Orion Nebula's hydroxyl masers were the first identified by physicists.
(Courtesy Anglo-Australian Observatory)
In the mid-Fifties the first successful maser was hailed as a dramatic technological breakthrough. Physicist Charles Townes, of the University of California at Berkeley, eventually received the Nobel Prize in physics for the invention. But by then, radio astronomers on the Berkeley campus found that Townes had been scooped by someone who didn't need a jungle of coaxial cables, power supplies, and strip-chart recorders to build a maser. Mother Nature had been doing it for billions of years.
Masers and lasers operate because of the way atoms within them absorb and release energy. 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.
By handling the atoms in just the right way, it is possible to "pump" the same amount of energy into a large number of them at once. But these "excited" atoms are highly unstable. 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.
In 1963 Harold Weaver and his radio astronomers at Berkeley were studying the radio waves emitted by hydroxyl (OH) groups in the Orion Nebula, the nearest region in the galaxy where stars are still being formed. 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.
Before long, however, the evidence was clear Mysterium was simply OH. 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. Since then, hundreds of interstellar OH masers have been discovered.
In 1968 radio astronomers at MIT discovered a second type of OH maser, in variable red giant stars. With diameters several hundred times that of the sun, these stars have burned their primary nuclear fuels and are in their death throes. As they tremble and quake, they eject gas that forms a shell around the star. When this cools to a few hundred degrees Kelvin, it forms molecules that can function as masers.
The next year brought the discovery of the stellar water maser. Soon after came the strangest one of all: the silicon monoxide (SiO) maser. Unlike the abundant oxygen and hydrogen of hydroxyl and water masers, silicon is relatively scarce. Stranger still, when astronomers identified the exact molecular energy levels in these masers, the SiO molecules proved to be between two highly excited states. Such stellar masers can exist only in regions of the circumstellar shell that are at about 1,000 degrees Kelvin.
So far, only molecular masers have been observed. But in principle there is no reason to believe that Mother Nature has not also built optical lasers, chemical lasers, free-electron lasers, and possibly even gamma-ray lasers, or grasers. They would probably form near hot stars, novas, and other violent objects that can emit high-energy radiation to pump the atoms and nuclei. Physicists all over the world are now working to build many of these devices. Learning how Nature accomplished it could do much to speed their success.
And it is a pretty good bet that she is operating exotic lasers that we humans haven't even thought of yet.