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# The Electromagnetic Spectrum

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## More about the Electromagnetic Spectrum

As it was explained in the Electromagnetic Spectrum - Level 1 of Imagine the Universe!, electromagnetic radiation can be described in terms of a stream of photons, each traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy. 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 low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and gamma-rays.

Actually, the amount of energy a photon has makes it sometimes behave more like a wave and sometimes 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 only in how it behaves. Low energy photons (such as radio) behave more like waves, while higher energy photons (such as X-rays) behave more like particles. This is an important difference for scientists to know when they design detectors and telescopes to try to 'see' EM radiation from very low to very high energies. In fact, scientists choose whichever description of light they need for their study.

The truth is, 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

(lambda and nu are just letters from the Greek alphabet that scientists like to use rather than l or f. It just helps them to keep things straight!) Both the speed of light and Planck's constant are, well, constant: They never change their values. Ever. The speed of light 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.

## Space Observatories in Different Regions of the EM Spectrum

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 radio telescopes on the Earth in some important ways.

There are a number of radio observatories in space. These include Polar, Cluster II, ISEE 1, ISEE 2, GOES 9 and Voyager 1. Most of these 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 is a special technique used in radio astronomy called "interferometry". Radio astronomers can combine data from two telescopes that are very far apart and create images which have the same resolution as if they had a single telescope as big as the distance between the two telescopes! That means radio telescope arrays can see incredibly small details. One such array is called the Very Large Baseline Array (VLBA): it consists of ten radio telescopes which reach all the way from Hawaii to Puerto Rico: nearly a third of the way around the world! By putting a radio telescope in orbit around the Earth, radio astronomers could make images as if they had a radio telescope the size of the entire planet! The Very Long Baseline Interferometry (VLBI) Space Observatory Program (VSOP) attempts to do just that. This Japanese mission was launched in February 1997, and renamed HALCA shortly thereafter.

### Microwave observatories

The sky is a source of microwaves in every direction, most often called the microwave background. This background is believed to be the remnant from the "Big Bang" scientists believe our Universe began with. It is believed that a very long time ago all of space was scrunched together in a very small, hot ball. The ball exploded outward and became our Universe as it expanded and cooled. Over the course of the past several billion years (the Universe's actual age is still a matter of debate, but is believed to be somewhere between ten and twenty billion years), it has cooled all the way to just three degrees above 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 direction to another. Astronomers have 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 of the theories about the Big Bang.

The Wilkinson Microwave Anisotropy Probe (WMAP), launched in the summer of 2001, measures the temperature fluctuations of the cosmic microwave background radiation over the entire sky with even greater precision. WMAP is answering such fundamental questions as:

### 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 which kept it in view of the ground stations at all times, a necessary arrangement since 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 Space Infrared Telescope Facility (SIRTF). SIRTF uses an passive cooling system (i.e. it radiates away its own heat rather than requiring an active refrigerator system like most other space infrared observatories) and it was placed in an earth-trailing, heliocentric orbit, where it will not have to contend with Earth occultation of sources nor with the comparatively warm environment in near-Earth space.

Another major infrared facility coming soon will be the Stratospheric Observatory for Infrared Astronomy (SOFIA). Although SOFIA will not be an orbiting facility, it will carry a large telescope within a 747 aircraft flying at an altitude sufficient to get it well above most of the Earth's infrared absorbing atmosphere. SOFIA will be replacing the Kuiper Airborne Observatory.

### 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, because it is above the Earth's atmosphere, it does not suffer distorted vision from the air. If the air was all the same temperature above a telescope and there was no wind (or the wind was 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 suffer tiny wobbles. It gets bent a little, much like light gets bent by a pair of glasses. But unlike 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 black road surface. This 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 and its perfect view gives it many many times better resolution than even the best ground-based telescopes on the best nights.

Another advantage of the Space Telescope is that without the atmosphere in the way, it can see more than just the visible spectrum. The Space Telescope can also see ultraviolet light which normally is absorbed by the Earth's atmosphere and cannot be seen by regular telescopes. So the Space Telescope can see a much wider portion of the spectrum.

### 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 very fairly small field of view. Until September 1996, the International Ultraviolet Explorer (IUE) was operating and observing ultraviolet radiation. Its demise, although unfortunate, was hardly premature: IUE was launched in January, 1978 with planned operations of three years. IUE functioned more or less like a regular ground based observatory save that 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 fairly 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 and so could take images of larger objects in the sky -- like 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

There are two extreme ultraviolet observatories in space at the moment. One of them is the very first extreme ultraviolet observatory ever, the Extreme Ultraviolet Explorer (EUVE). Astronomers have been somewhat reluctant to explore from space at the extreme ultraviolet wavelengths since all theory strongly suggests 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 further than theory predicted.

Another extreme ultraviolet observatory currently operating is the Array of Low Energy X-ray Imaging Sensors (ALEXIS). Although its name indicates that it is an X-ray observatory, the range of energy ALEXIS is exploring is at the very lowest end of the X-ray spectrum and often considered to be extreme ultraviolet. ALEXIS was launched on 25 Apr 1993 on a Pegasus rocket. During launch, a hinge plate supporting one of the solar panels broke. However, the satellite survived, and the panel remains connected to the satellite via the electrical cables and a tether, and it still provides the requisite power to the satellite. ALEXIS is spinning about an axis pointed approximately toward the sun. ALEXIS provides sky maps on a daily basis whenever the satellite is not in a 100% sunlight orbit. These sky maps are used to study diffuse x-ray emission, monitor the brightness of known EUV objects, and to detect transient objects.

### X-ray observatories

There are several X-ray observatories currently operating in space with more to be launched in the next few years.

The Rossi X-ray Timing Explorer (RXTE) was launched on December 30, 1995. RXTE is able to make very precise timing measurements of X-ray objects, particularly those which show patterns in their X-ray emissions over very short time periods, such as certain neutron star systems and pulsars.

Other X-ray observatories currently operating in space include ROSAT, 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 attached to the Russian space station Mir, and Beppo SAX, an Italian X-ray satellite.

NASA launched another major new X-ray astronomy satellite, the Chandra X-ray Observatory (CXO), in mid 1999.

### Gamma-ray observatories

The Compton Gamma-Ray Observatory (CGRO) was launched by the space shuttle in April 1991. The observatory's instruments are 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 which Compton has made was the observation of gamma-ray burst events coming from the Earth itself at the top of thunderstorm systems. The cause of this phenomenon is not known, but it is currently suspected to be related to "Sprites": lightning flashes which are occasionally seen jumping upward from cloud tops to the upper stratosphere.

The Russian gamma-ray observatory Granat has exhausted its control fuel. Its last maneuver in 1994 was to initiate a roll which allowed it to perform a continuous all-sky survey until Nov 1998.

The European mission INTEGRAL was launched in October 2002. It is studying gamma-ray bursts, and sources within our galaxy.

The next major gamma-ray missions in the near future include SWIFT and the Gamma-Ray Large Area Space Telescope (GLAST). SWIFT will study gamma-ray bursts, and be capable of quickly pointing narrow field X-ray and optical detectors in the direction of gamma-ray bursts detected by its large field detectors. GLAST will have a field of view twice as large as that of the Compton Gamma-Ray Observatory, and a sensitivity of up to 50 times greater than Compton's EGRET instrument. GLAST will study a wide range of gamma-ray objects, including pulsars, black holes, active galaxies, diffuse gamma-ray emission, and gamma ray bursts.

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