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

This photo taken by the Hubble Space Telescope on January 23, 1999, shows the remains of the most powerful cosmic explosion recorded to date. (Reproduced by permission of the

Corbis Corporation [Bellevue]

.)

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.

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

This image is made up of data taken by the first four years of the Compton Gamma-Ray Observatory spacecraft's telescope. The bright horizontal band along the center is the gamma-ray emission from the Milky Way galaxy. (Reproduced by permission of the

Corbis Corporation [Bellevue]

.)

(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 ]