1973

Gamma-Ray Bursts

Physicists studying the spontaneous decay of radioactive elements around the turn of the twentieth century recognized three kinds of particles or radiation that were emitted by Radioactive elements, such as uranium and radium. Some elements emitted helium nuclei (alpha particles) when they decayed; others emitted higher-energy electrons or positrons (beta particles); and when a third, even higher-energy form was discovered in still other radioactive decay events, physicists called those particles gamma rays. Like X-rays, gamma rays are a form of electromagnetic radiation of very high energy (very short wavelength). Later twentieth-century physicists discovered that X-rays are emitted by electrons surrounding an atom undergoing radioactive decay, and higher-energy gamma rays are emitted by the atomic nucleus itself.

Because gamma rays are generated in the nucleus, physicists predicted and observed the creation of gamma rays during nuclear fusion bomb tests. Indeed, the United States and the Soviet Union both deployed gamma-ray detectors in space in the 1960s to verify that the terms of the 1963 Partial Test Ban Treaty were being upheld.

The US satellites were part of the military’s Vela series, which could detect gamma rays coming from anywhere in space. Surprisingly, beginning in 1967, the satellites began to occasionally—a few times a year—detect mysterious short gamma-ray bursts (GRBs), lasting from milliseconds to several minutes, that were later determined to be coming from random directions in deep space. The military declassified the data and alerted civilian scientists to the phenomena in 1973.

Astrophysicists were puzzled by GRBs for decades, because the energies were far higher than gamma ray events produced by radioactive decay or stellar nuclear fusion reactions. The detailed nature of GRBs would remain mysterious until space telescope observations from NASA’s Compton Gamma Ray Observatory in the 1990s revealed their origin. GRBs appear to be created during supernova explosions when massive stars collapse, or during the mergers of pairs of colliding neutron stars. Like pulsars, GRBs occur when jets of highly focused energy stream out of the exploding star and are pointed toward Earth. Gamma-ray bursts appear to be the most violent and energetic events in the cosmos.

An artist’s impression of the gamma-ray burst event GRB 080319B, detected on March 19, 2008. The burst of energy is thought to have come from jets of gas accelerated to a speed of 99.9995 percent of the speed of light during the supernova collapse of a massive star 7.5 billion light-years away.