NASA provides 3D recreation of supernova remnant Crab Nebula
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The groundbreaking supernova discovery could help solve two cosmic mysteries, one of which has puzzled astronomers for nearly 1,000 years. Supernovas are among the biggest and most spectacular fireworks in the universe – the “last hurrah of a dying massive star”, according to the US space agency NASA. When supernovas erupt, their light is powerful enough to temporarily outshine all of the stars within a galaxy.
According to a newly published paper in Nature Astronomy, one such supernova may have lit up the skies nearly 1,000 years ago and gave birth to the beautiful Crab Nebula.
But this was not a regular type of supernova – one triggered by a massive star collapsing in on itself or a white dwarf star erupting in a thermonuclear explosion.
Instead, an international team of scientists led by the University of California, Santa Barbara has detected exciting evidence of a whole new type of so-called electron-capture supernova.
Scientists have been speculating about this third type of supernova for nearly half a century but there has been very little evidence of their existence.
Triggered by the explosion of a super-asymptotic giant branch or SAGB stars, an electron-capture supernova may be responsible for a peculiar flash of light observed in the year 1054 AD.
That year, Chinese and Japanese records recall a source of light so bright, it was seen in the daytime for 23 years.
Just as impressively, the supernova was seen at night for two years and eventually gave way to the Crab Nebula – a supernova remnant about 6,500 light-years from Earth.
Until now, scientists have recognised two different types of supernova: type I and Type II.
Type I supernovas erupt when a white dwarf in a binary star system syphons material from its partner until it triggers a runaway nuclear reaction.
A type II supernova occurs when a massive star runs out of nuclear fuel and the forces of gravity cause it to collapse and explode.
NASA: Hubble telescope captures fading supernova in NGC 2525
But in 1980, astrophysicists at the University of Tokyo proposed a third, theoretical type of supernova, an electron-capture supernova.
In this case, the supernovas occur in stars with lighter cores made out of oxygen, neon and magnesium.
Some of the electrons in these cores smash into the cores’ atomic nuclei – a process called electron-capture.
When the electrons are removed, the stars’ cores buckle and collapse under their own weight, triggering a supernova.
Scientists believe they have spotted evidence of this phenomenon in the supernova SN 2018zd, in the galaxy NGC 2146 – some 31 million light-years away from Earth.
The discovery was led by Daichi Hiramatsu, a graduate student at UC Santa Barbara and member of the international Global Supernova Project.
The researchers found the supernova had a number of exciting characteristics, such as a possible SAGB origin and unusual chemical composition.
Mr Hiramatsu said: “We started by asking ‘what’s this weirdo?’
“Then we examined every aspect of SN 2018zd and realised that all of them can be explained in the electron-capture scenario.”
Scientists have considered the Crab Nebula a good candidate for an electron-capture supernova.
But the supernova’s appearance some 1,000 years ago has made it hard to analyse.
The discovery of SN 2018zd helped draw comparisons between the two.
Ken Nomoto at the Kavli IPMU of the University of Tokyo said: “I am very pleased that the electron-capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years.
“I very much appreciate the great efforts involved in obtaining these observations.
“This was a wonderful case of the combination of observations and theory.”
Mr Hiramatsu added the discovery was a “Eureka moment” for him and his colleagues.
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