Gamma Ray (Encyclopedia of Science)
Gamma rays are high-energy subatomic particles formed either by the decay of radioactive elements or by nuclear reactions. The wavelength of a gamma ray is very shortess than the radius of an atomnd the energy they carry can measure millions of electron volts.
Terrestrial gamma rayshose produced on Earthre the only gamma rays we can observe here. A second class of gamma rays, called cosmic gamma rays, do not penetrate to the surface of Earth because the ozone layer absorbs high-energy radiation. The only way to detect cosmic gamma rays, which are created by nuclear fusion reactions that occur within the core of stars, is by sending a satellite-observatory into space.
Cosmic gamma rays were first discovered in 1967 by small satellites called Velas. These military satellites had been put into orbit to monitor nuclear weapon explosions on Earth, but they found gamma ray bursts from outside our solar system as well.
Several other small satellites launched in the early 1970s gave pictures of the whole gamma-ray sky. These pictures reveal hundreds of previously unknown stars and several possible black holes, the remains of massive stars. Thousands more stars were discovered in 1977 and 1979 by three large satellites called High Energy Astrophysical Observatories. They found that the entire Milky Way galaxy...
(The entire section is 840 words.)
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