The phenomena of astrophysical masers are not new by any means. The first

discovery was made in 1965, starting with the wider bands and gradually narrowing to

shorter wavelengths, eventually passing into visible frequencies. Their realization may

be old news, but the application of the theory behind them leads to new discoveries

daily.

A maser is caused by the stimulated emission of particles from a higher energy

level to a lower. In a conventional hydrogen maser, the hydrogen gas is heated causing the particles to become excited. The excited gas is then transferred into a resonant cavity, where the excited particles begin to emit photons spontaneously. These spontaneously emitted particles then interact with the unexcited particles moving them to higher energy levels where they quickly shift down to the lower energy level. When this happens each de-excited particle emits two photons at the same frequency of the excitation energy.

The first artificial maser was constructed in 1953 by Townes, Zeiger and Gordon.

It was believed at the time that no environment existed in nature that would produce the same effect. This belief led to the first observation of a naturally occurring maser being discovered and misidentified in 1965. They called the phenomenon Mysterium because they did not know what was causing it, but later identified the signal as emission lines from hydroxide molecules at 1665 MHz. Townes, Cheung and Rank aimed a 20 foot radio antenna at the sky in 1968 and on their second try, after looking towards the galactic center, they aimed the antenna at a mass of clouds called Sagittarius B2 and found the spectral line of ammonia. They found the spectral line of water in this cloud as well and were later seen very intensely in the Orion Nebula. Indeed, water produces the greatest signal gain of any naturally occurring maser. Masers were first discovered

in molecular clouds, and since have been recorded in the atmospheres of stars both in

this galaxy and surrounding galaxies. They have also been seen in the haloes of

comets, not surprisingly since comets consist of mostly frozen water.

Fundamentally, there is no difference between the artificial maser and the

astronomical except in size and the fact that the astrophysical maser has no resonance

cavity. The reaction takes place inside clouds of certain compounds copacetic to the

amplification of certain frequencies. The emission is stimulated and monochromatic,

the frequency corresponding to the energy difference between the two energy levels of

the type of compound in the gain medium. The emission results from one pass through the medium so unlike artificial masers, they usually lack spatial coherence and mode purity. However, since the gain medium stretches across kilometers and sometimes light-years of distance, the gain is extremely large. The lack of a resonance cavity leads to two phenomena. The first being amplifier noise which is caused by spontaneous emission. Even though the highest gain is recorded along the path of output photon flux, when the photons enter the cloud stimulated emission occurs, but spontaneous emission also occurs. The probability density of an atom in the N2 level spontaneously emitting a photon of frequency between v and dv is

Psp(v)dv = (1/tsp)g(v)dv

If the atomic density in the upper energy level is N2 then the average spontaneously

emitted photon density is N2Psp(v). When the density is multiplied by hv the result is the power density. This is emitted uniformly in all directions and though not nearly as powerful as the amplified photon flux of frequency v, it requires a filter to remove it from the amplified signal.

Astrophysical masers also resemble random lasers in that, unlike conventional masers and lasers, the radiation scattered back into the active medium has a random phase. When the scattering in the medium is very strong it can provide feedback, though it is incoherent and intensity based rather than field-based. When the large path length of the scattered photons is coupled with the already large path length of the medium substantial gain may be recorded.

The discovery of astronomical masers confirmed the existence of multiple compounds both beyond the Earth and much earlier in the age of the universe than ever recorded. So called “water masers” were discovered around the centers of galaxies billions of light years away. The centers of these galaxies are theorized to contain super massive black holes and as the radiation from the superheated accretion disk is emitted it passes through the clouds of surrounding molecules and is amplified. Due to this phenomenon water molecules can be seen at incredible distances, while the intensity of these masers rivals that of the Sun, and all in a single emission line, and the gain medium for this type of maser can be as large as the Solar System.

The molecule formaldehyde is a unique substance in that it is de-excited by collisions and because of this it absorbs radiation. It even absorbs the microwave background radiation left over from the Big Bang. This causes the cloud of formaldehyde to appear to a radio telescope as even cooler than the coldest regions of space. These areas were jokingly named dasars, or darkness amplification by stimulated absorption of radiation.

The discovery of naturally occurring masers has created new opportunities for

exploring the universe. They provide the ability to see into the center of some galaxies

obscured by dust and gas. The discovery of water masers in galaxies billions of light

years away showed that water molecules existed much earlier in the age of the universe

than originally theorized. It was thought that organic molecules could not exist in deep

space until the discovery of masers with organic molecular frequencies. The formation

of stars also creates masers. When a star collapses from a cloud of gas, mostly

hydrogen, it begins to heat up and spin. The outflow from the poles releases heated gas which creates masers from the lower density material that is not caught up in the accretion disk of the star. The star itself releases radiation through the disk of material making up the accretion disk. The masers from these materials reveal the composition of young planetary systems in formation. From these measurements inferences may be made as to the method of formation of the Solar System. Masers are also used in observing active galactic nuclei, also known as quasars, located billions of light-years away. The center of these distant galaxies is thought to contain a super-massive black hole around which a disk of gas and dust is circling inward. This disk is heated through friction to temperatures capable of emitting microwave to gamma radiation. The

radiation emitted from this region is passed through the surrounding clouds and is

amplified. Since this occurs across the diameter of the disk it is possible to measure the

size of these regions and also their rate of spin. This is then used to calculate their

mass. The overall signal is then measured for red-shift which tells how quickly the

galaxy is moving away. These measurements allow scientists to calculate the age of certain molecules relative to the age of the universe.