Scientists have gazed into an incredibly dense star cluster with NASA's Chandra X-ray Observatory and identified a surprising bonanza of binary stars, including a large number of rapidly rotating neutron stars. The discovery may help explain how one of the oldest structures in our Galaxy evolved over its lifetime.

These Chandra images provide the first complete census of compact binary stars in the core of the globular cluster known as 47 Tucanae. As the oldest stellar systems in the Milky Way Galaxy, globular clusters are laboratories for stellar and dynamical evolution. The left image shows over 100 X-ray sources, more than ten times found by previous X-ray satellites. Astronomers have long studied 47 Tucanae, but Chandra is the first X-ray satellite with enough the spatial resolution and sensitivity to detect all of these objects. The image on the right is a blow-up view of the central region of the Chandra field to the left. The different colors in the Chandra image represent the dominant X-ray energy range for each source: low-energy X-ray emission (red sources), intermediate energy X-ray emission (green sources), and high-energy X-ray emission (blue sources). The white sources are bright in each energy range. The faint red sources are mostly millisecond pulsars, while the bright white sources are mostly binaries containing white dwarfs pulling matter off normal stars. The two blue sources are also binaries containing white dwarfs. Pairs of normal stars that have undergone large flares induced by their close proximity are shown as objects with a mixture of red and white. Photo: NASA/CfA/J.Grindlay et al.

By combining Chandra, Hubble Space Telescope, and ground-based radio data, the researchers conducted an important survey of the binary systems that dominate the dynamics of 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 (CfA) and lead author of the report that appears in the May 18 issue of Science. "The relative number of neutron stars versus white dwarfs in these binaries tell 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," which contain neutron stars that are rotating extremely rapidly, between 100 to nearly 1000 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 CfA. "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 important populations of binary systems, including those with white dwarf stars and normal stars, and others where pairs of normal stars undergo large flares induced by their close proximity.

This picture, taken with the Hubble Space Telescope's Wide Field and Planetary Camera 2, depicts the stars' natural colors in 47 Tucanae. For example, the red stars denote bright red giants at the end of their lives, while the common yellow stars are similar to our middle-aged Sun. The Chandra image falls in a region seen in the upper-left corner of this HST image. Photo: NASA/STScI/R.Gilliland et al.

The Chandra data also indicate an apparent absence of a central black hole. Stellar-sized mass black holes -- those about five to ten times as massive as the Sun -- have apparently not coalesced to the center of the star cluster. All or most stellar-sized black holes that formed over the lifetime of the cluster have likely been ejected by their slingshot encounters with binaries deep in the cluster core.

"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 the CfA's Craig Heinke. "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 a period of 74,000 seconds with the Advanced CCD Imaging Spectrometer (ACIS).

The ACIS X-ray camera was developed for NASA by Penn State and the Massachusetts Institute of Technology. The High Energy Transmission Grating Spectrometer was built by MIT. NASA's Marshall Space Flight Center in Huntsville, AL, manages the Chandra program. TRW, Inc., Redondo Beach, CA, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, MA.