Gamma-ray burst (GRB) is an extremely energetic series of flashes of light observed to originate in a distant galaxy. A GRB consists mostly of X rays and gamma rays. A gamma ray is a form of light with even more energy than an X ray. Space-based observatories detect about one GRB per day, but many more go undetected. These bursts originate in all directions of the sky with an apparently random distribution. Astronomers have detected the light from GRB’s that took place in some of the first galaxies in the universe. This light traveled for over 13 billion years before reaching Earth.
Astronomers classify GRB’s as either short or long. Short bursts last less than 2 seconds. They may last for as little as a few milliseconds (thousandths of a second) but have an average duration of about 0.3 second. Long bursts last for an average of about 30 seconds but may continue for several minutes. Long bursts usually give off more total energy than do short bursts. No two GRB’s are identical—each burst gives off a unique series of flashes.
Scientists think that long GRB’s are produced by collapsing stars. When a star with at least 8 to 11 times the mass (amount of matter) of the sun reaches the end of its life, its core collapses, causing the star to explode in an event called a core-collapse supernova. Particularly massive stars can collapse to form a black hole, a region of space whose gravitational pull is so strong that nothing can escape it. Some collapsing cores fire jets of high-speed particles that penetrate the outer regions of the star. Collisions between particles within the jets can produce gamma rays. The jets of particles later collide with gas and dust outside the star. These collisions generate an afterglow in the form of X rays, visible light, and radio waves.
A star not massive enough to form a black hole can collapse into a neutron star, a small, dense star made mostly of neutrons. Some neutron stars called magnetars have extremely strong magnetic fields. A magnetic field is the influence that a magnetic object creates in the region around it. Scientists think that in some cases disturbances called starquakes can crack the magnetar’s crust, producing severe disruptions in the star’s magnetic field. The rapidly changing magnetic field can generate powerful bursts of X rays and gamma rays.
Scientists believe that short GRB’s occur when the compact remnants of two stars collide and merge. These collisions may involve two neutron stars or a neutron star and a black hole. The objects merge to form a single black hole, giving off large amounts of energy in the form of bright flares of gamma rays.
United States military satellites detected the first GRB’s in the late 1960’s. These satellites were designed to watch for gamma rays released during nuclear weapons tests. The discovery of GRB’s remained unannounced while scientists on the project studied the satellite data. In 1973, the American scientists Ray Klebesadel, Ian Strong, and Roy Olson reported the discovery.
See also Astronomy (Gamma-ray astronomy) ; Fermi Gamma-ray Space Telescope ; Gamma rays ; Supernova ; Telescope (Gamma-ray telescopes) .
