U.S. astronomers say they've discovered two supernovas 10 billion light-years from Earth that are 100 times more luminous than a normal supernova.

Scientists at the University of California, Santa Barbara, say the newly observed objects are puzzling because the mechanism that powers most supernovae -- the collapse of a giant star to a black hole or normal neutron star -- cannot explain their extreme luminosity.

The two unexpected supernovae were so unusual astronomers said initially they could not figure out what they were or even determine their distances from Earth.

"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," UCSB researcher and study lead author D. Andrew Howell said.

"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," he said.

Further study has suggested such supernovae are likely powered by the creation of a magnetar, an extraordinarily magnetized neutron star spinning hundreds of times per second, the researches said.

"What may have made this star special was an extremely rapid rotation," study co-author Daniel Kasen of UC Berkeley 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."

Such superluminous supernovae are rare, the scientists said, occurring perhaps once for every 10,000 normal supernovae.

"These are the dinosaurs of supernovae," Howell said. "They are all but extinct today, but they were more common in the early universe."