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Updated May 6th, 2021 at 18:57 IST

‘Oddball supernova’ oddly cool without hydrogen before explosion; stuns astrophysicists

Astrophysicists speculate that in the years preceding its death, the yellow star might have shed its hydrogen layer or lost it to a nearby companion star.

Reported by: Zaini Majeed
NASA
(Image Credit: NASA)  | Image:self
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In a never-before-seen cosmic event, NASA’s Hubble telescope spotted a fitful yellow star 2.5 years before it exploded into an oddball supernova. Located 35 million lightyears from Earth in the Virgo galaxy cluster, the mysterious yellow star has caused astrophysicists to reevaluate what's possible within the universe as it lacked a crucial hydrogen layer at the time of the explosion and had strangely remained cool at the time it burst into a massive cluster of energies. The phenomenon examined by NASA’s Hubble Space Telescope was documented by the Northwestern University on May 5. “At the end of their lives, cool, yellow stars are typically shrouded in hydrogen, which conceals the star's hot, blue interior. But this yellow star had none,” an international team of astrophysicists said the release. 

A researcher at Northwestern’s Interdisciplinary Exploration and Research in Astrophysics (CIERA) Charles Kilpatrick, who led the study, said: "We haven't seen this scenario before,” as his team studied the death throes of stars prior to their explosion into the supernova. Kilpatrick is also a member of the Young Supernova Experiment, which uses the Pan-STARSS telescope at Haleakalā, Hawaii, to catch supernovae right after they explode.

[Yellow supergiant in the last 100 years before it exploded as a supernova. Credit: AAAS journal]

In the study published in the American Association for the Advancement of Science journal, and the Monthly Notices of the Royal Astronomical Society, the scientists hypothesized that, perhaps, in the years preceding its death, the star might have shed its hydrogen layer or lost it to a nearby companion star. Lead author Kilpatrick explained that if a star explodes without hydrogen, it should be “extremely blue -- really, really hot.” But this was not the case with a yellow star and it is a scenario never witnessed. “It's almost impossible for a star to be this cool without having hydrogen in its outer layer,” said Kilpatrick. “We looked at every single stellar model that could explain a star like this, and every single model requires that the star had hydrogen, which, from its supernova, we know it did not. It stretches what's physically possible,” he further explained. 

"Astronomers have suspected that stars undergo violent eruptions or death throes in the years before we see supernovae," Kilpatrick said. "This star's discovery provides some of the most direct evidence ever found that stars experience catastrophic eruptions, which cause them to lose mass before an explosion. If the star was having these eruptions, then it likely expelled its hydrogen several decades before it exploded."

[Hubble Space Telescope (HST) imaging showing the explosion site. Credit: Charles Kilpatrick / Northwestern University/AAAS journal]

How did the yellow star explode without hydrogen?

Astrophysicists started the young Supernova Experiment after they spotted a yellow star in 2019 in the relatively nearby spiral galaxy NGC 4666. The team of scientists deployed deep space images captured by NASA's Hubble Space Telescope to closely observe this section of the sky. "What massive stars do right before they explode is a big unsolved mystery," Kilpatrick said. "It's rare to see this kind of star right before it explodes into a supernova.”

Hubble was able to capture intricate details of the supernova, including its massive star source just about a couple of years before the explosion. The progenitor star had exploded while it was cool and without hydrogen, a key component of combustion in the universe. Several months after the explosion, however, the team speculated that the ejecta from the star's final explosion travelling through its environment must have collided with a large mass of hydrogen on its way. And there is a possibility that the progenitor star might have expelled the hydrogen within a few years before its death. 

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Published May 6th, 2021 at 18:57 IST

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