dwarf star.  Stars with masses greater than 8 times the mass of the sun start their lives as hot OB stars, and end their lives in spectacular outbursts called core-collapse supernovae.  In between these stars undergo phases as Luminous Blue Variables (such as Eta Carina and S Doradus), yellow supergiants (such as Canopus), red supergiants (such as Betelgeuse), and/or Wolf-Rayet stars, all depending upon their initial mass and composition.

The evolution of massive stars is particularly hard to model.  Because they have very high luminosities (in some cases more than a million suns!) radiation pressure removes the outer layers of these stars. Yet, understanding the evolution of such stars is important, not only in its own right, but also to answer such basic questions as where the elements that make up our universe come from.  (The carbon, oxygen, and nitrogen atoms in your body were all made in the cores of massive stars.)

Phil Massey, Kathryn Neugent, and their collaborators use telescopes in Chile and Arizona (including the DCT) to try to better understand massive star evolution.  The nearby galaxies of the Local Group (such as the Magellanic Clouds and the Andromeda Galaxy) serve as their astrophysical laboratories for these studies.

Currently they are using the 4.3-meter DCT and the Smithsonian’s 6.5-meter MMT telescope to measure the orbits of Wolf-Rayet binaries in the Andromeda Galaxy, and using the Swope 1-m and Magellan 6.5-meter telescopes on Las Campanas in Chile to search for Wolf-Rayet stars in the Magellanic Clouds. So far they have identified a sample of a previously unknown type of Wolf-Rayet star in the Large Magellanic Cloud, which they are now studying using the Hubble Space Telescope.

Magellan 6.5-meter telescopes on Las Campanas, Chile. Photo by Kathryn Neugent.
The Magellan 6.5-meter telescopes on Las Campanas, Chile. Photo by Kathryn Neugent.

We can observe stars to better understand our own Sun, exploring its variability and its effects on Earth’s environment and climate.

Thirty years ago, stimulated by the new knowledge that the Sun’s brightness variations over the 11-year solar cycle were less than 0.1 percent, Wes Lockwood, Brian Skiff, and their colleagues began a systematic photometric study of the small brightness fluctuations of sunlike stars of various ages. Using the 21-inch telescope and a dedicated photometer, Brian Skiff observed several dozen sunlike stars for 16 consecutive seasons, finding that a majority of sunlike stars have detectable year-to-year variations from as small as 0.3 percent to several percent; (2) the amount of variability decreases with increasing stellar age.

Wes, Jeff Hall, Brian Skiff, and Len Bright have also observed these stars spectroscopically since 1994 using Lowell’s Solar-Stellar Spectrograph, an instrument fed by an optical fiber from a solar feed and from the 1.1-m J. S. Hall telescope at Anderson Mesa.  It is intended to characterize the magnetic activity of these stars and the Sun on the timescale of the 11-year solar cycle.

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