COLDDUST

New features may make southern sky's 'Coalsack' ideal for further study

Stargazers call a prominent dark black region in the Southern Hemisphere's night sky the Coalsack. Even for naked-eye observers, the cloud of cold gas that makes up the Coalsack is hard to miss: It covers a part of the misty luminescence of the Milky Way, blocking out distant stars of our galaxy with the deep black shades that have earned the Coalsack its name.

A newly discovered aspect of the Coalsack may soon have astronomers thinking of it more like a treasure chest. At an American Astronomical Society Meeting in Nashville this week, astronomers will reveal evidence that the Coalsack has hot gases on its perimeter, a finding that means the Coalsack will likely provide many outstanding opportunities to learn more about interactions between regions of hot and cold gas, processes that are essential to star formation and distribution of the elements that make up life forms and the planets. Findings from the Coalsack may also help scientists refine their models of energy production in the galaxy.

"Every once in a while, nature gives us a break and sets things up so that we can study the key processes fairly easily," says B-G Andersson, associate research scientist in physics and astronomy in the Krieger School of Arts and Sciences at Johns Hopkins and lead author of the AAS presentation. "For astronomers, this is a bit like finding a living dodo - instead of trying to make inferences about how the dodo walks, which is what we normally have to do, we can get direct, detailed observations of it walking."

The Coalsack is relatively nearby in cosmological terms, about 650 light years away in the same spiral arm of the Milky Way as Earth. Because it is close by, structures in the Coalsack can be studied in great detail. It's also nicely backlit from Earth's point of view by bright stars in the next arm of the Milky Way, allowing scientists to use spectroscopy to acquire a fairly comprehensive sense of the chemical ingredients of the cloud.

The hot gas on the perimeter may indicate that the Coalsack is contained within a region of active massive star formation and supernovae known as the Upper Centaurus-Lupus super-bubble. This region has produced large hot stars that burn out quickly and die explosively, sometimes heating interstellar gas to high temperatures. Naked-eye observers can still see the darkness of the cold gas in the Coalsack because most of the surrounding hot gas is too warm to emit light in the visible portion of the electromagnetic spectrum.