Issue 3, May 2001
Discovery of the First and Only Laser in Outer Space
Biology and Physics, Georgetown University
happened on the last day that NASA's Kuiper Airborne Observatory
(KAO) was supposed to fly before being permanently shut down. As
the world's only airborne astronomical research facility, this unique
observatory's 36-inch (91-cm) reflecting telescope allowed researchers
to observe deep space from anywhere on Earth, well above the atmospheric
cloud cover and water which blocks certain wavelengths of light.
Several major discoveries had been made aboard the KAO, including
the first sightings of the rings of Uranus and the definite identification
of an atmosphere on Pluto. A lack of future funding, however, was
forcing the KAO out of operation.
Vladimir Strelnitski, a researcher at the Smithsonian's National Air
and Space Museum in Washington, D.C., hoped for precious observation
time aboard the KAO. With only a few more days before the planned
shutdown, Strelnitski flew to Hawaii, where the KAO was stationed.
Although he was not scheduled to fly aboard the KAO, an informal promise
had been made: weather permitting, he would be allowed to use the
KAO before its final trip home to search for the elusive astronomical
Strelnitski is an expert in astronomical masers, which are produced
by the extreme conditions present in our Universe. A maser is the
non-visible light equivalent of a laser (light amplification of stimulated
emission of radiation). In both lasers and masers, a beam of light
of a single wavelength is amplified (How is the light
in masers and lasers amplified?). In a laser, visible or infrared
light is amplified, whereas in a maser, microwave radiation is amplified.
Microwave radiation has longer wavelengths than visible and infrared
light, from 300 millimeters to 1/3rd of a millimeter, compared to
1/3rd to 1/3,000th of a millimeter for visible and infrared light.
In both cases, the light produced comes from electrons emitting photons,
or light, as they drop down into lower-energy orbits around nuclei
(i.e. from electron transitions). Masers are very important because
they can give researchers valuable information about the chemical
and physical processes that occur in space. For example, masers can
be used to probe the conditions of disks of gases and dust surrounding
stars, from which planets may form.
Strelnitski was one of many astrophysicists working on astronomical
maser theory. In 1995, he was tackling a thirty-year-old puzzle: why
had no natural lasers been observed in space? Calculations had predicted
the conditions necessary for a laser to form in clouds of ionized
atomic hydrogen. Although scientists had observed thousands of strong
masers with amplification factors of up to a trillion, no natural
lasers had ever been found. Two possible lasers had been identified,
but their amplifications were so low that the phenomena could easily
have been ascribed to other processes such as fluorescence.
Strelnitski wanted to fly aboard the KAO to observe MWC 349A, a very
bright, hot star (~ 30,000 times brighter than our Sun) in the constellation
Cygnus. He had chosen this star because it is surrounded by a massive
disk of gases and dust in which many masers formed by ionized hydrogen
had been discovered, and because it is the largest known source of
radio waves in the sky. He had first observed the star while looking
for evidence of lasers about a year before; his target then had been
the hydrogen transition called the H10alpha (52.5 mm) recombination
Although he found that the light from the H10alpha transition was
bright enough to be a laser, Strelnitski could not state conclusively
that the light was laser-induced. The results would probably not convince
the scientific community; a larger amplification of the light beam
was required. This time, aboard the KAO, Strelnitski wanted to observe
the light produced by the hydrogen transitions H15alpha (169.4 mm)
and H12alpha (88.8 mm).
On the morning of August 16, 1995, Strelnitski awaited word of whether
he would be able to use the KAO's facilities. The weather that day
was beautiful, in contrast to the past few days of rain. It appeared
that bad weather would not prevent him from observing aboard the KAO.
Finally, Strelnitski received the good news: he would be able to use
the KAO to observe these two transitions in the disk surrounding MWC
349A. A few hours later, he and the observing crew took off aboard
the KAO. The weather that night was beautiful. Even if he didn't find
a laser, as he looked out the window at the beautiful night sky Strelnitski
knew it would be a remarkable moment for him, as one of the last people
ever to fly aboard the KAO.
At 41,000 feet, the KAO's telescope was pointed at the source: MWC
349A. If there were any lasers in the disk surrounding the star, there
would be no problem identifying them. The KAO's instruments were simply
not sensitive enough to observe light produced by electron transitions
in the absence of lasers. Slowly scanning the 169 mm energy band (H15alpha),
there was nothing at first. But, then suddenly around 169.25 mm, there
was a jump in the readings. Strelnitski and others aboard the KAO
waited in anticipation. The next reading, taken at about 169.33 mm,
was even higher. The tension and excitement aboard the KAO was mounting.
If lasers were present around the star, the observed line profile
should resemble a bell curve (why?), and
should peak at 169.4 mm, the true wavelength of the H15alpha transition.
The next reading then, at about 169.42 mm, should be the highest yet,
being close to the peak radiation intensity value. Five minutes later,
the reading was even higher. Their data appeared to indicate that
the observed H15alpha line was due to a laser. Strelnitski and his
colleagues had discovered the first unmistakable laser in space!
The mood turned from tense to joyous aboard the KAO, and the researchers
exchanged hugs, laughs and sighs of relief. After analysis, the H15alpha
line was found to be about 5 standard deviations above the expected
non-lasing level, with a calculated amplification of about 1000 -
an undisputable positive identification. The H12alpha and H10alpha
measurements were about 2-3 standard deviations above the expected
level, a probable identification by astrophysical standards. Strelnitski
knew that this would be big news in the astrophysical community.
One mystery remains: why are natural lasers so rare? In the paper
Strelnitski published in the journal Science, he speculated on possible
answers. According to the results obtained aboard the KAO, lasers
can only form below a critical density of gas and dust - a density
which is roughly a trillion times less dense than air on Earth. The
critical density is, however, unique for each type of laser or maser.
For lasers, this critical density is higher and can only be found
in a very small region surrounding the star. This would explain why
lasers are so hard to observe: the regions where they can form are
very small and may be quite rare in stars. Even if they do form, they
could be easily hidden by all the other light emitted by the star.
(For more detailed information on why lasers are so rare, click here.)
Since Strelnitski and his colleagues' discovery of the first natural
laser in space, no other lasers have been found. MWC 349A is an enormously
large and bright star with many masers discovered in its disk prior
to the discovery of the first laser. This is perhaps testimony to
the rareness of the conditions required for natural laser formation.
Whatever the case may be, it still remains a mystery why natural lasers
are such an anomaly.
One last note: although the KAO is no longer in operation, the
discovery of Strelnitski and colleagues helped obtain funding for
the creation of a new airborne astronomical facility, the Stratospheric
Observatory For Infrared Astronomy (SOFIA). SOFIA will begin operations
in the year 2002.
Masers and Lasers: In Depth
How is the light in masers and lasers amplified,
and what's this "amplification of stimulated emission of radiation"?
Intuitively, amplification implies that there is more of something
at the end of some process than at the beginning. Light amplification
means that we have more photons at the end of a process than at
the beginning. How can we get more photons out than we put in? A
process known as stimulated emission provides the mechanism. In
stimulated emission, the electric field of an incoming photon causes
a molecule in a higher energy level to emit a photon of the same
wavelength and thus to drop to a lower energy level; the incoming
photon is not affected in the process.
photon has the same wavelength and phase as the original photon and
travels in the same direction. We now have our amplification mechanism:
one incoming photon yields two outgoing photons. The problem now is
how to get stimulated emission on a scale large enough to have amplification.
If only a few molecules are in a higher energy state, then we cannot
amplify light to observable levels partly because spontaneous emission
will dominate over stimulated emission. Thus, we need to have more
molecules in a higher energy state than in the lower the energy state.
How can we get enough photons in a higher energy level so that
incident light can be amplified?
Let's take a population of molecules. If this population is in thermodynamic
equilibrium, most molecules are in the ground state. In this situation,
we say that very few of the upper levels are populated. "Upper
level" refers to any level above the ground state, i.e. any excited
state of the molecule.
But what would happen if this population of molecules was not in thermodynamic
equilibrium? The upper levels would become significantly populated.
When this happens, there is a chance for a population inversion to
occur. In a population inversion, a higher energy level is more populated
than another energy level that is lower in energy. If E3
> E2 > E1, and E2 is more populated
than E1, then E2 is inverted relative to E1.
If we can achieve population inversion, then we will have enough molecules
in excited states for stimulated emission to produce observable amplification.
But how can E2 ever become more populated than E1?
Consider E1, E2 and E3 as forming
a cycle. Molecules on energy level E1 are excited to energy
level E3, then fall down to E2 through the emission
of a photon, and finally return to E1 through the emission
of a second photon. This cycle repeats itself over and over. Molecules,
however, occupy a certain energy level only for a finite period of
time that is characteristic of that energy level. The median "occupation"
time is called the lifetime, t. If the lifetime, t3, of molecules
on E3 is shorter than the lifetime, t2, of molecules on
E2 (i.e. if t3 < t2), then more molecules will accumulate
on E2 relative to E1. This process will invert
level E2 relative to E1.
Additionally, if there are several pump states that feed into E2,
this will help more molecules to accumulate on E2 relative
to E1. In this scenario, E3 is called the pump
or feeder state, and E2 is an intermediate, metastable
state. The laser/maser photon is the "stimulated" photon,
and the transition from E2 to E1 is called the
working transition. A cycle with a pump state and at least one other
lower energy level is called a pumping cycle. For a pumping cycle
to form a population inversion, however, there must be at least three
levels. If there are only two levels in the cycle, a population inversion
can never be formed because, as Einstein proved, the probability of
absorption is equal to the probability of stimulated emission for
a given transition. Thus, the most pumping can achieve for a two-level
system is to equalize the number of molecules on two levels. A third
level, E3, provides the way for more molecules to accumulate
in an excited state, E2, relative to the ground or lowest
energy state, E1, by feeding molecules into E2.
But how can a population of molecules not be in thermodynamic
equilibrium? i.e., What natural processes can cause lasers and masers
The roundabout answer to this question is that a population of molecules
will not be in thermodynamic equilibrium if the molecules have not
had a chance to thermalize with the surrounding environment. According
to Maxwell-Boltzmann principles, a population of molecules reaches
thermodynamic equilibrium through collisions. If one part of the population
has a different temperature than another part of the population, then
through collisions, eventually the entire population will have the
The key here is that a population of molecules will not reach thermodynamic
equilibrium if the molecules do not collide enough. This translates
into the following: below a critical density*, which is unique to
the molecules and the transitions of the pump cycle, a population
inversion can be maintained because collisions are not frequent enough
to prevent the higher energy levels from becoming significantly populated.
Some examples of processes that can create population inversions include:
shock fronts (when particles are accelerated to supersonic speeds),
stellar winds/flares, stellar radiation, collisions between populations
of particles at different temperatures, and chemical reactions (the
products formed are usually in metastable states.
Why does the observed profile line resemble
a bell curve when lasers are present around a star?
The observed line profile should resemble a bell curve because of
the Doppler Effect. Recall that for sound, the Doppler Effect says
if a train is approaching an observer, its pitch is higher, but if
the train is moving away from an observer, its pitch is lower. The
same applies for light. For the side of a rotating disk that is "approaching"
the observer, the light emitted appears to be higher in energy. For
the side of the rotating disk that is moving away from the observer,
the light appears to be lower in energy. Thus, rotation of the disk
around the star causes a shift in energies of some of the emitted
light. Instead of a single wavelength, a bell curve of many wavelengths
surrounding the "true" wavelength is observed.
Why Might Astronomical Lasers Be So Rare?
Each recombination line has an optimum gas density for lasers or masers
to form. The lower the principle quantum number, n, the higher
the optimum density. Because the density in the disk surrounding MWC
349A falls off as R-3/2, where R is the distance to the
center of the star, the lower n is, the smaller the region in which
lasing can occur. The smaller the possible amplification region, the
lower the intensity of the lasing since there is simply less material
available for amplification. According to calculations, if the disk
does not extend inward of 40 AU (roughly the distance from the Sun
to Pluto) to MWC 349A, then the material in the disk is not dense
enough to support detectable lasing by Ha lines with n < 10.
Even if the disk extends inward of 40 AU or Astronomical Units (about
the distance from our Sun to Pluto), because the lasing regions for
n < 10 would be so small, the background radiation could easily
become dominant over any lasing. Detection equipment with better resolution
and higher sensitivity would thus be necessary to find these small
laser "hot spots". Lastly, Strelnitski speculates that because
any tilt in the disk would remove any small "hot spots"
from view, it is likely that any detectable lasing could simply not
be in our line of sight.
*Not all proposed pumping mechanisms require the population of molecules
to be below a certain critical density, but these other mechanisms
are rare or unproven.
of Young Investigators. 2001. Volume Three.
Copyright © 2001 by Courtney Peterson and JYI. All rights reserved.