My research focuses on using the Chandra X-ray Observatory to study compact objects, such as accreting neutron stars (low-mass X-ray binaries, often while in a low or quiescent state), accreting white dwarfs (known as cataclysmic variables--some are seen as novae) and millisecond pulsars, in globular clusters. 

Here are a few colorful images of some clusters I've worked on.

The globular cluster 47 Tucanae (47 Tuc), home of over a million stars.  The left image shows a ground-based photo; 47 Tuc is one of only two globular clusters that can be seen from the ground with the naked eye (unfortunately, only from the Southern Hemisphere!). The right image shows an X-ray observation of 47 Tuc with Chandra. 
    The two brightest objects are quiescent low-mass X-ray binaries containing neutron stars. Low-mass X-ray binaries are compact objects (black holes or neutron stars) pulling matter from a star in orbit around them; as this matter is sucked down onto the compact star, it emits X-rays.  In quiescence, we probably don't see matter continuing to fall onto the neutron star, but rather the neutron star surface, still hot from the last episode of accretion. 
    Many of the other bright objects are cataclysmic variables--white dwarfs pulling matter from a companion star.  Dozens of the faint red objects are millisecond radio pulsars. Radio pulsars are fast-spinning neutron stars that emit radio pulses once (or twice) per spin, as their magnetic pole(s) faces towards Earth. Millisecond pulsars spin faster than 100 times per second, and are believed to have reached these fast spin rates by accreting matter from a companion star.  They should thus be descendants of low-mass X-ray binaries.  Finally, the largest number of (usually faint) X-ray sources are pairs of normal stars.  Even stars like our Sun emit X-rays, but rather faintly.  The stars visible in X-rays in 47 Tuc have are in binary systems where both stars are tidally locked facing each other (like our moon to the Earth).  They are thus forced to spin more rapidly than they would otherwise, and this rapid spin induces greater X-ray activity.
 See Grindlay et al. 2001a, Science 292, 2290, and the NASA press release. Credit: J. Grindlay and NASA.

A deeper image of 47 Tuc, showing some 300 X-ray sources. In true-color x-ray images like this, red indicates lower-energy photons, blue higher-energy photons.


 
A Chandra image of the globular cluster Terzan 5.  Terzan 5 (located near the Galactic center) is roughly twice as far from us as 47 Tuc, and is hidden behind substantial dust and gas.  Terzan 5 is a denser cluster than 47 Tuc, and the interactions between stars in the center of the cluster have produced additional X-ray binaries.  Terzan 5 is may have the largest numbers of X-ray binaries of any globular cluster yet surveyed.