Milky Way starbirth: some far-out action.

Astronomers have identified the first young star known to hover at the fringe of our galaxy. The finding surprised some researchers, who thought such a remote part of the Milky Way could not harbor young stars because the region now contains relatively little gas and dust--the raw materials needed to form them.

In challenging that notion, the new finding should shed light on the nature of starbirth in an environment drastically different from that in the star-producing regions near the sun, says Stuart N. Vogel of the University of Maryland at College Park. Vogel, Eugene J. de Geus, now at the California Institute of Technology in Pasadena, Robert A. Gruendl of the University of Maryland, and Seth W. Digel of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., report their work in the Aug. 20 ASTROPHYSICAL JOURNAL LETTERS.

The youthful star lies perhaps 90,000 light-years from the Milky Way's center--some 25,000 light-years beyond the visible spiral arms of the galaxy. In contrast, the sun lies well within the visible disk, about 27,000 light-years from the center.

Vogel notes that before he and his colleagues conducted their study, only elderly stars were known to reside at the Milky Way's periphery. Indeed, it seemed unlikely that the small amounts of gas and dust at the galaxy's edge could congregate into clouds of material dense enough for starbirth. In addition, Vogel says, outlying regions of the galaxy contain much lower abundances of elements heavier than helium and much less radiation--conditions common to star formation in the sun's vicinity.

Stephen E. Strom of the University of Massachusetts at Amherst calls the study significant. He notes, however, that it's not surprising to find a young star at the edge of our galaxy. Astronomers have observed young stars in the low-density outskirts of nearby galaxies, including a satellite of the Milky Way known as the Small Magellanic Cloud.

"The potential of this discovery is that by studying star formation in the outer galaxy, where there is a lower abundance of heavy elements, we can learn about starbirth under very different physical conditions than in the solar neighborhood," he says. Finding youthful stars at the edge of our galaxy should allow researchers to study this population up close and in greater detail.

An intriguing finding prompted Vogel and his colleagues to begin their study. Digel and his Harvard-Smithsonian colleague Patrick Thaddeus last year reported evidence for a molecular cloud--a stellar nursery--at the extreme fringes of the Milky Way (SN: 7/4/92, p.13). Figuring that where there's a star-making cloud, there should be stars, Vogel and his colleagues examined the region near the cloud last December using the 1.5-meter telescope on California's Palomar Mountain.

Rather than looking for a young star directly, the team searched for evidence of its presence: red light emitted by surrounding hydrogen atoms. Hot, young stars ionize hydrogen gas, and the gas radiates red light when its electrons and protons recombine into atoms. After pin-pointing this telltale radiation, the team searched for a likely stellar source. They report that a blue supergiant first detected some 20 years ago fits the bill.

Vogel notes that the distance to this hot, young star (no more than a few million years old) had been undertermined when other astronomers first catalogued it in 1974. His team now estimates that the star lies in the plane of the galaxy beyond the visible disk, between 77,000 and 155,000 light-years from the Milky Way's center. The most likely distance is 90,000 light-years.

The study could only detect the most luminous young stars at the galaxy's edge, Vogel says, but more sensitive surveys should find fainter newborns there because a single molecular cloud gives birth to many stars.