Summer 2000 -- Vol. 17, No. 1

Feigelson Works With Group That Discovers Young Cluster of Stars

The discovery of an unusual cluster of very young stars near Earth in the sky over the South Pole has been announced by a group that comprises Eric Feigelson, professor of astronomy and astrophysics at Penn State, and two collaborators from the University of New South Wales, graduate student Eric E. Mamajek and senior lecturer Warrick A. Lawson.

Because of its location close to Earth and its highly unusual characteristics, the cluster is expected to become an important laboratory for understanding a number of mysteries concerning star formation and the early stages of stellar evolution.

"We have used the enhanced X-ray emission of young stars to discover an unusual compact cluster, about 8 million years old," Feigelson said. "Among the more than a thousand known open clusters of stars, it is the first to be discovered with X-rays and the nearest to Earth discovered this century. It also is odd because it appears isolated in space, far from any stellar nursery in which the stars could have been born. Young stars usually are found in or near the dark clouds of gas and dust from which they are born."

The star cluster, centered around the bright star, eta Chamaeleon, is located in the Chameleon constellation about 97 parsecs, or 316 light years, from Earth. The discovery reveals that these previously known stars are members of a previously unrecognized cluster of at least a dozen very young "pre-main-sequence" stars. Most stars quickly pass through their pre-main-sequence phase and spend most of their lives, typically billions of years, burning hydrogen during their "main-sequence" phase. Pre-main-sequence stars have not yet started to burn hydrogen but they nevertheless glow brightly, primarily as a result of the intense heat generated by the gravitational compression during their formation.

"It is very hard to find pre-main-sequence stars far from their clouds without an X-ray telescope," said Feigelson, who in 1981 discovered pre-main-sequence stars have a dramatically distinctive X-ray signature that can be used to locate them. "The intensity of X-ray signals emitted by pre-main-sequence stars is elevated far above that of normal main-sequence stars because they have more powerful magnetic flares," Feigelson said. "These flares are similar to those on the sun, but are thousands of times more powerful."

The astronomers made their X-ray observations with the German/U.S./English satellite called ROSAT, which was launched in 1990. They made spectroscopic measurements to confirm the stars' young age with the 2.3-meter optical telescope at Mount Stromlo and Siding Springs Observatory in New South Wales, Australia. They also used data from the European Space Agency satellite Hipparcos for gauging the distance to the stars in the cluster in order to determine their luminosity as another indicator of their age.

This research was supported by the J. William Fulbright program, the Australian-American Education Foundation, and NASA.

Barbara K. Kennedy

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