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Ouch! A spiral galaxy as viewed edge-on collides with a small blue galaxy. From the Hubble Ultra Deep Field. Photo by NASA, ESA, S. Beckwith (STScI) and the HUDF Team.


astro101 -- Multi-wavelength Astronomy

The Electromagnetic Spectrum

We can describe light either by giving its wavelength (the distance between peaks in a wave) or its frequency (the number of waves that go by per second). For visible light, the range in wavelength is between 400 nanometers (nm) for blue light and 700 nm for red light. This comprises visible light because the retinas of our eyes are sensitive to these wavelengths. But, visible light is only a small part of the electromagnetic spectrum, which also includes gamma rays, X-rays, ultraviolet radiation, infrared and radio waves. In astronomy, there is no need to limit ourselves to what we see with our eyes. Detectors can be made to record any type of radiation. Although celestial objects may produce a variety of types or radiation, most of it gets blocked by the Earth's atmosphere. For emission that cannot get through, we must rely on satellite observatories which orbit the Earth to collect celestial information. From the Earth we are essentially limited to visible light and radio waves.

The Andromeda Galaxy

Gamma Ray Astronomy

We start with the type of radiation with the highest frequency and smallest wavelength. Since Gamma Rays are unable to penetrate the Earth's atmosphere, the observations must be done from space. NASA's Compton Gamma Ray Observatory flew from 1991 to 2001. The mission studied the high energy celestial activity in objects such as pulsars, novae, supernovae, solar flares, black holes, quasars, and cosmic rays interacting with the interstellar medium.

The Andromeda Galaxy

X-Ray Astronomy

Also high energy, X-rays cannot penetrate the Earth's atmosphere. The Chandra X-ray Observatory was launched in 1999. The illustration shows a Chandra image of Cas A, a remnant of a supernova that went off in the constellation Cassiopeia.

The Andromeda Galaxy

Ultraviolet Astronomy

Because ultraviolet radiation is blocked by the Earth's ozone layer, it too must be done from space. The International Ultraviolet Explorer orbited the Earth between 1978 and 1996, making it the longest space mission ever.

The Andromeda Galaxy

  1. Optical Astronomy

Visible light is able to penetrate the Earth'atmosphere, at least when the weather is clear. As a result, most telescopes are located on the planet Earth. Photograph of Kitt Peak National Observatory (NOAO/AURA/NSF). Although light is able to penetrate the Earth's atmosphere, the atmophere blurs out the images slightly. The Hubble Space Telescope orbits the Earth to eliminate this problem.

The Andromeda Galaxy

Infrared Astronomy

As we go to the long wavelength side of visible light,the atmosphere becomes opaque again. Infared radiation is blocked by water vapor. It is possible to make some observations in the near part of the infrared by observing from mountain tops, which can get above most of the water vapor. The Infrared Astronomical Satellite (IRAS) orbited the Earth in the 1980's. Most of the radiation shown in the image of the entire sky is dust that is heated up by starlight, which produces an infrared glow. NASA's newest infrared mission is the Spitzer Space Telescope.


Radio Astronomy

At the long wavelength, low frequency end of the electromagnetic spectrum are radio waves. Like visible light, radio waves are able to penetrate the Earth's atmosphere. Therefore, there are many radio telescopes on the Earth. Pictured is the Very Large Array (VLA) which consists of 27 large dishes spread out on the plains of New Mexico to collect radio waves.