Chandra X-ray Observatory image of the center of Omega Centauri; each dot represents one X-ray photon

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My research focuses on globular clusters--big balls of about 100,000 to 1,000,000 stars each, held together by gravity. The Milky Way is home to more than 150 of these star clusters. They're more than 10 billion years old, having formed before the galaxy's disk took shape.

Here's one that you can see from the SFSU Observatory on a clear night.

I'm particularly interested in the dynamical evolution of globular clusters--how their structure changes over time, and whether intermediate-mass black holes form at their centers. Understanding cluster dynamics has sometimes been called the million-body problem. Here's an N-body simulation of a globular cluster created by theorist Simon Portegies Zwart a few years ago.

My students and I have been using Hubble Space Telescope and Chandra X-ray Observatory to peer into the dense centers of these clusters. That's where binary stars, which are thought to play a critical role in cluster dynamics, tend to congregate. Clusters, in turn, can alter the binary stars that inhabit them, through stellar interactions. Globluar clusters are one of the few places in the Galaxy where stars actually collide.

Here is a listing of my papers from ADS. Here are the refereed papers only.

Two of the clusters that my students and collaborators and I have studied have been chosen to be part of the Hubble Heritage project:
  • Omega Centauri is the largest globular cluster in the Milky Way--it may actually be the remnant of dwarf galaxy accreted by the Milky Way.
  • NGC 6397 is the nearest dense-core cluster; stars near its center are lightweeks apart--vs. lightyears in the solar neighborhood.