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By Alexander J. W.

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Sample text

If the universe contained only these elements, perhaps our story could stop here, but that notion is, of course, absurd. Earth itself consists of more than 90 additional elements, such as silicon, oxygen, carbon, iron, nitrogen, magnesium, sulfur, nickel, phosphorus, sodium, and chlorine. A few of these elements—carbon, nitrogen, and sulfur, for example—are to be found in the interstellar medium. So a fundamental puzzle has nagged astrochemists for decades: What is the mechanism by which the “heavy” elements are formed in the universe?

Low temperature and low density in the ISM mean that conditions favoring chemical reactions on Earth tend to be absent or rare, and that novel explanations must be developed for the formation of compounds, ions, and free radicals like those shown in the chart. The search for such explanations is likely to constitute an important line of research in astrochemistry for the foreseeable future. Tools for Studying the Chemical Characteristics of the ISM The preceding sections show how much astrochemists have learned about the composition of the interstellar medium, but how was all this information obtained?

For example, consider the reaction that occurs when molecular hydrogen in the ISM is bombarded by cosmic rays. indd 36 7/30/07 11:14:37 AM The Chemistry of Interstellar Space 37 H2+ + H2 k H3+ + H The formation of the H3+ ion marks the conclusion of this sequence of reactions since the H3+ ion itself does not readily react with other forms of hydrogen. It tends to accumulate in the ISM, where it appears to be rather abundant. Furthermore, the H3+ ion does react readily with other neutral atoms and molecules, providing a mechanism by which other more complex species, like those in the chart on page 24, can be produced.

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