An interstellar cloud is the environment in which solar systems are formed. It contains a few tens of billions of atoms per cubic centimetre (compared with our atmosphere which contains billions of billions) and stretches over hundreds of light-years. It contains the equivalent of several times the mass of the Sun in gaseous matter. It is composed mainly of hydrogen, with helium the second most abundant element; it also contains traces of heavier elements such as carbon, nitrogen and iron.
These elements are the result of the thermonuclear fusion occurring in the centre of stars. Another property of interstellar clouds is their temperature: 100 K at most (-173°C).
Cold interstellar clouds contain the main reserves of water in the universe, mainly in the form of ice.