Gamma-ray astronomy opens another window through which astronomers can observe the universe. Gamma rays are the most energetic
form of electromagnetic radiation, but they have the same properties as light and can be used to study distant
objects. Produced by nuclear processes such as fission and fusion reactions, radioactive decay (see radioactivity), and collisions of high-energy
particles (see fundamental particles), they provide a means for studying the most energetic processes taking place in the universe, such as material falling into black holes and the creation and destruction of elements within and between the stars.DevelopmentIn 1972 the first certain detection of celestial gamma rays was made with equipment aboard the U.S. OSO-3 satellite (see OSO). In that same year another satellite, OSO-7, was used to detect gamma-ray emission lines in the spectrum of the Sun. Thereafter, knowledge of the gamma-ray universe grew rapidly by means of such satellites as the U.S. SAS-2 (1972Ð73) and the European COS-B (1975Ð82). The instruments on the Compton Gamma Ray Observatory, launched in 1991 from the Space Shuttle, have yielded many exciting discoveries. They have surveyed our Milky Way galaxy (see Galaxy, The) and observed a number of discrete sources of gamma rays, such as pulsars. They have also observed gamma rays emitted from collisions of energetic particles called cosmic rays with the interstellar matter in our galaxy. Using gamma rays with energies between 100 MeV (million _electron volts) and several GeV (billion electron volts), the Compton observatory has mapped out the gamma-ray sky.In addition to galactic sources of gamma rays such as the Vela and Crab pulsars, a number of intense sources of high-energy gamma rays have been detected by the Compton instruments in the extragalactic objects known as active galactic nuclei (AGN)Ñthe most energetic objects known in the universe. (An AGN, which emits enormous quantities of energy in the form of gamma rays, is believed to be powered by matter falling into a massive black hole in the center of the galaxy's nucleus; see extragalactic systems.) At least two of these objects, Markarian (Mrk) 421 and Mrk 501, have been detected by telescopes, such as the Whipple Telescope in Arizona, that are sensitive to gamma rays with energies near 1 TeV (trillion electron volts). In 1988 the first such 1-TeV source, the Crab nebula, was detected. In May 1996, Mrk 421 was twice observed to brighten by more than a factor of ten for several hours. Similarly, in 1997, Mrk 501 showed several short-term outbursts during which it became the brightest TeV object in the sky. This short-term behavior is not yet understood. Observation by ground-based detectors of TeV gamma rays from AGNs has opened the possibility of using the measured absorption of rays from these very distant objects to probe radiation from the very early universe.Unexplained gamma-ray bursts (GRBs) lasting from fractions of a second to more than an hour have also been observed at energies of several thousand electron volts (KeV) to 20 GeV. One of the Compton instruments detects about one such burst each day. It had been expected that the GRBs would have their origin in the plane of our galaxy, but results have shown that they are equally likely to come from any direction in the sky. Scientists have been puzzled over the origin of these GRBs and have produced a wide range of theories about their source. Some suggest that the source is possibly in the region of the Oort cloud of cometary material (see comet) or else in the halo of our galaxy. Others suggest that the bursts are very distant, on a cosmological scale. On Feb. 28, 1997, a Dutch-Italian satellite called BeppoSAX detected a GRB. Using an X-ray telescope also onboard, it examined the burst's location in the sky and saw a bright, rapidly fading object. Within eight hours, astronomers in the Canary Islands looked at this location and saw a point source of light fading in tensity. The source appeared to be contained in a distant galaxy. This suggests that the burst may in fact be at a cosmological distance and implies that such bursts must give off enormous quantities of energy. Toward the end of 1997 the BeppoSAX satellite also detected the most powerful GRB yet known, a tremendous cosmic blast that for 40 seconds outshone the rest of the universe. What caused such an explosion is not known, but speculations include the possibility that a black hole had collided with a neutron star.InstrumentsDevices for detecting gamma rays include assemblies of scintillators of various types (see scintillation counter). More complicated techniques use large area detectors that, in effect, supply pictures of the tracks produced by the charged particles produced by high-energy photons. These spark chamber telescopes make it possible to distinguish gamma-ray events and determine their direction of arrival. Higher-energy rays can produce detectable flashes of light in the atmosphere, through the Cherenkov effect of bombarded particles being caused to move at speeds faster than that at which light moves through air (see Cherenkov radiation).