Embryo From a Collapsing Star

Astronomers for decades have offered a persuasive argument to explain how stars are born: one of the huge, tenuous clouds of gas and dust that pervade the galaxy collapses under its own weight, heats up dramatically and bursts into nuclear flame. Until now though, this has been only a model. But in a report to be published in the Oct. 1 issue of Astrophysical Journal Letters, a team of astronomers will announce that they finally have supporting evidence. Says Charles Lada, professor of astronomy at the University of Arizona: "We've detected what we believe to be the actual collapse of a cloud to form a star very much like our own sun."

It has been difficult to find a newborn star because outer regions of the collapsing cloud hide the new star within. Ordinary light cannot penetrate the haze. The long-wavelength infrared and radio waves produced by a warming embryonic star can pierce it, however, just as a radar signal can cut through the densest fog.

For that reason, the immense catalog of data gathered by the Infrared Astronomical Satellite (IRAS) was an ideal place to start. Although the satellite operated for only ten months in 1983, it sent back information on more than a quarter-million cosmic sources of infrared radiation. One of them was in the constellation Ophiuchus, some 520 light-years away. "What the IRAS survey indicated," says Team Astronomer Bruce Wilking of the University of Missouri, "was that this source was 40 degrees above absolute zero (-233 degreesC), extremely cold by our standards but warm enough by interstellar | standards that it led us to think this would be an interesting object to look at." Best of all, the object, designated IRAS 16293-2422, was relatively close, making it easier to see finer detail.

The astronomers then switched from infrared to radio observations, using the twelve-meter radio telescope atop Kitt Peak, Ariz. Reason: infrared radiation gives information on the cloud's overall temperature, but radio waves carry more detailed data on the motion within. Interstellar clouds are made up of dozens of different types of molecules, and each emits radio waves of a specific frequency when heated or otherwise "excited." By tuning their telescope to the right frequency, astronomers monitor the behavior of different molecules and consequently learn more about conditions within the cloud.

According to Arizona Astronomer Erick Young, "We chose carbon monosulfide because it is a probe of the densest parts of these clouds." The molecule is most excited when it is most compressed. In the center of the cloud, says Lada, "we found that we were seeing carbon monosulfide in a very excited state." In the outer reaches, though, the molecules were much calmer. There was a dense core at the center of the cloud. It was also clear that there was systematic motion inside. Just as a train whistle is higher in pitch as it approaches than when it recedes, radio waves also vary in frequency according to direction of motion. Using the radio telescope like a police-radar detector, the astronomers measured the movement within the cloud. "We didn't detect any motion in the outer, cooler regions," reports Wilking. "It was as we probed deeper and deeper that we began to see evidence of gas falling inward."

From the data they have gathered so far, astronomers estimate that the central, collapsing portion of the cloud is roughly 400 billion miles across, 50 times the size of our solar system. The innermost core is about three times as big as the sun but has one-fourth the mass. The astronomers speculate that the star will probably burst into view in another 100,000 years, when it drives the outer parts of the cloud off into space.

By Michael J. Lemonick. Reported by David Bjerklie/New York