A 'zombie star' which refuses to die has left astronomers stumped and questioning everything they know about supernovae.
The star, which is 500 million light years away, was discovered in 2014 as it exploded and resembled a basic supernova that was getting fainter over time.
But astronomers at the Las Cumbres Observatory in California were surprised to see it getting brighter a few months later.
The star named iPTF14hls, which also exploded in 1954, has brightened and dimmed five times since its latest explosion.
Usually, a supernova fades within 100 days but the 'zombie' star has been going strong for more than 1,000 days. It is currently fading slowly.
The baffling find was published in the Nature journal.
Astrophysicist Iair Arcavi, who led the study, called the discovery "very surprising" and "very exciting".
He said: "We thought we've seen everything there is to see in supernovae after seeing so many of them, but you always get surprised by the universe. This one just really blew away everything we thought we understood about them."
Astronomers say the phenomenon could be down to the explosions happening so frequently that they run into one another or that it is a result of a single explosion getting brighter and fainter.
Another theory is that the star, which was once 100 times larger than the sun, was so big and its core so hot that an explosion blew away its outer layers and left the centre intact to be able to repeat the process.
Avi Loeb, Harvard University's astronomy chairman, was not involved in the study but offered his own theory.
He said a black hole or magnetar – a neutron star with a strong magnetic field – might be behind the never-before-seen behaviour.
It is not known how unique the 'zombie' star is but no others have ever been observed.
Mr Arcavi said: "We could actually have missed plenty of them because it kind of masquerades as a normal supernova if you only look at it once.
"Eventually, this star will go out at some point – energy has to run out eventually."