Download The phenomena of astrophysical masers are not new by any means

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.