Space supernova discovery sheds new light on mysterious medieval phenomenon

A rare new supernova has seen observed by astronomers for the first time – in a "eureka moment" four decades after it was predicted in 1980.

Scientists used to think only one of two things happen when stars run out of fuel and explode but could never explain a mysterious phenomenon reported in China nearly 1,000 years ago.

Until recently, there was no hard evidence that anything other than core-collapse supernovae – which leave behind black holes and neutron stars – and thermo nuclear supernovae, which form white dwarf stars, happen in nature.

But Ken'ichi Nomoto of the University of Tokyo's prediction that there is a third possibility – which he dubbed the electron capture supernova – has finally been proved correct.

And what's more, in 1054 a supernova happened in the Milky Way Galaxy that, according to Chinese and Japanese records, was so bright that it could be seen in the daytime for 23 days, and at night for nearly two years.

The resulting remnant, the Crab Nebula, has been studied in great detail.

The Crab Nebula was previously the best candidate for an electron-capture supernova, but its status was uncertain partly because the explosion happened nearly a thousand years ago.

The new result increases the confidence that the historic SN 1054 was an electron-capture supernova.

Andrew Howell, a staff scientist at Las Cumbres Observatory and adjunct faculty at UCSB, said: "The term Rosetta Stone is used too often as an analogy when we find a new astrophysical object, but in this case I think it is fitting.

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"This supernova is literally helping us decode thousand-year-old records from cultures all over the world.

"And it is helping us associate one thing we don't fully understand, the Crab Nebula, with another thing we have incredible modern records of, this supernova.

"In the process it is teaching us about fundamental physics: how some neutron stars get made, how extreme stars live and die, and about how the elements we're made of get created and scattered around the universe."

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A core-collapse supernova occurs when a massive star, more than 10 times the mass of the Sun, runs out of fuel and its core collapses into a black hole or neutron star.

A thermonuclear supernova occurs when a white dwarf star – the remains of a star up to eight times the mass of the Sun – explodes.

In 1980, Ken'ichi Nomoto of the University of Tokyo predicted a third type called an electron capture supernova.

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Stefano Valenti is professor of physics and astronomy at the University of California, Davis, and a member of the team that discovered and described supernova 2018zd.

He said: "One of the main questions in astronomy is to compare how stars evolve and how they die.

"There are many links still missing, so this is very exciting."

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The energy produced in their central core is what keeps most stars from collapsing under their own gravity.

In an electron capture supernova, as the core runs out of fuel, gravity forces electrons in the core into their atomic nuclei, causing the star to collapse in on itself.

The new supernova, SN 2018zd, was detected in March 2018, about three hours after the explosion, and is relatively close to Earth, at a distance of about 31 million light years in galaxy NGC2146.

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