Astronomy
The Cause of Gamma-Ray Bursts in Distant Galaxies
Quick fact
Long-duration gamma-ray bursts are likely produced when a star over 20 times the Sun's mass collapses directly into a black hole, while short bursts come from merging neutron stars—some of which emit gravitational waves.
Why this is interesting
Imagine an explosion so bright it outshines an entire galaxy for a few seconds, and it happens a billion light-years away. What could possibly cause that?
Read the full explanation
Understanding The Cause of Gamma-Ray Bursts in Distant Galaxies
Gamma-ray bursts (GRBs) are sudden, incredibly intense flashes of gamma-ray radiation. They are observed in distant galaxies because they are so powerful that, even from billions of light-years away, they appear as brief bright spots in gamma-ray telescopes. There are two main types: long bursts (lasting more than 2 seconds) and short bursts (lasting less than 2 seconds). Each type has a different cause. Long bursts are thought to happen when a very massive star, one more than 20 times the Sun's mass, runs out of fuel and collapses. The core collapses so fast that it forms a black hole, and incoming matter spirals into it, releasing colossal energy along a narrow beam. Short bursts, on the other hand, are caused by the collision of two ultra-dense neutron stars—the leftover cores of dead stars. When they orbit closely and finally merge, they create a black hole and fire off a burst of gamma rays.
A deeper explanation
The mechanism behind both long and short GRBs is the formation of a black hole and the launch of a relativistic jet—a stream of matter and radiation moving at almost the speed of light. In the 'collapsar' model for long bursts, the massive star's core collapses into a black hole, but material outside forms a swirling accretion disk. This disk becomes so hot and magnetically stressed that it drives two narrow jets out along the rotation axis. The jets bore through the outer layers of the star and explode into space, producing gamma rays. The jet only shines brightly if it is aimed at us, which is why we detect many GRBs as brief flashes. In short bursts, the neutron star merger also produces a disk and a jet, and it is a primary source of gravitational waves (as first observed in 2017). Understanding these events matters because they provide a way to study the extreme physics near a black hole and to find the birthplaces of heavy elements like gold. Because they are so bright, GRBs also serve as beacons that illuminate the distant early universe, helping astronomers probe conditions just a few hundred million years after the Big Bang.