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Superlong Supernova Explosion Defies Scientists' Understanding of Stars

Started Nov-11 by BlearyIrish; 16 views.
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From: BlearyIrish

Nov-11

The appearance of a years-long supernova explosion challenges scientist's current understanding of star formation and death, and work is underway to explain the bizarre phenomenon.

Stars more than eight times the mass of the sun end their lives in fantastic explosions called supernovas. These are among the most energetic phenomena in the universe. The brightness of a single dying star can briefly rival that of an entire galaxy. Supernovas that form from supermassive stars typically rise quickly to a peak brightness and then fade over the course of around 100 days as the shock wave loses energy.

In contrast, the newly analyzed supernova iPTF14hls grew dimmer and brighter over the span of more than two years, according to a statement by Las Cumbres Observatory in Goleta, California, which tracked the object. Details of the discovery appeared on Nov. 8 in the journal Nature. [First Supernova Shock Wave Image Snapped by Planet-Hunting Telescope]

An inconspicuous discovery

Supernova iPTF14hls was unremarkable when first detected by a partner telescope in San Diego on Sept. 22, 2014. The light spectrum was a textbook example of a Type II-P supernova, the most common type astronomers see, lead author Iair Arcavi, an astronomer at the University of California, Santa Barbara, told Space.com. And the supernova looked like it was already fading, he said.

The observatory was in the middle of a 7.5-year collaborative survey, so Arcavi focused on more-promising objects. But in February, 2015, Zheng Chuen Wong, a student working for Arcavi that winter, noticed the object had become brighter over the past five months.

"He showed me the data," Arcavi said, "and he [asked], 'Is this normal?' and I said, 'Absolutely not. That is very strange. Supernovae don't do that,'" Arcavi said.

At first, Arcavi thought it might be a local star in our galaxy, which would appear brighter because it was closer, he said. Many stars are also known to have variable brightness. But the light signature revealed that the object was indeed located in a small, irregular galaxy about 500 million light-years from Earth.

And the object only got weirder. After 100 days, the supernova looked just 30 days old. Two years later, the supernova's spectrum still looked the way it would if the explosion were only 60 days old. The supernova recently emerged from behind Earth's sun, and Arcavi said it's still bright, after roughly three years. But at one one-hundredth of its peak brightness, the object appears to finally be fading out.

"Just to be clear, though, there is no existing model or theory that explains all of the observations we have," said Arcavi. The supernova may fade out; it may grow brighter, or it may suddenly disappear.

One reason for Arcavi's uncertainty is that a supernova was seen in the same location in 1954. This means that the event Acavi has been observing, whatever it is, may actually be 60 years running. There's a 1 to 5 percent chance the two events are unrelated, but that would be even more surprising, said Arcavi. Astronomers have never observed unrelated supernova in the same place decades apart. "We are beyond the cutting-edge of models," Arcavi said.

Supernova iPTF14hls dwarfs typical supernovas in both brightness and longevity. And the event's dramatic fluctuations pose an exciting challenge for the astronomical community to explain.
Credit: Credit: S. Wilkinson/LCO

Beyond cutting edge

"I'm not sure, and I don't think anyone else is sure, just what the hell is happening," astrophysicist Stanford Woosley, at University of California, Santa Cruz, told Space.com. "And yet it happened, and so it begs explanation."

Woosley is not affiliated with the study, but he is among the theoreticians working to understand the event. Two hypotheses show promise in explaining it, he said.

The first involves the famous equation E = mc2. With this formula , Albert Einstein demonstrated that matter and energy are fundamentally interchangeable. Stars burn by converting matter into energy, fusing lighter elements like hydrogen and helium into heavier elements, which build up in the star's core and also release energy. When a star more than 80 times the mass of the sun reaches a temperature of 1 billion degrees Celsius (1.8 billion degrees Fahrenheit), this energy-matter equivalence produces pairs of electrons and their antiparticle counterparts, positrons, Woosley said. The process robs the star of energy, and so the object shrinks.

But as this happens, the temperature rises in the star's core. At 3 billion C (5.4 billion F), oxygen fuses explosively, blowing off massive amounts of material and resetting the cycle. This process repeats until the star reaches a stable mass, explained Woosley. When the front of an ejected shell of material hits the trailing edge of a previous shell, it releases energy as light.

The star continues to fuse oxygen and the elements of greater masses, up until iron, at which point the reaction fails to release enough energy to keep the star from collapsing in on itself.Eventually, a star like the one that gave rise to iPTF14hls will collapse into a black hole without another explosion, said Woosley.

This image depicts a simulated collision between two shells of matter ejected by subsequent pulsation pair instability supernova explosions.
Credit: Ke-Jung Chen/School of Physics and Astronomy, University of Minnesota

This phenomenon, called a pulsation pair instability (PPI) supernova, could account
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