Caltech/Scott Kardel/Robert Quimby

Billions of years ago in a dwarf galaxy far, far away, a gigantic star exploded in a burst of light 10 billion times brighter than the sun. Four years ago, that light reached the eyes of Robert Quimby, an astronomer at Caltech. He called this distant stellar paroxysm 2005a; it was the brightest supernova ever seen. And soon he would find out this wasn't just an odd loner—it's part of an entirely new class of supernova, one that's shrouded in mystery.

Right around the time Quimby saw this brilliant remnant of a stellar explosion, the Hubble Space Telescope witnessed a similarly destructive event (called SCP 06F6), except that it occurred even farther away and longer ago, and remained more mysterious. Hubble's find was so bright that researchers weren't sure at first whether it was even a supernova. But Quimby saw a connection. After he lookedclosely at the stars' spectra—the chemical fingerprint of stellar light—something clicked. "I realized the wiggles in the spectra matched," Quimby says. These two supernovas, along with four more that his team discovered in the last couple years, represent a different kind of stellar explosion previously unknown to astronomy. "We have a whole new class of objects that can't be explained by any of the models we've seen before," he says.

In a new study in Nature today, Quimby describes the extraordinary find. They are extremely bright: 10 times more brilliant than a type 1A supernova, the brightest class previously known, and 100 times brighter than most supernovas. And their composition is peculiar, too: These new stellar detonations don't contain hydrogen, the universe's most abundant element and a key ingredient that astronomers see in many supernovas.

Quimby has two guesses about what's going on. The first possibility is that these supernovas are the death pangs of stars about 100 times the size of the sun, which is about as enormous as stars can get. But the problem with this scenario is accounting for that missing hydrogen. Typically, astronomers would expect light coming from the collapse of such an enormous star to bear traces of the ubiquitous gas, because current models would expect hydrogen to remain in significant amounts in its outer layers (and, by reacting with the star's other expelled material, to cause much of the supernova's brightness). Perhaps, then, the hydrogen from these supernovas was stripped away by the electromagnetic field of a neighboring binary star, or cast off into space by enormous wind storms or eruptions on the star. Astrophysicist Mario Livio at the Space Telescope Science Institute in Baltimore suspects the latter might be the case. "There is more and more evidence that very [large] stars lose mass via episodes of dramatic mass ejections," he says.

In the second scenario, a slightly smaller star —but still tens of times bigger than our sun—collapses and then explodes, but a fragment of the core remains and forms a magnetic neutron star, or magnetar, the most magnetic object in the universe. All neutron stars are incredibly dense, packing a mass greater than that of the sun into a space just about 10 miles across, though it is not it clear why only a small percentage of neutron stars (perhaps one in 10) form magnetars. Quimby says a magnetar's rapidly spinning magnetic field could drastically heat up the material ejected during a stellar explosion, causing the intense burst seen in this extraordinary class of supernovas.

As astronomers learn about this new kind of celestial pyrotechnics, these supernovas could also teach scientists a thing or two about their far-off homes. In another strange twist, all the supernovas in Quimby's new study come from dwarf galaxies, which contain about 10 times fewer stars than the Milky Way. Like lamplights along an empty highway, these supernovas could help us to see these little-known stellar neighborhoods by shining their intense light through the galaxies' gas clouds, revealing clues about what dwarf galaxies are made of.

Meanwhile, Quimby waits for more of these dazzling flashes from deep in the cosmos. He works at Caltech's Palomar Transient Factory, a project that looks for transients, or brief flashes of light in the night sky, many of which are from supernovas. So far, the group has confirmed more than 1000 overall, but just a precious few have belonged to this new and enigmatic group—a group that Columbia University astronomer Maryam Modjaz says could open up intriguing possibilities for research. "It is the first systematic work that presents the discovery of a class of exotic and overluminous explosions that are confounding both observers and theorists," Modjaz says.

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