Traditionally astronomers studied stars, planets, galaxies, and other celestial objects using visible light and optical telescopes. Visible light is simply the range of electromagnetic radiation that the human eye can detect. There is nothing astronomically special about visible light compared to other forms of electromagnetic waves.
By the middle of the twentieth century the technology to detect other types of electromagnetic waves became available. Astronomers also began to realize that these other wavelength ranges of electromagnetic waves could help us probe the universe. Astronomers now study the entire electromagnetic spectrum from celestial objects.
Radio Astronomy
Radio telescopes are large concave metal dishes that reflect radio waves and focus them into a focal point in a manner analogous to optical reflecting telescopes. A radio receiver and amplifier at this focal point detects radio waves from celestial objects and measures their intensity. The receivers are usually tunable, just like your radio, so radio astronomers can study specific radio frequencies.
Quasars, active galaxies, pulsars, molecular clouds, star forming regions, and the cosmic microwave background radiation can all be studied with radio telescopes.
Infrared Astronomy
Infrared telescopes (or IR telescopes) are slightly modified large reflecting telescopes. The mirrors in optical reflecting telescopes reflect infrared light in the same way that they reflect visible light. Infrared telescopes use special detectors to measure the intensity of the infrared light.
Objects at temperatures comparable to average temperatures on Earth emit large amounts of infrared light. Therefore the telescope, its surroundings, and Earth's atmosphere emit infrared radiation that is usually at least millions of times more intense than the infrared signals from celestial objects. For use in the infrared, optical telescopes must be modified to compensate for this strong infrared background.
Water vapor and other molecules can absorb infrared light, so infrared telescopes must be located on very dry high altitude sites to maximize the atmospheric transparency. Some infrared wavelengths can only be studied from space.
Among other things, infrared astronomy helps astronomers understand star forming regions and circumstellar dust shells around stars.
Ultraviolet Astronomy
Like infrared telescopes ultraviolet telescopes (or UV telescopes) can be very similar to optical telescopes. They however have special detectors designed to measure the intensity of ultraviolet light.
Earth's atmosphere, particularly the ozone layer, blocks ultraviolet light. Therefore ultraviolet astronomy must be done from space, and ultraviolet telescopes are launched in satellites.
Hot stars and other celestial objects emit large amounts of ultraviolet light, so ultraviolet astronomy is most useful for studying these objects.
X-ray and Gamma Ray Astronomy
X-ray and gamma ray telescopes do not resemble traditional telescopes. High energy X-rays and gamma rays do not reflect as easily as other forms of electromagnetic waves, so the traditional reflecting telescope design does not work. In addition, detecting X-rays and gamma rays requires detectors similar to those used by high energy particle physicists. Hence these telescopes are more similar to high energy particle and radiation detectors than to optical telescopes.
Our atmosphere blocks most X-rays and gamma rays, so these telescopes must be launched into space on satellites.
X-rays and gamma rays are most useful for studying high energy astrophysical objects such as black holes and gamma ray bursts.
Further Reading
Zeilik, M. Astronomy: The Evolving Universe, 9th ed. Cambridge, 2002.
Chaisson, E. and McMillan, S. Astronomy Today, 5th ed. Pearson Prentice Hall, 2005.
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