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Carbon monoxide (CO) is a compound with molecules each consisting of one oxygen and one carbon atom. In astronomy, CO emission lines are used to detect molecular clouds. Though the clouds are primarily molecular hydrogen, the hydrogen lines are weak and only detectable in nearby clouds, and CO lines serve as a tracer.
A series of very useful CO lines is due to the steps in CO's possible rotational energy (CO transitions), a series of millimeter/submillimeter lines termed the CO ladder. The individual lines are useful for determining redshifts, protoplanetary disks and very distant (z>6) molecular clouds. When observed together (yielding their spectral line energy distribution (SLED)), can yield qualities of the gas, in particular, its temperature.
From CO line widths, using the virial theorem, if virial stability is assumed, the mass of the cloud can be calculated. Assuming a particular CO to H2 factor (XCO), the mass of a distant molecular cloud can be calculated.
Clouds above a certain size have sufficient CO to saturate the lines in which case they reveal no more than a minimum amount of the CO present. The specific C and O isotopes affect the wavelength of the CO lines sufficiently that each isotope combination can sometimes be independently measured and by determining the amount of some of the rarer isotope combinations such as C17O, C18O, or 13CO, and applying typical ratios of isotopes, the total mass of the CO can be extrapolated, and from that, the mass of H2 using XCO.