Super-Duper Supernovas in a Class All By Themselves

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We are finding that the universe is such an extraordinarily energetic place that it leaves us at a loss for superlatives.

Take the latest discovery announced on Wednesday in the journal Nature: Astronomers have uncovered a new class of objects they are calling “ultra-luminous supernovas.”

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But hold on. Supernovae — exploding massive stars — are the biggest bangs since the Big Bang. Their light flashes across billions of light-years like cosmic flashbulbs: it’s brief and intense.

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A supernova sky survey called the Palomar Transient Factory, has uncovered four of these super-duper explosions that are ten times brighter than normal supernovas, and unusually bluer. Using stellar forensics, a team of astronomers also reclassified two previously mysterious flashes in the night as belonging to the same new class of brighter-than-bright supernovas.

“Our survey is finding a new ultra-luminous supernova every two weeks,” Shri Kulkarni of Caltech told Discovery News. Statistically, that means that one in every 10,000 normal supernova is, well, a super-supernova.

How does nature make an explosion that seems far more powerful than conventional supernovas that commonly arise from the implosion of a iron core inside a massive star?

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Kulkarni says that nobody’s quite sure, but clues come from the explosions’ spectral fingerprints, captured by follow-up observations with the Keck, Hale, and William Herschel telescopes. The new supernovas are unusually blue, peaking at ultraviolet wavelengths. Even more puzzling, they take 50 days to fade away. That’s much longer than the decay rate for a typical supernova.

Kulkarni says that the extra brightness is, in part, powered by huge quantities of nickel in the explosion. And this points to the progenitor star weighing in at over 100 times the mass of our sun. It manufactures enormous quantities of nickel through nucleosynthesis.

But a complete explanation for the super-brightness remains elusive. One clue may come from simply looking into the night sky. This time of year the bright red star Antares can easily be found low on the southern horizon. The star is cool but bright because it is a swollen red giant. Giants are many times our sun’s diameter and have a large glowing surface area.

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Likewise, Kulkarni suggests the super-supernova’s progenitor star was embedded inside a huge bloated shell of hydrogen 10,000 times the star’s diameter. The shell was furiously ejected into space before the explosion.

Such a petulant outburst was seen in the southern star Eta Carina (pictured below), which briefly flared to become the second brightest star in the night sky in the mid 1800s.

According to this model the massive helium stellar core gets so hot that powerful gamma rays create “dead” pairs of electrons and positrons, which bled off the pressure. Like a deflating hot air balloon, the core “just caves in,” says Kulkarni.

The heat of that implosion triggers a titanic explosion. Expanding shock waves furiously ripple through successive shells of gas out to the huge hydrogen envelope. The shock heating of this vast envelope greatly boosts the supernova’s luminosity.

A suspected pair-instability supernova was first seen by the Palomar Transient Factory in 2007. The star’s spectrum was oddball and one of a kind. But team member Robert Quimby, in a classic case of supernova forensics, found that he could match the strange spectral signature to the newly discovered supernovas. This was after he factored in how the spectra would be stretched by the effect of the expansion of the Universe on the light of the farther away, newly discovered supernovas.

What also fits into this sequence is the bizarre light signature of one of the strangest phenomenon ever seen by the Hubble Space Telescope. In 2006, a transient specter of light, called SCP 06F6, was spotted while Hubble was pointed at the distant Bootes galaxy cluster. It increased its brightness by a factor of 120 over 100-days, and then faded away over the same period. A solution has eluded scientists until now.

If the pair-instability model is correct, then even more of these ultra-luminous supernovas should be seen the farther back in time we look.

The earliest stars in the universe were hundreds of times the mass of our sun. That’s because the newborn universe was deficient in heavier elements (they hadn’t been cooked up in star factories yet) and so stars could grow to gargantuan masses before cooling off.

This means the next generation of large space and ground based telescopes should seen more of these ultra-luminous supernovas popping off at increasingly farther distances.

This presents a staggering image for the imagination: an early universe ablaze with super-supernovas popping off like an arena full of flashbulbs at the Super Bowl.

Image credits: Caltech, NASA, iStock