A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2).
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.
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 galactic orbit. The bulk of the molecular gas is contained in a molecular ring between 3.5 to 7.5 kiloparsecs from the center of the galaxy (the Sun is about 8.5 kiloparsecs from the center). Large scale carbon monoxide maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. 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, Z, of approximately 50–75 parsec, much thinner than the warm atomic (Z=130–400 pc) and warm ionized (Z=1000 pc) 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.
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