An interstellar nebula, unusually rich in molecules (as opposed to atoms), detected from microwave radiation. The Galactic centre and Orion Nebula have very rich clouds of this sort.
A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2).
Occurrence
Within our own Galaxy molecular gas accounts for less than one percent of the volume of the interstellar medium )(ISM), yet it is also the densest part of the medium comprising roughly one-half of the total gas mass interior to the Sun's galatic orbit. The bulk of the molecular gas is contained in a molecular ring between 3.5 to 7.5 kiloparsecs from the centre of the galaxy (the Sun is about 8.5 kiloparces from the center). That molecular gas occurs predominantly in the spiral arms argues that molecular clouds must form and dissociate on a timescale shorter than 10 million years - the time it takes for material to pass through the arm region.
Vertically, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height of approximately 50–75 parsec, much thinner than the warm atomic (Z=130-400pc) and hot ionized (Z=1000pc) gaseous components of the ISM. The exception to the ionized gas distribution are HII regions which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have approximately the same vertical distribution as the molecular gas.
This smooth distrubtion of molecular gas is averaged out over large distances, however the small scale distribution of the gas is highly irregular with most of it concentrated in discrete clouds and cloud complexes.
Types of Molecular Cloud
Giant Molecular Clouds (GMCs)
Vast assemblages of molecular gas with masses of 10 times the mass of the sun are called Giant molecular clouds (GMC).
The densest parts of the filaments and clumps are called "molecular cores", whilst the densest molecular cores are, unsurprisingly, called "dense molecular cores" and have densities in excess of 10 particles per cubic centimeter. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). The most massive collection of molecular clouds in the galaxy, the Sagittarius B2 complex, forms a ring around the galactic centre at a radius of 120 parsec.
Small Molecular Clouds
Isolated gravitationally bound small molecular clouds with masses less than a few hundred times the mass of the sun are called Bok globule. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and often included in the same studies.
High Latitude Diffuse Molecular Clouds
In 1984 IRAS identified a new type of diffuse molecular cloud.
Processes
Star Formation
Main article: Star Formation
To our knowledge, the creation of newborn stars in the current Universe occurs exclusively within molecular clouds. This is a natural consequence of their low temperatures and high densities, and of the observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity (like stars, planets, and galaxies) rather than external pressure (like clouds in the sky).
Physics
The physics of molecular clouds are poorly understood and much debated.
Molecular clouds, and especially "Giant" molecular clouds (GMCs), are often the home of astronomical masers.
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