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Extra Bright Supernova Could Change the Way We Measure the Universe

This could change the way we view standard candles.
Image: NASA/Hubble

The discovery of an extra bright supernova has baffled researchers. The research, available on the arXiv pre-print server, could change the way we view these cosmic explosions.

When stars die, they produce some of the most violent explosions in the Universe, called supernovae. Astronomers refer to these massive stellar explosions as "standard candles", and use them to measure distances in the Universe.

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Standard candles are all thought to be the same, emitting a specific amount of light. Since their brightness is known, astronomers are able to estimate their distance from the Earth by measuring the strength of their light. Picture this: You're standing on a street that's lined with street lamps. According to the inverse square law, the second lamp will be one-quarter as bright as the first lamp. The third will then be one-ninth as bright as the first, and so on down the street.

In our own galaxy, astronomers use young stars known as Cepheid variable stars, which pulse at regular intervals, as standard candles. However, once we go beyond the Milky Way, astronomers cannot pick out individual stars to use as standard candles, so they need another option. This is where supernovae come in.

Supernovae, specifically Type 1a supernovae, are incredibly important to our understanding of physics and the Universe itself. "Type 1a supernovae are very uniform," said Steve Howell, a project scientist on NASA's Kepler mission. "The explosions are very predictable, essentially emitting the same amount of light and allowing us to accurately measure distances in the Universe."

However, astronomers still do not know what causes these explosions. Until very recently, the leading theory for standard candle supernovae included a white dwarf—the dying embers of a Sun-like star—that was feeding off of a companion. Once the white dwarf gobbled up as much material as it could hold, a thermonuclear explosion was triggered, resulting in a supernova. However, new data from Kepler shows that Type 1a supernovae may in fact be caused by the merger of two white dwarfs.

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Basically stars are like massive nuclear reactors, fusing together elements until they run out of fuel. When they explode, supernovae help seed the Universe with material to form new stars by ejecting large amounts of heavy elements into space. The light we see from the explosion is a result of radioactive decay. Radioisotopes like nickel are unstable, meaning the atoms cannot hold themselves together and they break down. In a supernova explosion, certain nickel isotopes (56Ni) will first decay into cobalt isotopes (56Co) and then again into a more stable iron isotope (56Fe).

A few days after the explosion, the light we see reaches peak brightness. At this stage, a supernova explosion can outshine an entire galaxy. Although the light from a supernova will radiate for years, astronomers predict there will be a sharp reduction in brightness about 500 days following the initial explosion.

However, astronomers have observed a supernova that doesn't follow these rules. Discovered three years ago, supernova SN2012cg continues to shine brighter than its Type 1a counterparts. This has astronomers puzzled. If all Type 1a supernovae aren't all the same, then are they really the standard candles science relies on?

There could be multiple explanations as to why this supernova still shines so brightly. The first is that we could be seeing a light echo. This means that the light from the explosion hit a large cloud dust and is bouncing off in different directions. If this is the case, the light from the supernova would be reaching us twice: once after the initial explosion, and again after the light bounced off the dust cloud.

Another explanation is that during the explosion, a heavier cobalt isotope (57Co) is produced. It takes longer to decay than 56Co, providing extra energy to the supernova, that could kick in about 2-3 years after the initial explosion. This is the first time the heavier cobalt isotope has been detected in a Type 1a supernova.

According to Or Graur, lead author on the study, and a research associate in the American Museum of Natural History's Department of Astrophysics, supernovae are extremely important to physics. Despite this he says, "We still do not know exactly what type of star system explodes as a type 1a supernova or how the explosion takes place. A lot of research has gone into these two questions, but the answers are still elusive."

Astronomers need to study more supernovae, especially ones that are closer to Earth, in order to determine which one of these explanations is the right one. By studying the chemical composition of these supernovae, scientists can better understand how they explode.