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 (H2). 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 H2 is carbon monoxide (CO). 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.
Image: ESO / VVV Team / A. Guzmán
Text: Wikipedia contributors. “Molecular cloud.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 13 Mar. 2016. Web. 18 Jun. 2016.
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