Hazy, hot, and hidden: dust-laden clouds at the centers of some galaxies may enshroud titanic starburst or baby quasars. (Out There).

Publication: Natural History

Publication Date: 01-JUL-03

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COPYRIGHT 2003 Natural History Magazine, Inc.

Galaxies illuminate the universe, and not just with visible light. Think, for a moment, about an incandescent light bulb--it's not only bright, it's hot, too; and the heat we feel comes mostly from infrared radiation emitted by the bulb's filament. Similarly, stars in galaxies pour out visible light, as well as plenty of radiation at wavelengths beyond the visible, such as infrared and ultraviolet. A typical galaxy (or more precisely, its constituent stars) emits that invisible radiation much the way the light bulb does: roughly proportional to the amount of visible light it emits. Our Milky Way, with its several hundred billion stars, is a fine example of a galaxy that emits light broadly over the range of the electromagnetic spectrum.

There are, of course, some spectacular exceptions. In the 1980s surveys by NASA's Infrared Astronomical Satellite revealed an entire class of galaxies shining far more intensely in infrared light than they did in visible light. We astronomers, straightforward as always, named the group "luminous infrared galaxies"--LIRGs for short--and the...

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