The space between the
stars, which is called the interstellar medium is not empty. Although the
density of matter is very low (about 10,000 particles per cm3), there is a rich
variety of atoms and molecules that populate it. The most abundant material, as
in the rest of the universe is hydrogen (in three variants: ionized, atomic and
molecular) and helium. But there are also molecules of all kinds, to over a
hundred have come to find. Since the most abundant and simple, water (H2O), to
large, complex organic chains as acetone or benzene.
