quick facts
What it is: Gamma-ray burst GRB 250702B
Where it is: 8 billion light-years away, in the constellation Scutum
When it was shared: Dec. 8, 2025
A gamma-ray burst (GRB) — the most energetic type of explosion in the universe since the Big Bang — is detected once every day, on average. But what happened on July 2, 2025, was highly unusual: NASA‘s Fermi Gamma-ray Space Telescope, which has been orbiting Earth since 2008, recorded an unusually long-lived GRB that continued emitting in bursts for more than seven hours.
The event, called GRB 250702B, was the longest-duration gamma-ray burst ever recorded. Astronomers now think it came from a previously unobserved or rare type of explosion that launched a narrow jet of material in the direction of the solar system, traveling at least 99% the speed of light.
GRB 250702B was not easy to figure out. Researchers used all kinds of telescopes to track its origin in all wavelengths of light, including the twin 8.1-meter Gemini telescopes in Chile and Hawaii, the Very Large Telescope in Chile, the Keck Observatory in Hawaii, and the Hubble Space Telescope.
GRBs come from the depths of the universe; even the closest one originated more than 100 million light-years away, according to NASA. GRB 250702B came from a massive galaxy 8 billion light-years distant that, critically, is so dusty that it blocked all visible light.
The only light detected by telescopes was infrared and high-energy X-ray wavelengths. Due to thick dust in its host galaxy, the GRB was almost invisible in ordinary visible light, the researchers reported in a study published Nov. 26 in The Astrophysical Journal Letters.
“This was the longest gamma-ray burst that humans have observed — long enough that it does not fit into any of our existing models for what causes gamma-ray bursts,” Jonathan Carney, lead author of the study and doctoral student in physics and astronomy at the University of North Carolina at Chapel Hill, said in a statement.
Analysis shows that GRB 250702B may have been caused by the death of a massive star, a star being ripped apart by a black hole, or the merger of a helium star and a black hole, where the black hole spirals into the core of the massive star, triggering an explosion from within.
“But we can’t yet tell which explanation is correct,” Carney said. “In the future, this event will serve as a unique benchmark — when astronomers discover similar explosions, they’ll ask whether they match GRB 250702B’s properties or represent something different entirely.”
For more sublime space images, check out our Space Photo of the Week archives.