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Hydroxyl (OH) is a compound with molecules each consisting of one oxygen and one hydrogen atom. When neutral, it is a radical, i.e., compound with an unpaired electron, specifically a hydroxyl radical. Negative ions with these two atoms are common, one being hydroxide.
Hydroxyl has spectral lines in the cm range and shorter, including an 18cm line, thus is commonly detected in radio astronomy. Some masers show it, and it is also visible in some molecular clouds and can serve as a tracer.