The Electromagnetic Spectrum
More about the Electromagnetic Spectrum
As it was explained in Electromagnetic Spectrum - Level 1 of Imagine the
Universe!, electromagnetic
radiation can be described as a stream of photons, each traveling in a wave-like pattern,
carrying energy and moving at the speed of light. In that
section, it was pointed out that the only difference between radio
waves, visible light and gamma rays is the energy of the photons. Radio waves have photons with the
lowest energies. Microwaves have a little more energy than radio
waves. Infrared has
still more, followed by visible, ultraviolet, X-rays and gamma rays.
The amount of energy a photon has can cause it to behave more like a
wave, or more like a particle. This is called the "wave-particle duality" of light. It is important to understand that we are
not talking about a difference in what light is, but in how it behaves.
Low energy photons (such as radio photons) behave more like waves, while
higher energy photons (such as X-rays) behave more like particles.
The electromagnetic
spectrum can be expressed in terms of energy, wavelength or
frequency. Each way of thinking about the EM spectrum is related to the
others in a precise mathematical way. The relationships are:
the wavelength
equals the
speed of
light divided by the frequency
or
lambda = c / nu
and
energy equals Planck's constant times
the frequency
or
E = h x nu
The Greek alphabet letters lambda and nu are used by scientists
instead of l and f) Both the speed of light and Planck's constant are
actually constant — they
never, ever change in value. The speed of light in a vacuum is equal to
299,792,458 m/s (186,212
miles/second). Planck's constant is equal to 6.626 x 10-27
erg-seconds.
Show a
chart of the wavelength, frequency, and energy regimes
of the spectrum
Space Observatories in Different Regions of the EM Spectrum
Radio observatories
Radio waves
can make it through the Earth's atmosphere
without significant obstacles. In fact, radio telescopes can
observe even on cloudy days. However, the availability of a space radio
observatory complements
Earth-bound radio telescopes on Earth in some important ways.
There are a number of radio observatories in space.Most of them study the
ionospheres of the planets down to 3 x 10-4 Hz. Some have
been used to monitor radio signals given off by earthquakes.
One special technique used in radio astronomy
is called "interferometry." Radio astronomers can combine data from two
telescopes that are very far apart and create images that have the same
resolution as if they had a single telescope as big as the distance
between the two telescopes. This means radio telescope arrays can see
incredibly small details. One example is the Very Large Baseline
Array (VLBA), which consists of 10 radio telescopes that reach from
Hawaii to Puerto Rico, nearly a third of the way around the world.
By putting a radio telescope in orbit around Earth, radio astronomers can make
images as if they had a radio telescope the size of the entire planet.
That, and more, was accomplished by the Very Long Baseline
Interferometry (VLBI) Space Observatory Program (VSOP). The Japanese
mission was launched in February 1997, renamed
HALCA shortly thereafter. The mission lasted until November 2005,
exceeding its expected three-year lifespan. This virtual radio
telescope had an aperture about three times the size of Earth's radius,
and it conducted observations in conjunction with ground-based radio
telescope networks.
Microwave observatories
The sky is a source of microwaves in every direction, most often
referred to as the microwave background. This background is the remnant of
the "Big Bang," a term used to describe the beginning of the universe.
A very long time ago, all the matter in existence
was scrunched together in a very small, hot ball. The ball expanded
outward and became our universe as it cooled. Since the
Big Bang, which is estimated to have taken place 13.7 billion years
ago, it has cooled all the way to just three degrees above absolute
zero. It is this "three degrees" that we measure as the microwave
background.
From 1989 to 1993, the Cosmic Background
Explorer (COBE) made very precise measurements of the temperature
of this microwave background. COBE mapped out the entire microwave
background, carefully measuring very small differences in temperatures
from one place to another. Astronomers had many theories about the
beginning of the universe, and their theories predict how the
microwave background would look. The very precise measurements
made by COBE eliminated a great many theories about the Big
Bang. Dr. John Mather (NASA) and Dr. George Smoot (University of
California, Berkeley) won the 2006 Nobel Prize in Physics for their
work on COBE.
The Wilkinson Microwave Anisotropy Probe (WMAP),
launched in the summer of 2001, measured the temperature fluctuations
of the cosmic microwave
background radiation over the entire sky with even greater precision.
WMAP answered such fundamental questions as:
WMAP operated until October 2010.
Tell me more about WMAP
!
Infrared observatories
The most recent infrared observatory currently in orbit was the
Infrared Space
Observatory (ISO), launched in November 1995 by the European Space
Agency, and operated until May 1998. It was placed in an elliptical orbit with a 24-hour period that
kept it in view of the ground stations at all times. This arrangement
was necessary because ISO transmitted observations as it made them,
rather than storing information for later playback. ISO observed from
2.5 to 240 microns.
In August 2003, NASA launched the Spitzer Space Telescope,
formerly known as the Space Infrared Telescope Facility
(SIRTF). Spitzer uses a passive cooling system, which means it radiates
away its own heat rather than requiring an active refrigerator system
like most other space infrared observatories. It was placed in an
earth-trailing, heliocentric orbit, where it does not have to contend
with Earth occultation
of sources, or with the comparatively warm environment in near-Earth
space.
Another major infrared facility is the Stratospheric Observatory for
Infrared Astronomy (SOFIA). Although SOFIA is not be an orbiting
facility, it carries a large telescope inside a 747 aircraft flying at
an altitude sufficient to get it well above most of the Earth's infrared
absorbing atmosphere.
SOFIA replaces the Kuiper Airborne Observatory.
The James
Webb Space Telescope (JWST) will be a large infrared telescope with
a 6.5-meter primary mirror. Launch is planned for 2015. JWST will study
every phase in the history of our universe, ranging from the first
luminous glows after the Big Bang, to the formation of solar systems
capable of supporting life on planets like Earth, to the evolution of
our own Solar System. JWST was formerly known as the "Next Generation
Space Telescope" (NGST).
Visible spectrum observatories
The only visual observatory in orbit at the moment is the Hubble Space
Telescope (HST). Like radio observatories in space, there are visible
observatories already on the ground. However, Hubble has several
special advantages over them.
HST's biggest
advantage is that it
does not suffer distorted vision from the air because it is above the
Earth's atmosphere. If the air was all the same
temperature above a telescope and there was no wind, or if the
wind were perfectly constant, telescopes would have a perfect view
through the air. Alas, this is not how our atmosphere works. There
are small temperature differences, wind speed changes, pressure
differences, and so on. This causes
light passing
through air to wobble slightly. It gets bent a little, much like light
gets bent by a pair of glasses. But unlike with glasses, two light beams
coming from the same direction do not get bent in quite the same way.
You've probably seen this before — looking along the top of the
road on a hot day, everything seems to shimmer over the heated road
surface. This effect
blurs the image
telescopes see, limiting their ability to resolve objects. On a good
night in an observatory on a high mountain, the amount of distortion
caused by the atmosphere can be very small. But the Space Telescope has
no distortion from the atmosphere. Its perfect view gives it many, many
times clearer images than even the best ground-based telescopes on the
best nights.
Another advantage of the HST is that without the atmosphere in the
way, it can see a much wider portion of the spectrum. In addition to the
visible spectrum, it
can also see ultraviolet light that is
normally absorbed by Earth's atmosphere and cannot be seen by regular
telescopes.
Ultraviolet observatories
Right now, there are no dedicated ultraviolet observatories in
orbit. The Hubble Space Telescope can perform a great deal of observing
at ultraviolet wavelengths, but it has a
fairly small field of view. From January 1978 to September 1996, the
International Ultraviolet Explorer (IUE) operated and observed
ultraviolet radiation. Its demise, although unfortunate, was hardly
premature: IUE was launched
with planned operations of three years. IUE functioned like a regular
ground-based observatory, with one exception: the telescope operator and
scientist did not actually visit the telescope, but sent it commands
from the ground. Other than some care in the selection of materials for
filters, a UV telescope like IUE is very much like a regular visible
light telescope.
In addition to IUE, there have been some important recent UV space
missions. A reusable shuttle package called Astro has been flown twice
in the cargo bay of the space shuttle. It consisted of a set of three UV
telescopes. Unlike HST, the Astro
UV telescopes had very large fields of view, so they could
take images of larger objects in the sky, such as galaxies. For
instance, if the Hubble Space Telescope and the Astro telescopes were
used to look at the Comet Hale-Bopp, Hubble would be able to take
spectacular pictures of the core
of the comet. The Astro telescopes would be able to take pictures of
the entire comet, core and tail.
Extreme Ultraviolet observatories
Currently, some extreme ultraviolet studies are being carried out by
the Solar Dynamics Observatory (SDO), launched February 2010. It is the
first mission to be launched as part of NASA's Living With a Star (LWS)
Program, which is designed to understand the causes of solar
variability and its impacts on Earth. Some examples of what the
observatory studies include the Sun's interior, atmosphere and magnetic
field, the plasma of the corona, and the irradiance that creates the
ionospheres of the planets in the solar system.
An earlier ultraviolet observatory was the Array of Low
Energy X-ray Imaging Sensors (ALEXIS). After 12 years in orbit, the ALEXIS satellite
reached the end of its career. Its solar arrays degraded in
charge-producing ability, and two of the four battery packs failed. On
April 29, 2005, its solar arrays were intentionally tipped away from
the sun, placing the Alexis system in the lowest power state for safety
purposes, after which it stopped being tracked.
Although the name indicates that it was an X-ray observatory, the range of energy ALEXIS
explored was at the very lowest end of the X-ray spectrum, often
considered to be extreme ultraviolet. ALEXIS was launched April 25, 1993
on a Pegasus rocket. During launch, a hinge plate supporting one of the
solar panels broke. However, the satellite survived. The panel remained
connected to the satellite by electrical cables and a tether, so it was
still able to provide the requisite power to the satellite. ALEXIS spun
about an axis pointed approximately toward the sun. It provided sky
maps on a daily basis whenever the satellite was not in a 100% sunlight
orbit. These sky maps were used to study diffuse X-ray emission, monitor
the brightness of known EUV objects and to detect transient objects.
The very first extreme ultraviolet observatory ever was the Extreme Ultraviolet Explorer (EUVE). The
observatory operated from June 1992 to January 2001. Astronomers were
somewhat reluctant to explore from space at the extreme ultraviolet wavelengths
since theory strongly suggested that the interstellar medium (the tiny traces
of gases and dust between the
stars) would
absorb radiation in this portion of the spectrum. However, when the
Extreme Ultraviolet Explorer (EUVE) was launched, observations showed
that the solar system is located within a bubble in the local
interstellar medium. The region around the Sun is relativity devoid of
gas and dust which allows the EUVE instruments to see much farther than
theory predicted.
X-ray observatories
NASA launched a major new X-ray astronomy
satellite, the
Chandra X-ray Observatory (CXO), in July 1999. It orbits Earth in
an elongated orbit that reaches more than a third of the distance to
the Moon. This orbit allows for long, uninterrupted observations, as
long as 55 hours. Chandra is designed to observe high-energy regions of space,
such as nebulae. It is also able to create images that are 25 times
sharper than any X-ray telescope preceding it.
The Rossi X-ray
Timing Explorer (RXTE) was launched December 30, 1995. RXTE is able
to make very precise timing measurements of X-ray objects, particularly
those that show patterns in their X-ray emissions over very short time
periods, such as certain types of neutron star systems and pulsars.
Suzaku was
launched by Japan in July 2005. It was jointly developed by the
Institute of Space and Astronautical Science of the Japan Aerospace
Exploration Agency (JAXA) and NASA's Goddard Space Flight Center.
Astronomers are using Suzaku to study galaxies, black holes, supernova
remnants, and galaxy clusters.
Europe has also had stake in the X-ray observation field, starting
with the EXOSAT
satellite in the 1980's. More recently, there is the European Space
Agency's (ESA) X-ray Multi-Mirror Mission, now known as XMM-Newton. Like
Chandra, it was launched in 1999. It has recently been used to observe
ultraluminous X-ray sources and find evidence of intermediate-mass black
holes.
Some X-ray observatories no longer in operation include ROSAT, which was a
joint venture between the United States, Germany and the United Kingdom;
the Advanced Satellite
for Cosmology and Astrophysics (ASCA), a joint U.S.-Japan venture;
the Kvant
astrophysics module, which was attached to the Russian space station
Mir, which completed its mission and was taken out of orbit to fall to
Earth in March 2001; and
Beppo SAX, an Italian X-ray satellite.
Gamma-ray observatories
Swift is a part
of the NASA Explorer Program designed with help from American
universities and NASA's international partners. It launched in November
2004. Swift studies gamma-ray bursts and is capable of quickly pointing
its narrow-field X-ray and optical detectors in the direction of
gamma-ray bursts that are detected by its large field detectors.
The Fermi Gamma-Ray
Space Telescope is the latest high-energy gamma-ray observatory launched
by NASA. It is designed to study energetic phenomena from a variety of
celestial sources. Fermi is a collaboration between NASA, the Department
of Energy and science communities in six other nations. Fermi studies a
wide range of gamma-ray objects, including pulsars, black holes, active
galaxies, diffuse gamma-ray emission and gamma ray bursts. While under
development, the satellite was known as the Gamma-ray Large Area Space
Telescope (GLAST).
The European mission INTEGRAL (INTErnational
Gamma-Ray Astrophysics Laboratory) was launched in October 2002, and
is the first space observatory that can collect data in the visible,
X-ray and gamma ray spectra. It targets gamma-ray bursts, supernovae and
black holes.
The Compton Gamma-Ray
Observatory (CGRO) was launched by the space shuttle in April 1991
and was deorbited in June 2000. The observatory's instruments were
dedicated to observing the gamma-ray sky, including locating gamma-ray burst sources, monitoring solar
flares, and other highly energetic astrophysical phenomenon. An
unexpected discovery that Compton made was the observation of gamma-ray
burst events coming from Earth itself at the top of thunderstorm
systems. The cause is not known, but it is currently suspected to be
related to "sprites," which are lightning flashes that jump upward from
cloud tops to the upper stratosphere. Fermi continues to monitor and
study this phenomenon.
The Russian gamma-ray observatory Granat has exhausted
its control fuel. Its last maneuver in 1994 was to initiate a roll that
allowed it to perform a continuous all-sky survey until November
1998.
Updated: November 2010
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