- 3974 -
ELECROMAGNETIC SPECTRUM - more
than meets the eye. Parts of the
electromagnetic spectrum invisible to human eyes reveal a vast amount of
information about the universe, but it took a long time for astronomers to learn
how to view it.
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-- 3974
- ELECROMAGNETIC SPECTRUM
- more than meets the eye.
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- For
thousands of years, humans were looking up at the star-studded night sky using
just their eyes sensitive to the optical wavelength of the electromagnetic spectrum.
The first telescopes, invented in the early 17th century, enhanced the ability
of human eyes by magnifying distant objects.
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- As
physicists started discovering in the 19th century that there are other,
invisible, types of light in the natural world around us, astronomers realized
that there must be such light emanating also from the universe.
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- Today,
astronomers know that the majority of radiation, or light, present in the
universe is invisible to human eyes. By looking at the universe in all possible
wavelengths, scientists are piecing together a complex picture of the
unfathomably vast cosmic environment that we are a part of. It took decades,
for instruments to be developed that could detect this invisible radiation from
celestial sources.
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- Astronomers
use different instruments to study different types of electromagnetic radiation
emitted by the universe. Radio astronomy
studies cosmic radiation with the longest wavelengths (from less than 0.4
inches to several miles, or 1 centimeter to several kilometers) and was the
first kind of astronomy developed that relies on wavelengths other than optical
light.
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- The
discovery that radio waves from bodies in the universe was made completely by
accident. In 1933, a young American radio engineer Karl Jansky, an employee of
the famous telephone company Bell Laboratories, was tasked to search for
sources of unexplained hiss that sometimes interfered with transmissions of
radio messages across the Atlantic Ocean.
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- Jansky
found that while some of this noise was coming from sources on Earth, such as
nearby thunderstorms, there was a type of signal, constantly picked up by his
experimental antennas, that appeared to be coming from what we know today is
the center of our Milky Way galaxy, the region where the black hole Sagittarius
A* resides. Systematic exploration of the radio universe began soon thereafter.
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- Astronomers
have discovered since that radio waves are emitted by spinning electrons and
emanate from all sorts of environments that have the ability to make those
electrons spin. Typically, we detect
radio waves, by looking at electrons moving through a magnetic field. But ionized gas can emit radio waves as well.
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- By tracing
the structure of radio wave-emitting clouds, astronomers were able to map out
the entire structure of our galaxy, the Milky Way, as well as other nearby
galaxies. They could determine areas with high concentrations of hot young
stars, but also study objects obscured by dust, such as black holes that hide
in galactic centers.
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- Highly
magnetized bodies, such as fast-spinning stellar remnants called “pulsars” are
prime targets for radio astronomy as they send out powerful flashes of radio
waves as they spin like superfast cosmic lighthouses.
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- The “Square
Kilometer Array's” site in Australia will rely on 130,000 Christmas-tree like
dipole antennas to listen to radio waves emitted by objects in the most distant
universe.
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- As radio
waves are the type of electromagnetic radiation with the longest wavelengths,
radio telescopes have to be large. Vast arrays of radio-antennas, such as the
“Karl G. Jansky Very Large Array” in New Mexico that consists of 28 dishes each
82-foot-wide (25 meters), are the technological standard today.
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- By
combining multiple antennas, astronomers create telescopes that have immense
apertures that equal the distance between the array's most distant parts, thus
enabling the scientist to detect the faintest signals with the best possible
resolution.
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- The “Square
Kilometer Array” (SKA), currently constructed across two locations in Australia
and South Africa, will be the world's largest radio telescope by a significant
margin once it comes online around 2028. With its thousands of dishes and
dipole antennas spanning thousands of square miles of remote land, SKA will
survey large areas of the sky at once and detect the faintest signals coming
from the farthest reaches of the universe.
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- The “Event
Horizon Telescope”, famous for taking photographs of black holes, is also a
radio telescope, or rather a worldwide network of radio telescopes with an
aperture equalling the size of our planet.
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- Unlike some
other types of wavelengths, radio waves mostly penetrate Earth's atmosphere
with ease, allowing astronomers to base their equipment on the planet's
surface.
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- Astronomers
are now seriously considering building a radio telescope on the far side of the
moon. Removed from Earth-based sources of human-made radio noise, as well as
from Earth's ionosphere (the upper part of the atmosphere which contains
ionized gas that absorbs and distorts some cosmic radio signals), such an
observatory would provide scientists with the deepest and most undisturbed
views into the earliest epoch of the universe.
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- The next
electromagnetic spectrum band after radio waves are “microwaves”. As microwaves
cover wavelengths between 3.3 feet and 0.04 inches (1 meter and 1 millimeter),
the first discoveries of cosmic microwaves were actually made by radio telescopes.
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- Microwaves
have a special, although rather limited place in astronomy. According to the
European Space Agency (ESA), the whole sky glows uniformly in microwaves with
what has been identified as the cosmic microwave background.
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- This uniformness
is unseen in other wavelengths, which reveal the sky in dots and regions of
varying brightness. In fact, cosmic microwave radiation is so odd that the
researchers who first discovered it in the 1960s , completely by accident
during experiments with echo balloons,
originally thought it was produced by a telescope defect.
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- Subsequent
research confirmed that the microwave hum was coming from space and that it was
nothing less than the residue of radiation released by the Big Bang, the enormous
explosion which created the universe some 13.8 billion years ago.
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- This
radiation was originally released in the form of highly energetic,
short-wavelength X-rays, but since it took so long to reach us, the “redshift
effect” caused by the expansion of the universe has stretched this wavelength
all the way into microwaves.
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- Microwaves
reveal the universe as it looked in its earliest stages. The most sensitive
surveys were able to go as far as distinguishing the denser regions of gas and
dust that subsequently produced the first galaxies.
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- Microwaves
get mostly absorbed by Earth's atmosphere, which means they are best studied by
space-based telescopes. In 1989, after
the initial crude Earth-based detections of cosmic microwaves, NASA sent the
first dedicated microwave-observing satellite, the “Cosmic Background Explorer”
(COBE), into space.
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- COBE
measured differences in the temperature of the microwave background in various
regions. COBE's successor, the “Wilkinson Microwave Anisotropy Probe” (WMAP),
launched in 2003, further improved the level of detail of this cosmic microwave
map. These observations helped to determine the universe's age with greater
precision, and define the amounts of different types of matter that the
universe contained in its earliest years.
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- ESA's
“Planck mission”, launched in 2009, then completed the task of creating the
most accurate map of the cosmic microwave background.
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-
Submillimiter waves have a limited but fascinating role in astronomy.
They reveal the sources of natural laser lights. The submillimeter wavelength sits between the
millimeter and infrared ranges.
Submillimeter waves have lengths shorter than 1 mm, or 0.04 inches, and
up to a few hundred micrometers. Observations in this range partially overlap
with the longest wavelengths of the infrared spectrum.
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- The use of
submillimeter wavelengths in astronomy is relatively recent. Detectors used to
detect submillimeter radiation are quite similar to those used in radio
astronomy, but thousands of times smaller. Technology therefore had to progress
enough to make these detectors possible.
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Submillimeter waves penetrate through clouds of molecular gas and dust
into star-forming regions, which are obscured from the view of of optical
telescopes. In submillimeter waves,
astronomers can observe universe's "natural lasers," regions where
highly charged electrons emit laser light as they discharge some of their
energy. These natural lasers, called “masers”, are usually observed in a
special type of pulsating variable stars called the Mira stars.
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- Because
submillimeter waves get absorbed by water in Earth's atmosphere, observatories
that study sources of submillimeter radiation in the universe need to be built
in high and dry places to prevent water vapor from obscuring their views. In
essence, you will find submillimeter telescopes in the same places on Earth
where you find the best optical telescopes.
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- The
“Atacama Large Millimeter/submillimeter Array” (ALMA), operated by the European
Southern Observatory is located on the Chajnantor plateau in northern Chile at
an altitude of 16,400 feet.
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- The
Submillimeter Array on Hawai'i's Maunakea, which is operated by the Harvard
Smithsonian Center for Astrophysics, sits somewhat lower, at 13,450 feet above
sea level.
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- Unlike
submillimeter waves, infrared light spans a vast range of the electromagnetic
spectrum from 0.04 inches (just below 1 millimeter) on the side bordering with
microwaves to 0.75 micrometers on the side bordering with the visible light.
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- The James
Webb Space Telescope (JWST), launched on Christmas Day, 2021, thrust infrared
astronomy into the spotlight with its ability to see the farthest reaches of
the universe.
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- Infrared light,
which is essentially heat, was the first non-visible wavelength discovered,
completely by accident, by British astronomer William Herschel in 1800 during
his experiments with the visible light spectrum.
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- The first
crude observations of celestial objects in the infrared spectrum focused on the
moon and the sun. Astronomers in the second half of the 19th century were able
to measure the temperature of the sun's atmosphere as well as the various
temperature zones on the moon's surface.
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- By the
turn of the century, technology progressed to the level that it was possible to
detect heat from the solar system's giant planets Jupiter and Saturn.
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- Infrared
astronomy didn't fully take off until the second half of the 20th century when
more sophisticated detectors were developed, allowing astronomers to analyze
heat sources across the Milky Way.
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- Infrared
light is good at many things. Thanks to
its ability to penetrate through dust and gas, infrared light reveals what's
going on inside of thick dust and gas clouds where stars form. Stars emerging
in the middle of these clouds are not yet hot enough to emit visible light, but
are warm enough to be detected by infrared sensors.
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- Astronomers
can observe matter that is only several degrees warmer than absolute zero, the
temperature of minus 459.67 degrees Fahrenheit (minus 273.15 degrees
Fahrenheit), where the motion of atoms stops.
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- When
viewing the Milky Way in infrared light, a hidden galaxy of failed stars,
called brown dwarfs, emerges. Brown dwarfs are bodies that are too big to be
called planets but are not quite massive enough to ignite nuclear fusion in
their cores.
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- Bodies in
the farther reaches of the solar system that receive too little solar
illumination also spring into view. Even the interstellar medium, the cool gas
and dust dispersed between stars and galaxies, can be mapped in the infrared
spectrum.
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- Webb was
built with the aim to detect the first light that lit up the universe a few
hundred million years after the Big Bang. Although this light had been emitted
in the optical wavelength range, the accelerating expansion of the universe had
stretched this light into the infrared range thanks to the effect known as
“redshift”. Optical telescopes, even if they were as sensitive as Webb, could
therefore no longer see this light.
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- But the
James Webb Space Telescope sees only a small fraction of the infrared spectrum,
the so-called mid and near-infrared light, which spans wavelengths from 28.5
micrometers to 0.6 micrometers where the visible spectrum begins.
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- NASA's
recently retired flying telescope SOFIA was a specialist in the longer
wavelength type of infrared light, the “far infrared”, which reaches all the
way to 612 micrometers and is best for observing the cool interstellar medium.
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- NASA's
“Spitzer Space Telescope” surveyed the universe in the mid-infrared and parts
of the far infrared spectrum from 2003 to 2020. ESA's “Herschel spacecraft”
complemented this work in the far-infrared spectrum between 2009 and 2013. An earlier airborne telescope, the “Kuiper
Airborne Observatory”, studied the infrared sky from 1974 to 1995.
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- Optical
astronomy has made enormous leaps since those first early 17th-century telescopes. From giant telescopes occupying remote
mountain tops and highland plateaus to orbiting super-eyes such as the iconic
Hubble Space Telescope, optical observatories reveal the universe with an
ever-increasing level of detail.
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- Optical
telescopes show the universe as it would appear to human eyes. Colors in
optical images correspond to the colors human eyes would see. Images from other
types of telescopes, such as those
imaging the universe in infrared and ultraviolet light, have to be processed by
astronomers on the ground, with colors artificially assigned to different
wavelengths.
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- To be
visible in the optical wavelengths, objects need to either emit their own
visible light or be illuminated by other objects. Planets, moons and asteroids
in our solar system are only visible to optical telescopes (and to human eyes)
because of the vicinity of our sun.
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- Optical
light can't pass through obstacles, such as thick clouds of dust, which hide
some of the most interesting areas of the universe (such as centers of galaxies
where supermassive black holes devour huge amounts of material or star-forming
nebulas).
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- Optical
light is also somewhat affected by Earth's atmosphere, even though not as much
as the infrared and submillimeter wavelengths. While infrared and submillimeter
radiation gets mostly absorbed, optical rays get a little dispersed by the
molecules in the atmosphere, which means that observed objects don't appear as
sharp as they would if the atmosphere wasn't present.
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- This
atmospheric blurring limits the accuracy of observations that Earth-based
optical telescopes can achieve, even though modern adaptive optics systems
installed on the world's best telescopes can to a certain extent make up for
this shortcoming.
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- The Hubble
Space Telescope is the undisputed king of optical astronomy and the source of
many images that have gained iconic status. The telescope, launched in 1990, is
still going strong and still may have a decade or so of life and fabulous
astronomy ahead of it.
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- The “Very
Large Telescope” (VLT) operated by the “European Southern Observatory” (ESO) in
Chile is one of the most advanced Earth-based optical telescopes. VLT consists
of four main telescopes, each with a 27-foot-wide (8.2 meter) mirror, and four
5.9-foot-wide (1.8 m) auxiliary telescopes.
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- The four
main telescopes can each detect light that is four billion times fainter than
what human eyes can see. The telescopes can also work together as a so-called
interferometer, which increases the resolution to a level that would be
achievable with a single telescope with a 426-foot-wide mirror.
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- ESO is
currently building the next-generation “Extremely Large Telescope” (ELT), also
in Chile. With a single 130-foot-wide mirror, ELT will be the world's largest
optical telescope. Once completed, the observatory will be able to gather 100
million times more light than the human eye and provide images 16 times sharper
than the Hubble Space Telescope.
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- The twin
Keck Telescopes on the Hawaiian island of Maunakea are fitted with
32.8-foot-wide mirrors that forced the technical teams that designed and built
them in the late 1980s to develop some ingenious technical solutions.
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- Since it
wasn't possible at that time to accurately operate a single solid mirror of
such a size, engineers made the Keck mirrors from 36 hexagonal segments that
work together as a unit with the help of an active optics system. This
segmented mirror design is quite similar to the one used for the 21-foot-wide mirror
of the James Webb Space Telescope.
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- The “Large
Binocular Telescope” in Arizona features the world's largest non-segmented
mirror, measuring 28 feet in diameter.
The Gran Telescopio Canarias on the Spanish island of La Palma off the
coast of western Africa, is the world's largest single-aperture optical
telescope, featuring a 10.4 meter wide mirror.
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- Hubble is
also the world's main observer of ultraviolet light that emanates from sources
in the universe. Ultraviolet light has shorter wavelengths and carries higher
energies than visible light and points astronomers to hot, energetic processes,
such as those taking place in young stars and in young star-forming galaxies.
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- Massive
stars that orbit each other in binary systems also emit ultraviolet light and
so do powerful auroras on giant gaseous planets like Jupiter.
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- Ultraviolet
light gets absorbed by the ozone layer in Earth's atmosphere, which is good for
organisms living on Earth (as these wavelengths are known to cause tissue-damage
and cancer). For astronomy the limited ability of ultraviolet light to
penetrate the atmospheres means that telescopes designed to study it need to
orbit in space.
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- Apart from
the Hubble Space Telescope, solar observatories such as the “European Solar
Orbiter” or NASA's “Solar Dynamics Observatory” carry ultraviolet imagers to
observe highly energetic processes on the sun. NASA's Jupiter explorer Juno
also carries an instrument for studying ultraviolet light.
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- Things get
even more heated and energetic with X-rays. Discovered accidentally by German
physicist Wilhelm Roentgen in 1895, these matter-penetrating rays are generated
in vast amounts during some of the most extraordinary processes in the
universe, such as when supermassive black holes or extremely massive neutron
stars suck in matter from their surroundings, or during supernova explosions of
dying stars.
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- X-rays come
from the hottest places in the universe including black hole and neutron stars'
accretion disks where matter spirals at extreme speeds. High-temperature plasma
that fills space between galaxies in galaxy clusters also emits X-rays, and so
do stars including our sun.
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- Astronomers
recently discovered that comets can emit X-rays, and that Jupiter, in addition
to its ultraviolet aurora, also produces an aurora that shines in X-rays.
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- X-rays are
a really powerful part of the spectrum because you get fluorescence in X-rays.
Rocky surfaces of moons and planets give off X-rays for fluorescence. The atmospheres
around terrestrial planets also fluoresce and X-rays, the gas giants scatter
solar X-rays, so they act like a mirror to the solar X-rays. Fluorescence is the ability of a surface to
absorb and subsequently emit light that originally arrived from another source.
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- Infamous for
their potential to cause DNA mutations that may lead to cancer, X-rays get,
just like ultraviolet rays, fortunately filtered out by Earth's atmosphere.
X-ray astronomy could therefore only take off once humans were able to send
objects to space.
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- One of the
problems with the detection of cosmic X-rays is their ability to penetrate
matter. Just like they penetrate human tissue to reveal broken bones, X-rays
also pass through mirrors that astronomers may want to use to concentrate them.
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- Building
sensitive X-ray detectors therefore requires some engineering ingenuity.
Scientists have to design mirrors for X-ray telescopes in a way that the
energetic rays hit the reflecting surface at a shallow angle "like a stone
skipping across the surface of a pond".
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- X-ray
telescopes require multiple mirrors positioned at gradually increasing angles
to deflect the X-rays onto a detector. Such contraptions, however, tend to be
rather chunky and require large satellites to accommodate them. NASA's Chandra,
for example, at 45-feet-long, is the largest satellite launched by the Space
Shuttle, about a three feet longer than Hubble.
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- The
matter-penetrating ability of X-rays, however, also has its advantages, as
these rays easily escape from dust-shrouded regions, such as galactic centers
where black holes munch on the infalling matter.
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- NASA's
“Chandra X-ray observatory” is the current flagship X-ray telescope. In space
since 1999, Chandra travels around Earth on an elliptical orbit that takes it
as far as 83,000 miles away from the planet's surface where no residual
atmosphere obstructs the X-ray views.
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- During its
more than two decades in orbit, Chandra has imaged jets of matter shooting from
supermassive black holes in galactic centers and even traced the separation of
dark matter from normal matter in the collisions of galaxies in galactic
clusters.
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- Gamma-rays
are the highest energy type of radiation present in the universe. Just like
X-rays, they come from extremely hot and energetic processes in the universe,
such as supernova explosions and accreting black holes.
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- Even more
capable of penetrating matter than X-rays, gamma-rays are also produced during
nuclear explosions on Earth, and, in smaller quantities, in thunderstorms and
during radioactive decay. Stars such as our sun also produce occasional
gamma-ray flashes in the form of solar flares.
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- Just like
many other types of astronomy, gamma-ray astronomy came about by accident. In
the 1960s, American military satellites were looking for signs of the USSR's
testing of nuclear weapons, when they detected inexplicable flashes of
extremely energetic gamma-rays. Lasting from fractions of seconds to several
minutes, these gamma-ray bursts, as they became known, were coming regularly
from all parts of the universe.
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- It took
until the 1990s for astronomers to figure out that these bursts come from
extremely powerful explosions that mark the birth of new black holes when
massive stars die. The shorter types of gamma-ray bursts are produced in
collisions of superdense stellar remnants called the neutron stars.
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- Gamma-ray
bursts point astronomers to the fact that a cataclysmic event has just occurred
somewhere in the universe. By measuring the intensity of the burst, astronomers
can learn something about the intensity and distance of the event.
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- However,
they need to search for the source of the flash afterward, using other types of
telescopes. When they manage to locate the region in the sky where the burst
has come from, they can then observe the area in other parts of the
electromagnetic spectrum to gain more insight into the processes involved.
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- NASA's
space telescopes “Fermi' and “Swift” together with ESA's “Integral” are the
world's current gamma-ray burst spotting workhorses. However, only Swift, which
covers about 9% of the sky, has the ability to locate sources of these giant
explosions.
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- Astronomers
are therefore looking for new approaches to gamma-ray burst detection. In 2021,
a team of scientists from Hungary and Slovakia launched a tiny cubesat called
“GRB Alpha”, that has been successfully detecting gamma-ray bursts ever since.
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- In October
2022, GRBAlpha made an accurate detection of the peak intensity of the
brightest gamma-ray burst ever seen, while the event completely blinded
detectors on NASA's Fermi.
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- The
researchers envision that a fleet of such cubesats would make it possible to
find sources of gamma-ray bursts across the entire sky through the so-called
triangulation, the same method used to pinpoint a location on Earth with the
help of GPS.
April 26, 2023
ELECROMAGNETIC SPECTRUM 3974
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------ “Jim Detrick” -----------
--------------------- --- Thursday, April 27,
2023 ---------------------------
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