The ancient balls of stars known as globular clusters are a favorite place for astronomers to test ideas of stellar evolution. Past studies of their cores revealed little more than a flecked smudge, however, compared with new results reported online by Science this week. And the sharp, color-coded x-ray map of the heart of a globular cluster revealed some surprises.
The newly mapped cluster, known as 47 Tucanae, is one of about 150 globular clusters sprinkled through our galaxy. Because the million or so stars in a cluster all formed at around the same time and are all at about the same distance from Earth, globular clusters are a perfect space lab for astrophysicists to study how stars mature as they age.
The researchers, led by Jonathan Grindlay of the Harvard-Smithsonian Center for Astrophysics, picked out 108 distinct x-ray sources in the central core of the cluster by using the Chandra X-ray Observatory. They used x-ray intensities and energy levels to estimate the relative numbers of four different types of x-ray sources in the cluster's core.
Some of the numbers came as a surprise. The researchers saw many more neutron stars than expected. Neutron stars derive from heavy stars, and astrophysical models predict that clusters should contain many more lightweight stars than heavy ones. And neutron stars travel at speeds of several hundred kilometers per second--so fast that they should just "zip out" of a cluster, Grindlay says. But Andrew Fabian of the Institute of Astronomy in Cambridge, United Kingdom, thinks the problem may be an illusion. Relatively lightweight white dwarfs may well outnumber neutron stars in the cluster, he says. But because they emit few x-rays and don't form pulsars, the x-ray census may simply have undercounted them.
Another mystery is why millisecond pulsars so vastly outnumber the handful of quiescent low-mass x-ray binaries (LMXBs). Most astrophysics think that millisecond pulsars start out as LMXBs and sometimes change back again. If so, Grindlay explains, the population of millisecond pulsars and LMXBs should show a delicate balance--a balance that Chandra does not see.
In any case, "it is a big step in x-ray astronomy to have actually resolved what is happening in the middle of a globular cluster," says Fabian. And astrophysicists agree that puzzles, at least, are one thing 47 Tucanae is likely to keep producing in abundance.
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