Duke Physics

Senior Thesis

Variable Stars in the Core of the Globular Cluster M3

Jay Strader

December 7, 2001

Thesis Committee: Henry Everitt, Henry Greenside, John Kolena, Steve Danford (UNC-Greensboro)

The oldest known astronomical objects in the galaxy are globular clusters, sphericalagglomerations of around 100,000 stars bound together by their mutual gravitational attraction.Galactic globular clusters are rich in RR Lyrae variable stars, whose luminosity ("brightness")varies cyclically with typical periods of 0.2-0.8 days. These luminosity variations are drivenby instabilities in the balance of radiation pressure and gravitational attraction in theouter hydrogen atmosphere of the star. Studying RR Lyraes allows us to test theories ofstellar structure and evolution and probe the interiors of globular clusters.

For my thesis research, I worked under Henry Everitt (Duke) and Steve Danford (UNC-Greensboro).Using images taken from the 0.81-meter Three College Observatory (TCO) telescope, managed by Prof.Danford, I studied RR Lyraes in the crowded inner core of the variable-rich globular cluster M3 (Fig. 1). We used a sophisticated image subtraction method (ISM) to determine which stars were variable.First, a composite reference image of several of the best images was created. Then, each of theremaining images was subtracted from the reference frame. When stacked, these residual imagesrevealed which of the stars in our images were variable. ISM is much more effective than traditionalphotometric methods when working in crowded fields.

We discovered twelve new variables, ten of which were RR Lyraes. We also made the firstperiod determinations for thirty-six previously known variables. Future plans for TCOinclude studying the galactic globular clusters M10 and M14, and exploring other possibleuses of ISM, such as asteroid light curves.

My thesis consists of two parts: a preprint that will be submitted for publication, and a primer on variable star research at Duke using the Three College Observatory.