A supernova brighter than any other ever observed, and perhaps brighter than current theories about the death of giant stars might allow, is still puzzling scientists months after its discovery.
First spotted by telescopes in June 2015, supernova Asassn-15lh was shining around 20 times brighter than the 100 billion stars making up all of the Milky Way galaxy, about as bright as 570 billion suns.
That's at least 200 times more powerful than an average supernova and more than twice as bright as any supernova previously observed, astronomers say.
"We have to ask, how is that even possible? It takes a lot of energy to shine that bright, and that energy has to come from somewhere," says astronomer Krzysztof Stanek with Ohio State University, a co-author of a study published in the journal Science.
Supernovas are the final act of a dying star, a massive explosion that rips the star apart and sends its material spinning into the surrounding cosmos.
Heavy elements created within the massive explosions become the seed material for new stars and planets.
Supernovas can sometimes outshine the entire galaxy they occur within and can last several weeks before they fade.
Asassn-15lh, however, has truly stolen the astronomy spotlight by being twice as bright as any supernova seen before and lasting for several months, astronomers say.
It is what scientists call a superluminous supernova, incredibly bright kinds of exploding stars that have only begun to be detected by telescopes in recent decades.
Some astronomers have suggested they are so bright because they are an explosion of a particular kind of star known as a magnetar, rapidly spinning neutron stars whose immensely-powerful magnetic fields can supercharge their luminosity when they go off.
Asassn-15lh really strains that explanation, astronomers say, and would require a "perfect storm" of events including an incredibly powerful magnetic field coupled with an unprecedented spin rate.
It would need the magnetar to be spinning at least a thousand times a second, something that would challenge the known laws of physics, they say.
If that's what created the supernova, "this record might hold for a while because it's kind of a really extreme case of what can happen if everything goes just right for producing a very powerful supernova," Stanek says.
The supernova, 3.8 billion light-years distant, is so bright that if it were as close to Earth as the star Sirius — 8.6 light-years away — it would shine almost as bright in our sky as the sun, astronomers say.