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The Electromagnetic Spectrum

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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
lambda = c / nu
Animation of the characteristics of a wave

energy equals Planck's constant times the frequency
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

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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 BackgroundExplorer (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.

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Infrared observatories

The most recent infrared observatory currently in orbit was the Infrared SpaceObservatory (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

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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 LowEnergy 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

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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All material on this site has been created and updated between 1997-2012.

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