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clouds of gas and dust in the spaces between the stars: the interstel- 

 lar medium. As we explore these clouds, we must make some allow- 

 ances for the further changes that have occurred since the Sun was 

 born; however, most of what we find today appears characteristic of 

 conditions that must have existed 4.6 b.y. ago. 



This exploration, still in its infancy, is yielding a continuous 

 stream of new information about the chemical composition, mass, 

 and distribution of these clouds, and about the relationship of differ- 

 ent types of interstellar clouds to the process of star formation. It is 

 already clear that the sheer mass of the low-density interstellar 

 material dictates that most of the chemistry that goes on in the 

 Universe takes place in interstellar clouds. 



If the material in the clouds were spread uniformly over all 

 space, the concentration of matter would amount to about three 

 hydrogen atoms per cubic centimeter. We consider two basic types of 

 clouds: (1) diffuse clouds, which contain little dust, and where con- 

 centrations of gas molecules are very low and single hydrogen atoms 

 are the dominant species; and (2) dark, dense clouds, which contain 

 abundant dust, and where molecular hydrogen gas (H 2 ) is the domi- 

 nant species. In the latter clouds the gas concentrations range from 

 about one thousand to about ten million molecules per cubic centi- 

 meter. Presumably, the material in dense interstellar clouds is utilized 

 in star formation. Relatively small stars, such as our Sun, are prob- 

 ably able to form almost anywhere in these massive interstellar 

 clouds. The dust in interstellar clouds is not well characterized; there 

 is evidence to suggest that it is composed of ice, silicates, graphite, 

 and both simple and complex carbon-containing compounds. While 

 there is relatively little information about the dust, there is a growing 

 body of data about the molecules present in these clouds. 



How are interstellar molecules produced? First, it is necessary 

 to consider the environment in which synthesis might occur. The 

 temperatures are very low, from -236° C to -173° C. In addition, the 

 extremely low concentrations of molecules in the gas phase means 

 that collisions (and therefore chemical reactions) are generally 

 restricted to those that involve only two species (that are binary). 

 These and other constraints have led to a model for the synthesis of 

 interstellar molecules in dense clouds in which reactions are initiated 

 by collisions of ubiquitous, high-energy cosmic rays with hydrogen 

 and helium. These encounters produce reactive, positively charged 



