gamma-ray burst
/ GRB /
For a few seconds, somewhere in the sky, a flash of gamma rays appears from nowhere, briefly outshining every other gamma-ray source in the universe combined — and then it is gone. These are gamma-ray bursts, the single most luminous explosions known. They were discovered by accident in the late 1960s by satellites watching for secret nuclear tests; the satellites saw flashes that were not from Earth, but from deep space.
A gamma-ray burst lasts anywhere from a fraction of a second to a few minutes, and in that brief time it can release more energy than the Sun will emit in its entire ten-billion-year life. The trick is that the energy is not blasted out evenly in all directions — it is focused into narrow jets moving at almost the speed of light, so we only see a burst when one of its jets happens to point at Earth. That beaming makes them look far more luminous than they truly are, and means many bursts go unseen because their jets miss us. They come in two broad families distinguished by how long they last and by their physical cause.
Gamma-ray bursts matter as cosmic beacons and as messengers. Because they are so bright, we detect them from across most of the observable universe, making them probes of the early cosmos and of star death in distant galaxies. The two families turned out to come from two of the most dramatic events in nature — the collapse of a massive star, and the merger of two neutron stars — and the short kind, tied to neutron-star mergers, became a cornerstone of multi-messenger astronomy when one was seen alongside gravitational waves in 2017.
A typical gamma-ray burst pops up unannounced; within seconds a space telescope alerts astronomers worldwide, who swing ground telescopes onto the spot to catch the fading afterglow. From those follow-ups we learn the burst lay in a galaxy billions of light-years away, meaning that brief flash crossed most of the visible universe to reach us.
A seconds-long flash of gamma rays, briefly the brightest thing in the universe, from a beamed jet.
A burst's apparent brightness is exaggerated by beaming: because the energy is funneled into a narrow jet, the true energy released is far less than if it shone equally in all directions, and most bursts whose jets miss Earth we never see at all.