Wiktionary
n. (context astronomy English) A large and relatively dense cloud of cold gas and dust in interstellar space from which new stars are formed
Wikipedia
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H is carbon monoxide (CO). The ratio between CO luminosity and H mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.
Usage examples of "molecular cloud".
If it had been in a molecular cloud, it would have been impossible to find.
She had discovered what seemed to be a new interstellar molecular cloud complex, and had obtained some very fine high time-resolution data on the pulsar at the center of the Crab Nebula.
A haze surrounded them as Streaker worked its way slowly through a dense molecular cloud, in that far-off place called the Shallow Cluster.
The Snake seems attached to a giant molecular cloud at one end, and merges with the spherical rim of a supernova at the other.
This adjustment will take us straight into a giant molecular cloud, Maserd commented, when he saw the proposed course change.
Not a black hole, this one is long and stringy -- an exceptional relic of creation, a faceted flaw in spacetime, deadly, gorgeous only to those who know its language of pure math -- having already stirred turmoil when it passed through an immense molecular cloud, spinning vortices that self-gravitate, flattening to ionized skirts that whirl and merge into newborn systems -- then on again we speed, past spiral arms that gleam like diamond dust, until -- we find ourselves zooming down to a modest yellow sun .
This one had been discovered in a murky molecular cloud that came coasting by the Isis star many centuries before.
A diffuse molecular cloud spread its cold veil across the emptiness.