What we call "light" is actually just a tiny fraction of the broad range of radiation on the electromagnetic radiation spectrum. The entire span stretches from very-low-energy radio waves through microwaves, infrared light, visible light, ultraviolet light, X rays, and finally to very-high-energy gamma rays. The processes producing photons (single particles of electromagnetic radiation) of each type of radiation differ, as do their energy, but all of the different forms of radiation are still just part of the electromagnetic spectrum's family. The only real difference between a gamma-ray photon and a visible-light photon is the energy. Gamma rays can have over a billion times the energy of the type of light visible to our eyes. They are produced by the hottest and most energetic objects in the universe, such as neutron stars and pulsars, supernova explosions, and regions around black holes. On Earth, gamma rays are generated by nuclear explosions, lightning, and the less dramatic activity of radioactive decay.
In fact, gamma rays are so energetic that they are harmful to life on Earth. Luckily, Earth's atmosphere absorbs gamma rays, preventing them from affecting life on the ground. But this poses a problem if you want to observe the Universe in gamma-ray light. The very atmosphere that protects us from gamma rays prevents us from directly observing them from the ground. Astronomical observations of gamma-ray sources are therefore done with high-altitude balloons or satellites, above the protective blanket of Earth's atmosphere.
The high energy of gamma rays poses another problem: they can pass right through any lens or mirror, making it very difficult to focus them in a telescope. Astronomical observations, therefore, must rely on a different technology to view the gamma-ray universe. Scientists must make use of methods developed by particle physicists, who have long understood techniques for measuring high-energy particles. The Fermi Gamma-ray Space Telescope's specialized astronomical instruments employ detectors used and perfected by physicists interested in the interactions of subatomic particles.
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