Research Areas: Michael Bolte

Part of the incredible image of NGC 602 obtained with the Advanced Camera for Surveys on the Hubble Space Telescope

Globular Clusters and the Age of the Universe: Research from the Past

For many years my research involved observations of stars in Galactic globular clusters. These beautiful objects are part of the halo of the Galaxy and, based on the comparison of stellar evolution models to the observed color-magnitude diagrams, they are ancient relics of the early formation period of the Galaxy. A very lively controversy in astronomy circles in the 1990s revolved around the ages derived for globular cluster stars. At the time the best ages were around 15 billion years which was older than the favored "expansion" age of the Universe based on the value of the Hubble Constant and a "flat" Universe with no Dark Energy. One day I will add my version of the history of this topic. For now, here is a Nature paper I wrote with Craig Hogan on the controversy in 1995. Note the prediction of Dark Energy and the call to use Type 1a supernovae to observationally confirm the prediction!

Bolte & Hogan Nature paper

Extremely Metal-Poor Stars, White Dwarfs and Supernovae: Current Research

My more recent research has concentrated in two areas. The first is the study of stars that are very deficient in the elements with higher atomic number than helium (that is everything but hydrogen and helium). Like globular clusters, these stars are relics from the earliest times in Galactic history. They are a laboratory for testing equilibrium and explosive nucleosynthesis in stars, they allow us to trace the early epoch of chemical evolution of the Galaxy and Universe, and the pattern of chemical abundances in these stars may be our best clue identifying the nature of the first stars to form in the Universe.

This work started with Jennifer Johnson's (now at Ohio State University) dissertation, David Lai has just finished up his dissertation (June 2008) in the same field and we have been fortunate to work with Sara Lucatello and it is a privilege to sometimes collaborate with Chris Sneden and Bob Kraft, two of the most important pioneers in this field.

The second area is based on observations of white dwarf (WD) stars in young open clusters. The princpal question we are addressing here is "what is the mass for main-sequence stars above which they explode as core-collapse supernovae and below which they evolve into WDs". This is an important question for understanding the early history of galaxies, particularly low-mass galaxies. SNII insert a significant amount of kinetic energy into the ISM and the lower limit on galaxy mass may be set by the gravitational potential required to stay bound during the initial star-formation epoch during which the stars about this critical mass rapidly evolve and explode. The brains behind this program is Kurtis Williams (now at U Texas, Austin). Santa Cruz graduate student Kate Rubin has made a nice contribution and long-time WD experts Detlef Koester and Jim Leibert (U. Arizona) are on-going collaborators.

I also have a long-standing program with Eric Sandquist and students at San Diego State University analysing large samples of stars in globular clusters. We use wide-field CCD images, a battery of "artifical star" experiments and generate very accurate luminosity functions for essentially all the evolved stars in each cluster. This allows very precise tests of stellar evolution for stars with around 1 solar mass.

Messier 5 CMD

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