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Giant Molecular Clouds

Orion Nebula

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Interstellar Gas

Matter in the universe is sparse. Aside from our own Sun, the nearest star to us is over 4 light years (24 trillion miles) away. The space between stars is not empty. It is thinly filled with gas and dust comprising the interstellar medium.

Jack Welch, Univ. of California at Berkeley, on-camera
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Interstellar gas consists mainly of hydrogen and helium. Many scientists believe these primordial gases were formed within minutes of the Big Bang. However, small amounts of carbon, nitrogen and oxygen are present in the gas between the stars. Forged in the cores of ancient stars, these heavier elements combine under the right conditions to form a variety of molecules.

Emergence of Giant Gas Clouds

Though extremely thin, interstellar gas may accumulate under its own gravitation to form a giant molecular cloud or GMC. As the name suggests, GMC's consist mostly of molecular gas, but these clouds also contain smaller quantities of dust. Our atmosphere is extremely dense compared to GMC's, about a trillion times more so! Spread across great distances, up to tens of light years across, GMC's are very much thinner than even the wispiest clouds in the sky. On the other hand, GMC's are over one thousand times denser than the interstellar gases which surround them.

GMC's are also very frigid, with an average temperature of 10 to 100 degrees Kelvin, that's -266 degrees Fahrenheit!

GMC's as Stellar Nurseries

Astronomers believe GMC's are the birthplace of stars.

Lew Snyder, University of Illinois, on-camera
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A typical GMC consists of a large, cool, low-density cloud of molecular gas surrounding a small, warm, dense core.

Core Region of a GMC

The density of gas at the core is about one thousand times greater than in the surrounding region. Further compression of the core resulting from its own gravity or other forces may trigger starbirth.
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NCSA. Last modified 11/11/95