Astronomers have discovered two mystery supernovae that are the brightest and most distant ever found.
The supernovae are 10 billion light years away and are 100 times brighter than an average supernova.
Published in the Astrophysical Journal, the researchers say the supernovae are particularly puzzling because the mechanism that powers them – the collapse of a giant star into a black hole or neutron star – does not explain how they are so bright.
The supernovae were initially discovered in 2006 and 2007, but they were so abnormal astronomers could not even work out what they were or their distance from Earth.
Lead author D Andrew Howell, from Las Cumbres Observatory Global Telescope Network and University California Santa Barbara, said: "At first, we had no idea what these things were, even whether they were supernovae or whether they were in our galaxy or a distant one.
"I showed the observations at a conference, and everyone was baffled. Nobody guessed they were distant supernovae because it would have made the energies mind-bogglingly large. We thought it was impossible."
The more distant supernova, SNLS-06D4eu, is now believed to be the most luminous supernova ever, belonging to a new class of cosmic events called superluminous supernovae.
Researchers believe the supernovas are powered by a magnetar, a highly magnetised neutron star that is spinning hundreds of times per second. Magnetars have the mass of the sun packed into a star the size of a city. Their magnetic field is about 100 trillion times that of the Earth.
Superluminous supernovae were announced in 2009 and since then, the astronomers have worked to show the two mystery supernovas fitted into this category.
Study co-author Daniel Kasen, from the University of California Berkeley and Lawrence Berkeley National Lab, said the star was probably much bigger to start but shed its outer layers, leaving a small naked core.
"What may have made this star special was an extremely rapid rotation," he said. "When it ultimately died, the collapsing core could have spun up a magnetar like a giant top. That enormous spin energy would then be unleashed in a magnetic fury."
The authors believe the supernovae exploded when the universe was around four billion years old, before the Sun even existed.
Howell said: "There was another star here that died and whose gas cloud formed the sun and Earth. Life evolved, the dinosaurs evolved and humans evolved and invented telescopes, which we were lucky to be pointing in the right place when the photons hit Earth after their 10-billion-year journey.
"These are the dinosaurs of supernovae. They are all but extinct today, but they were more common in the early universe. Luckily we can use our telescopes to look back in time and study their fossil light. We hope to find many more of these kinds of supernovae with ongoing and future surveys."