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molecular cloudsCool dense regions of interstellar matter within which atoms tend to be combined into molecules. The clouds are composed principally of molecular hydrogen (H2 ), with between 300 and 2000 molecules/cm3. There is also a small admixture of cosmic dust comprising about 1% of the mass, with gas temperatures between 10 and 20 K. Hydrogen molecules do not usually emit at radio and infrared wavelengths, and molecular clouds were discovered only in the mid-1970s in surveys of radio emission from carbon monoxide, CO, which is 10 000 times less common than hydrogen molecules in the clouds. The 2.6-and 1.33-mm CO lines are still the prime means of mapping and investigating the clouds. A wide variety of molecules, in addition to H2 and CO, have been found in the clouds (see molecular-line radio astronomy); more than 80 types have been detected in the largest clouds, such as Sagittarius B2.
Several dense core regions, with about 105 hydrogen molecules/cm3 and mass about 102 to 103 solar masses, may be found within one molecular cloud. In giant molecular clouds, these dense cores contain infrared sources, H II regions, maser sources, and the peak CO temperatures, which suggest that these regions are sites of massive star formation. There are also smaller clouds, containing about 500 solar masses of molecular hydrogen, throughout which low-mass stars are forming and which may be relatively isolated, as in the Taurus–Auriga star-forming region.