Gamma-Ray Astronomy in the Compton Era: Introduction
Looking into the night sky, one sometimes feels that the visible Universe is a calm, comfortable place. NASA's Compton Gamma Ray Observatory looks at a very different cosmos. This is the Universe viewed through gamma-ray photons, the highest energy portion of the electromagnetic spectrum. This is a place of explosive energy, cosmic particle accelerators, and exotic environments such as collapsed stars and mysterious bursters. A place of nuclear decays, particle collisions, extraordinary temperatures, and violent explosions. For the first time, the entire sky has been comprehensively studied using this high-energy radiation. From the Sun to the furthest galaxies - from diffuse clouds of interstellar gas to the cores of powerful, gigantic black holes - the Compton Observatory has reshaped the way scientists view nature. Each gamma-ray photon (which is the basic "piece" of light, much like electrons are one of the basic pieces of normal matter), carries at least 10,000 times the energy of a common photon of visible light. This energy is usually measured in electron- volts (or eV). A typical optical photon has around 2 or 3 eV's of energy. The gamma-ray spectrum begins at energies of around 50,000 eV (or 50 keV) and extends up to 1 TeV (1,000,000,000,000 eV) or even higher! 

Arthur Compton

The window provided by gamma-ray astronomy gives researchers valuable information about a wide variety of astrophysical phenomena, information which is impossible to get by other means. The high-energy part of the electromagnetic spectrum reveals information about nuclear interactions and decays, about how elementary particles are created and accelerated in astrophysical sources, and clues to the origin of cosmic rays.

Gamma rays are not easily scattered or destroyed. So once created, gamma rays can provide unambiguous information about very important astrophysical environments. Over the years, astrophysicists have realized that in order to fully understand nature, one must observe in as many ways as possible to provide a unified view. With the launch of the Compton mission, gamma-ray astronomy has, for the first time, become an integral part of this multi-wavelength approach. This success did not come quickly or easily. Gamma-ray astronomy is a very difficult discipline to pursue for several reasons. First, it cannot be accomplished from the ground like radio or optical astronomy. The earth's atmosphere is opaque at gamma-ray energies. Cosmic gamma rays are absorbed high up in the atmosphere. Only at the highest energies are gamma rays detectable from the ground, through the large showers of particles caused by interaction with the atmosphere. Also, gamma rays are scarce. Even the brightest sources emit relatively few photons at these energies as compared to other energy bands. The Crab pulsar, a bright source, emits about one gamma-ray photon for every 10,000 optical photons. 
Gamma rays occupy the highest energy portion of the electromagnetic
spectrum. TOP: Arthur Holly Compton, the Nobel Prize winning American physicist  for whom the Gamma Ray Observatory is named. 

LEFT: Gamma rays occupy the highest energy portion of the electromagnetic spectrum. Only at the highest energies do remnants of the cosmic gamma rays survive to ground level.

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