Star-Birth Engines –The Black Clouds of Deep Space


Infrared dark clouds (IRDCs) are dark patches in the sky seen against the continuous, bright infrared background produced by our galaxy. IRDCs are rich in molecules and relatively dense, cool gas, and they are natural sites for future star birth. Studies of IRDCs to date have emphasized those candidates that already have star formation underway within them, but astronomers are increasingly interested in probing younger, colder clouds to probe earlier stages in the star formation process.

One tool to use is the gas, ammonia. In 1969, astronomers discovered that ammonia (NH3) was present in large quantities in interstellar gas clouds. The species was most apparent in regions of star formation where the density and temperature of the gas enabled it to emit bright radio-wavelength radiation. Since then, ammonia has become one of the staple diagnostic probes of the regions where new stars are forming. One issue has been that radio telescopes capable of detecting ammonia have relatively poor spatial discrimination; this means many IRDCs appear as point sources, without structure.

CfA astronomer David Wilner and two colleagues teamed up to use a combined set of telescopes: one group in New Mexico and the second one in West Virginia. Operating together coherently, the combination is able to see small sub-structures in IRDCs, including regions within the clouds where new stars may be forming.

They find strong signals from ammonia, and calculate from them that the gas temperatures are only about ten degrees above absolute zero. Although a few of their sample of IRDCs are known to contain young stars, the ammonia gas in all cases was cold – apparently the new stars have not heated up the cloud. Of particular importance is their conclusion that the density of the gas is high (none has frozen out onto dust, for example, as can happen to molecules like carbon monoxide).

The paper concludes from the overall physical conditions that these clouds are probably resistant to collapsing into new stars. New stars do form in them, they argue, largely because of pressure from the much warmer gas reservoir surrounding them in their galactic neighborhoods.

Image at top of page: An infrared image of an infrared dark cloud, seen against a glowing background of warm infrared dust. This IRDC has a young star forming within (seen as green spot). Credit: NASA Spitzer.

Some scientists have speculated that a life form could be based on radiation emitted by isolated atoms and molecules in a dense interstellar cloud, similar to the one physicist Fred Hoyle described in his scifi thriller, The Black Cloud. Such clouds can have a long lifetime lasting millions of years before they collapse.

The Daily Galaxy via Smithsonian Astrophysical Observatory


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