Atoms can't take in simply any amount of energy, they hold it only in fixed amounts. So they absorb and emit only radio waves or light whose energy is exactly equal to the difference between any two of these fixed energy states. When light of just the right energy passes through an excited gas, each photon can force an atom to give up its energy in the form of an identical photon. The two photons trigger other atoms, and before long an electromagnetic avalanche is under way. The resulting light can be millions of times brighter than an incandescent lamp burning at the same temperature.
Hydroxyl emits some of the brightest radiation of all the molecules.
Weaver and his colleagues were astounded to find that one part of the nebula showed a bizarre emission spectrum. Its line intensities were all wrong, and one was so strong that Weaver suspected it of belonging to a different molecule altogether. They dubbed it Mysterium. But the OH had formed a natural maser . The molecules were being pumped by infrared light from nearby gas clouds that were collapsing to form stars.