A GRB, or gamma-ray burst, is the most powerful type of explosion in the known universe. In just seconds to minutes, a single gamma-ray burst can release more energy than our sun will produce over its entire 10-billion-year lifetime. These bursts are brief, intense flashes of gamma radiation (the highest-energy form of light) that originate from distant galaxies and were one of the biggest mysteries in astronomy for decades after their accidental discovery.
How GRBs Were Discovered
Gamma-ray bursts were first detected in 1967 by the Vela satellites, a series of U.S. military spacecraft designed to monitor for secret nuclear weapons tests in space. The satellites picked up flashes of gamma radiation that clearly weren’t coming from Earth or from nuclear detonations. The discovery remained classified until 1973, when it was published and opened up one of the longest-running puzzles in astrophysics. For nearly 30 years, scientists debated whether these bursts came from the edges of our own solar system, from within the Milky Way, or from billions of light-years away.
The mystery wasn’t solved until the late 1990s, when satellites capable of quickly pinpointing a burst’s location allowed ground-based telescopes to follow up and identify the host galaxies. The bursts turned out to be cosmological, meaning they originate in galaxies billions of light-years from Earth. That distance made their brightness almost incomprehensible, and the race to explain what could produce that much energy reshaped our understanding of how massive stars die.
The Two Types of GRBs
Astronomers divide gamma-ray bursts into two categories based on how long the flash lasts, and each type has a fundamentally different cause.
Long GRBs
Long gamma-ray bursts last more than two seconds, with some stretching to several minutes. They are produced when a very massive star, typically at least 20 to 30 times the mass of our sun, reaches the end of its life and its core collapses. But this isn’t an ordinary supernova. In a long GRB, the collapsing core forms a black hole, and the infalling material generates two narrow jets of matter and radiation that punch through the star at nearly the speed of light. If one of those jets happens to point toward Earth, we detect it as a gamma-ray burst. Long GRBs are found in galaxies with active star formation, which supports the link to young, massive stars that burn through their fuel quickly.
Short GRBs
Short gamma-ray bursts last less than two seconds, often just a fraction of a second. These are caused by the collision and merger of two neutron stars, or in some cases a neutron star and a black hole. Neutron stars are the ultra-dense remnants of earlier supernovae, and when two of them spiral into each other after orbiting for millions or billions of years, the final merger releases a brief, intense burst of gamma rays. This explanation was dramatically confirmed in 2017 when the LIGO and Virgo gravitational wave detectors picked up ripples in spacetime from a neutron star merger, and telescopes simultaneously detected the associated short GRB (known as GRB 170817A). That single event linked gravitational waves to gamma-ray bursts for the first time and also showed that neutron star mergers forge heavy elements like gold and platinum.
Why GRBs Are So Bright
The extreme brightness of gamma-ray bursts is partly an illusion of geometry. The energy isn’t radiating equally in all directions like a lightbulb. Instead, it’s concentrated into tight beams, or jets, that span only a few degrees. Think of it like the difference between a bare bulb lighting a room and a laser pointer shooting a narrow beam across a stadium. Because the energy is focused, any observer looking down the barrel of the jet sees an incredibly intense flash, while someone off to the side would see nothing at all. This means for every GRB we detect, many more are happening that we never see because their jets aren’t aimed at us. Estimates suggest we only detect a small fraction of all gamma-ray bursts that actually occur.
Even accounting for beaming, the total energy involved is staggering. A typical long GRB releases somewhere around 10^44 joules of energy in its jets alone. The material within the jet moves at 99.99% or more of the speed of light, creating shock waves that produce the gamma rays we observe.
What Happens After the Initial Burst
The gamma-ray flash itself is just the opening act. After the initial burst fades, an “afterglow” appears across longer wavelengths of light: X-rays first, then ultraviolet, visible light, and eventually radio waves. This afterglow is produced as the jet slams into the gas and dust surrounding the explosion, creating a shock front that gradually cools and fades over days, weeks, or even months. The afterglow is what allows astronomers to pinpoint the burst’s location, measure its distance, and study the environment where it occurred.
Some long GRBs are also accompanied by a supernova that becomes visible days after the burst. The supernova is the explosion of the outer layers of the star, while the GRB jet came from the collapsing core. These associated supernovae tend to be unusually energetic, sometimes called hypernovae, and are spectroscopically distinct from ordinary stellar explosions.
How Often GRBs Happen
Current satellites detect roughly one gamma-ray burst per day on average across the observable universe. NASA’s Swift Observatory (launched in 2004) and the Fermi Gamma-ray Space Telescope (launched in 2008) have been the primary workhorses for detecting and studying these events, collectively cataloging thousands of bursts. Because gamma rays are absorbed by Earth’s atmosphere, GRBs can only be detected from space.
Within any single galaxy the size of the Milky Way, a GRB is estimated to occur perhaps once every few hundred thousand to million years. The overwhelming majority of detected bursts come from galaxies billions of light-years away, which is good news for us.
Could a GRB Affect Earth
A gamma-ray burst occurring nearby and aimed at Earth would be catastrophic. The gamma radiation would strip away a significant portion of the ozone layer, exposing the surface to intense ultraviolet radiation from the sun for years afterward. This could trigger mass extinctions through damage to photosynthetic organisms at the base of the food chain and widespread DNA damage in exposed life. Some researchers have proposed that a relatively nearby GRB may have contributed to the Ordovician mass extinction roughly 450 million years ago, though this remains speculative.
The practical risk is extremely low. For a GRB to threaten Earth, it would need to occur within a few thousand light-years and have its jet pointed directly at us. The stars in our galactic neighborhood that are candidates for producing long GRBs are not currently aimed in our direction, and the rate of nearby events is vanishingly small on human timescales. It’s the kind of hazard that matters over hundreds of millions of years of evolutionary history but not something that factors into any realistic near-term threat assessment.
The Brightest GRB Ever Recorded
On October 9, 2022, satellites detected a gamma-ray burst so extraordinary it was quickly nicknamed the “BOAT” (Brightest Of All Time). Formally designated GRB 221009A, it originated about 2.4 billion light-years from Earth and was so intense it temporarily disrupted Earth’s ionosphere, the electrically charged upper layer of the atmosphere. Analysis suggested an event this bright occurs only once every 10,000 years or so. It was produced by the collapse of a massive star and sent a jet almost directly toward Earth, giving astronomers an unprecedented close look at the physics of these extreme explosions.