By combining data from NASA's Chandra X-Ray observatory, the Hubble Space Telescope and ground-based radio telescopes, a team of researchers discovered that binary stars predominate at 47 Tucanae, a globular cluster about 12 billion years old located in our Milky Way Galaxy.

Most of the binaries in 47 Tucanae are systems in which a normal, Sun-like companion orbits a collapsed star, either a white dwarf or a neutron star. White dwarf stars are dense, burnt-out remnants of stars like the Sun while neutron stars are even denser remains of a more massive star. When matter from a nearby star falls onto either a white dwarf or a neutron star, as in the case with the binaries in 47 Tucanae, X-rays are produced.

"This Chandra image provides the first complete census of compact binaries in the core of a globular cluster," said Josh Grindlay of the Harvard-Smithsonian Center for Astrophysics in a prepared statement. Grindlay is the lead author of a report on the findings in the May 18 issue of the journal Science.

"The relative number of neutron stars versus white dwarfs in these binaries tells us about the development of the first stars in the cluster, and the binaries themselves are key to the evolution of the entire cluster core."

Many of the binaries in 47 Tucanae are exotic systems never before seen in such large quantities. Perhaps the most intriguing are the "millisecond pulsars" -- neutron stars that rotate extremely rapidly, between 100 to nearly 1,000 times per second.

"The Chandra data, in conjunction with radio observations, indicate that there are many more millisecond pulsars than we would expect based on the number of their likely progenitors we found," said co-author Peter Edmonds, also of the Center for Astrophysics. "While there is a general consensus on how some of the millisecond pulsars form, these new data suggest that there need to be other methods to create them."

In addition to the millisecond pulsars, Chandra also detected other binary systems, including those with white dwarf stars and normal stars, and others where pairs of normal stars in close proximity release large flares.

Along with what they found, the astronomers were surprised at what they didn't find -- a central black hole. Instead, it seems like the black holes that could have formed in the lifetime of the cluster were ejected due to slingshot encounters with the abundant binary stars.

"These results show that binary star systems are a source of gravitational energy which ejects stellar mass black holes and prevents the collapse of the cluster's core to a more massive, central black hole," said Craig Heinke, also of the Center for Astrophysics. "In other words, binary systems -- not black holes -- are the dynamical heat engines that drive the evolution of globular clusters."

Chandra observed 47 Tucanae on March 16-17, 2000, for more than 20 hours with its Advanced CCD Imaging Spectrometer (ACIS) developed for NASA by Penn State and the Massachusetts Institute of Technology. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Massachusetts.