The hidden lives of massive stars.

The hidden lives of massive stars

The formation of massive, hot stars takes place in the warm, dense cores of thick molecular clouds. Such hot, bright stars generally remain hidden until they move out of their clouds or disrupt the clouds sufficiently to thin out the obscuring dust and gas. But even the hidden stars signal their presence. They ionize the surrounding gas (mainly hydrogen) to form compact regions visible at radio wavelengths and heat up circumstellar dust, which eventually radiates light at infrared wavelengths. Now astronomers are combining the results of radio-telescope surveys and data from the Infrared Astronomical Satellite to obtain a more complete picture of how these short-lived stars form and evolve.

Using these techniques, Edward B. Churchwell of the University of Wisconsin-Madison and Douglas O.S. Wood, now at the Harvard-Smithsonian Center for Astrophysics in Cambridge Mass., obtained a direct measure of the population and distribution of embedded massive stars in the Milky Way galaxy. Their results, scheduled to appear in the May 1 ASTROPHYSICAL JOURNAL, show that 10 to 20 percent of all massive stars are surrounded by clouds. This implies that a typical massive star spends a like fraction of its lifetime inside a molecular cloud.

Using the Very Large Array radio telescope near Socorro, N.M., the researchers observed cloud-enshrouded stars, some only a few tens of thousands of years old. Such stars survive just a few million years before they explode as supernovas. Churchwell and Wood estimate the Milky Way galaxy contains about 20,000 of these hot, bright, massive stars. Molecular clouds within the galaxy spawn at least two such stars every thousand years or so, and one explodes into a supernova roughly every 25 years.