Dusty Ingredients

Astronomers have been able to analyze the content of interstellar gas quite accurately by studying spectral absorption lines, the fingerprint elements create by allowing some wavelengths to pass while absorbing others (see Chapter 7, "Over the Rainbow"). The precise composition of the interstellar dust is less well understood; however, astronomers have some clues.

The 1 percent of interstellar gas that isn't hydrogen or helium contains far less carbon, oxygen, silicon, magnesium, and iron than would be expected based on the amounts of these elements found in our solar system or in the stars themselves. It is believed that the interstellar dust forms out of the interstellar gas, in the process drawing off some of the heavier elements from the gas. So the dust probably contains silicon, carbon, and iron, as well as ice consisting mainly of water, with traces of ammonia and methane as well as other compounds. The dust is rich in these substances, while the gas is poor in them.

Details of the W49 star-forming region. These radio-frequency images were made with the Very Large Array and have a resolution of .04 arcsecond (40 milliarcseconds). The emission comes from the hot gas that surrounds a number of young O and B stars. The bar in each corner has a length of 5,000 A.U. The letters help astronomers refer to individual regions of hot gas.

(Image from authors' collection)

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