Outer space

The supernova explosion that failed to kill it’s star

Scientists studying a galaxy 120 million light years away have found remnants of a star that appears to have survived a supernova explosion.

Curtis McCully of Las Cumbres Observatory told a meeting of the American Astronomical Society last week that the find was startling because it was a type of supernova, known as a type 1a supernova, generally believed to be caused by a titanic thermonuclear explosion.

Such blasts, McCully said, occur in white dwarf stars, in which a star the size of the Sun had collapsed to a white-hot remnant about the size of the Earth.


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Usually, McCully said, these blasts signalled the complete destruction of the white dwarf. But in this case, not only did follow-up observation reveal a star at the site of the blast, but that star was actually brighter after the explosion than it had been beforehand.

The blast, known as SN 2012Z, occurred in the galaxy NGC 1309, in the constellation Eridanus. It was spotted in 2012 (hence the 2012 in its name). Like all supernovas, it caught attention at the time, but when McCully’s team took a look at it afterward, they were not only able to find Hubble Space Telescope images taken of the progenitor star before the blast, but also ones taken of the same area in 2016, four years later.

“This was very fortuitous, because it gave us a chance to see the entire life cycle of the supernova from before it exploded to much later,” McCully says.

Supernova
Galaxy NGC 1309. Image Credit: NASA, ESA, The Hubble Heritage Team (STScI / AURA), and A. Riess (JHU / STScI).

It was from these images that his team was able to find that the original star was not only still there, but brighter than ever.

One of McCully’s colleagues, Andy Howell (also of Las Cumbres Observatory), compared it to the resurrection of Obi-Wan Kenobi in Star Wars: “Nature tried to strike this star down, but it came back more powerful than we could have imagined. ”

“Nature tried to strike this star down, but it came back more powerful than we could have imagined.”

The reason is a bit of a mystery, but it appears to have been a “failed” supernova in which the blast was not strong enough to destroy the star.

Why this type of supernova occurs at all is not quite clear, but a leading theory is that the white dwarf has a more normal-sized companion star, which it orbits closely enough to be able to “steal” mass from it. Eventually, the stolen mass reaches a point where a cascade of thermonuclear reactions ignite in the white dwarf’s core, leading to a runaway explosion that blasts it to smithereens.


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SN 2012Z’s survival, McCully said, might be due to turbulence within the star that altered how the stolen mass mixed with its core, producing a failed explosion.

Understanding this is important, he added, because type 1a supernovae are used by astronomers as “standard candles”, the apparent brightness of which from Earth can be used to determine the distance of faraway galaxies.

From this, astronomers have been able to fine-tune their estimates of the expansion of the universe, including the Nobel Prize-winning discovery that the universe’s rate of expansion appears to be slowly increasing – a finding attributed to the otherwise-undetected force known as dark energy. “So, type 1a supernovae are extremely important for astronomers,” McCully said.



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