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gamma-ray astronomy

gamma-ray astronomy, study of astronomical objects by analysis of the most energetic electromagnetic radiation they emit. Gamma rays are shorter in wavelength and hence more energetic than X rays (see gamma radiation) but much harder to detect and to pinpoint. X rays and some gamma rays are produced throughout the universe by the same catastrophic astrophysical events, such as supernovas and black holes, and gamma-ray astronomy can be considered an extension of X-ray astronomy to the extreme shortwave end of the spectrum.

Gamma rays are difficult to observe from ground-based telescopes due to atmospheric interference, and high-altitude balloons, sounding rockets, and orbiting observatories are therefore used. Some ground-based facilities, including a large 33-ft (10-m) dish with many small mirrors at Mount Hopkins, Ariz., are successful gamma-ray collectors because they record the radiation emitted by very-high-energy gamma rays as they generate high-speed electrons in the upper atmosphere. Another approach to detecting this radiation is the Milagro detector in the Jemez Mountains of New Mexico. It consists of hundreds of phototubes floating within a pond containing 6 million gallons of water; through interactions with the water, the radiation generates weak trails of light that are detected by the phototubes, yielding data about the energy and direction of the gamma rays.

Cygnus X-3 and the Crab and Vela pulsars are well known gamma-ray sources. In addition, gamma rays have been detected as general background radiation concentrated along the plane of the Milky Way. These gamma rays may result from cosmic rays interacting with gaseous matter in the interstellar medium. Gamma rays from outside the Milky Way have been found emanating from radio galaxies (galaxies whose radio emissions constitute an extraordinarily large amount of their total energy output), Seyfert galaxies (galaxies with extremely bright cores—called Active Galactic Nuclei [AGN]—that are strong emitters of radio waves, X rays, and gamma rays), and supernovas.

The first gamma-ray telescope was carried into orbit on the Explorer XI satellite in 1961. Additional gamma-ray experiments flew on the OGO, Vela, and Russian Cosmos series of satellites. The Orbiting Solar Observatory OSO-3 made the first certain detection of celestial gamma rays in 1972, and OSO-7 detected gamma-ray emission lines in the solar spectrum. However, the first satellite designed as a "dedicated" gamma-ray mission was the second Small Astronomy Satellite (SAS-2) in 1972. In 1975 the European Space Agency launched the COS-B satellite to survey the sky for gamma-ray sources. SAS-2 and COS-B confirmed the earlier findings of gamma-ray background radiation and also detected a number of point sources, but the poor resolution of the instruments made it impossible to associate most of these point sources with individual stars or stellar systems. The third High Energy Astronomy Observatory (HEAO-3), launched in 1979, studied both cosmic rays and gamma radiation. A number of satellites launched during the 1980s carried gamma-ray experiments into orbit. The Compton Gamma-Ray Observatory (CGRO), launched in 1991, carried a collection of four instruments that were larger and more sensitive than any gamma-ray telescope previously orbited. In addition to creating a comprehensive map of celestial gamma-ray sources and demonstrating that gamma-ray bursts are evenly distributed across the sky (which suggests that the radiation is coming from the distant reaches of the universe and not just from within the Milky Way), CGRO detected a number of "firsts," such as the first gamma-ray quasar. During the 1990s a number of planetary probes, such as Mars Observer (1983), and earth-orbiting satellites, such as Minisat 1 (1997), carried gamma-ray detection and measurement devices as part of their instrumentation.

The turn of the century saw designs for gamma-ray astronomy satellites that allow for imaging resolution and spectral resolution powers never before possible. Launchings of orbiting gamma-ray observatories include missions such as the High Energy Transient Explorer (HETE-2), launched in 2000, the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, the Swift Gamma Ray Burst Explorer, launched in 2004, and the Fermi Gamma-Ray Space Telescope, launched in 2008. Swift detected (2009) an extremely distant gamma-ray burst (more than 13 billion light-years from Earth) that may be associated with the supernova of a blue giant star of the early universe, and Fermi has discovered hundreds of gamma-ray sources.

In 1967 a Vela military satellite designed to detect nuclear explosions discovered the first gamma-ray bursts (GRBs). These events are very short-lived, lasting from about 50 milliseconds to, in extreme cases, several minutes, and occur on an almost daily basis. It has been suggested that the formation of black holes is associated with these intense gamma-ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting—from a few days to several years—electromagnetic radiation (called the afterglow) in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance.

See G. E. Morfill, ed., Galactic Astrophysics and Gamma-Ray Astronomy (1983); P. Murthy and A. Wolfendale, Gamma-Ray Astronomy (1993); N. Gehrels, Gamma Ray Astronomy (1995); T. Weekes, Very High Energy Gamma Ray Astronomy (2003).

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Gamma-ray Burst

Gamma-ray burst

Gamma-ray bursts are unexplained intense flashes of light that occur several times a day in distant galaxies. The bursts give off more light than anything else in the universe and then quickly fade away. They were first detected in the late 1960s when instruments on orbiting satellites picked them up. No known explosion besides the big bang is more powerful than a gamma-ray burst. (The big bang is a theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago.) Gamma-ray bursts are mysterious because scientists do not know for sure what causes them or where in the sky they will occur.

Types of bursts

There are two types of gamma-ray bursts: short and long. Short bursts last no more than two seconds. Long bursts can last up to just over fifteen minutes. The shorter life of a short gamma-ray burst makes it more difficult for astronomers to study. Short bursts leave no trace of light because they have no detectable afterglow (a gleam of light that remains briefly after the original light has dissipated). In addition, weaker gamma-ray

bursts tend to be observed as shorter, since only the higher parts of the emission are observable.

Astronomers believe that each type of burst may come from a different type of cosmic explosion. To learn more about the sources of gamma-ray bursts, scientists at the National Aeronautics and Space Administration (NASA) studied the time histories of short and long bursts. They did this by counting the number of gamma-ray pulses (particles of light, called photons, that arrive at about the same time) in each burst, and by measuring the arrival time of lower-energy and high-energy pulses. The astronomers learned that short bursts had fewer pulses than long bursts and that their lag times were twenty times shorter than those of longer bursts. This suggested that both long and short bursts were produced in physically different objects.

Words to Know

Big bang theory: Theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago.

Black hole: Single point of infinite mass and gravity formed when a massive star burns out its nuclear fuel and collapses under its own gravitational force.

Gamma ray: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.

Neutron star: The dead remains of a massive star following a supernova. It is composed of an extremely dense, compact, rapidly rotating core composed of neutrons that emits varying radio waves at precise intervals.

Supernova: The explosion of a massive star at the end of its lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.

Theories of gamma-ray burst origin

Scientists' theories about the source of gamma-ray bursts are many. Some believe that they are a result of a fusion of black holes or neutron stars. (A black hole is the remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity. A neutron star is a dead remnant of a massive star; a star dies when it uses up all of its nuclear fuel.) Others believe that supernovae or hypernovae are the cause of a gamma-ray burst. (A supernova is a typical exploding star; a hypernova also is an exploding star, but with about 100 times more power as that of supernova.) In 2000, two sets of astronomers found evidence of an iron-rich cloud near gamma-ray bursts. Since stars at the supernova stage produce iron, the scientists theorized that a supernova emitted the iron cloud just before the gamma-ray burst.

The power of a gamma-ray burst is astounding. A satellite launched by NASA in 1991 detected gamma-ray bursts at a rate of nearly one a day for almost two years. The energy of just one burst was calculated to be more than 1,000 times the energy that the Sun would generate in its almost 10-billion-year lifetime.

The most distant gamma-ray burst measured so far is one that scientists detected on January 31, 2000. To determine how far the gamma-ray burst had traveled, astronomers measured the burst's spectrum (the range of individual wavelengths of radiation produced when light is broken down by the process of spectroscopy). Astronomers estimated that the explosion that caused the burst took place near the time the Milky Way

(a galaxy that includes a few hundred billion stars, the Sun, and our solar system) was formed, or 6 billion years before our solar system was born. Viewed another way, this particular gamma-ray burst has traveled through 90 percent of the age of the universe.

Scientists study gamma-ray bursts as a way of helping to better understand the evolution of the universe.

[See also Gamma ray; Star ]

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