A rare long-lasting gamma ray flash has been sighted after a Sun-like star was shredded by a monster black hole.
Gamma ray bursts of energy typically flare up and end in seconds or even milliseconds, often when a collapsing star is dying.
Scientists predicted that the ongoing flash, which has already lasted more than two months, would not be seen again for a million years.
"This is truly different from any explosive event we have seen before," said Joshua Bloom of the University of California-Berkeley, a co-author of research on the blast published in the journal Science.
NASA's Swift spacecraft, which is trolling the universe for gamma ray bursts, initially spotted the flash at the end of March, Bloom said.
Scientists are puzzled as the black hole, located in the constellation Draco (The Dragon) about 4 billion light years from Earth, at first appeared inactive when a star about the mass of our Sun moved into range.
"We have this otherwise dormant black hole, not gobbling up an appreciable amount of mass, and along comes this star which just happens to be on some orbit which puts it close to the black hole," Bloom said.
"This was a black hole which was otherwise quiescent and it sort of has an impulsive feeding frenzy on this one star," he said, adding that the strange occurrence could happen perhaps once per black hole per million years.
Active black holes, thought to be dotted around most galaxies including our own Milky Way, normally suck in everything their vast gravity can pull in, including light.
Black holes are invisible, but astronomers can infer their existence because the material they pull in lights up before it gets sucked in.
However the black hole in this case tore apart the star, about the same size as our Sun, when it pulled it in, emitting powerful gamma ray jets from its centre as parts of the dying star were turned into energy.
The black hole's gravitational pull was so great that it exerted what's called a tidal disruption on the passing star.
Astronomers could use this observation to help them learn more about how black holes grow, Bloom said.
"We still don't understand how black holes and the universe grow," he said.
"We think most black holes start off as being no more than the mass of our Sun.
"How they go from 10 solar masses to a billion solar masses is critical."