Chemical evolution in space

Most of the complex molecules in interstellar space ar probably contained in small, frozen interstellar dust grains which are about as old as the earth and have been photochemically converted into large organic molecules. These molecules' maximum molecular weight is limited only by the approximately 0.1-micron grain size. Their evolution leads from cool, evolved stellar atmospheres' formation of seedlings to destruction through incorporation into the material of new stars. Organic dust constitutes about 0.1 percent of the total mass of the Milky Way, far outweighing any estimates of total planetary mass in the Galaxy. Because comets may be virtually pure, aggregated interstellar dust, they offer a source of interstellar organic material for detailed study.