Astronomers have found leftover 'zombie star' from supernova shone in the night sky 1,000 years ago

For six months in 1181, a dying star left a mark in the night sky.

The striking object appeared as bright as Saturn in the vicinity of the constellation Cassiopeia, and historical chronicles from China and Japan recorded it as a "guest star."

Chinese astronomers used this term to signify a temporary object in the sky, often a comet or, as in this case, a supernova - a cataclysmic explosion of a star at the end of its life.

The object, now known as SN 1181, is one of a handful of supernovas documented before the invention of telescopes, and it has puzzled astronomers for centuries.

Now, a new study has for the first time described SN 1181 in detail by creating a computerized model of the supernova's evolution from immediately after the initial outburst appeared until today. The research team compared the model with archival telescope observations of its nebula - the giant cloud of gas and dust, visible to this day, that is the remnant of the monumental event.

The researchers said the analysis strongly suggested that SN 1181 belongs to a rare class of supernovas called Type Iax in which the thermonuclear flare-up could be the result of not one but two white dwarfs that have violently collided yet fail to detonate completely, leaving behind a "zombie star."

"There are 20 or 30 candidates for Type Iax supernovas," said Takatoshi Ko, lead author of the study published July 5 in the The Astrophysical Journal. "But this is the only one that we know of in our own galaxy." Ko is a doctoral student of astronomy at the University of Tokyo.

What's more, the study also found that, inexplicably, high-speed stellar wind, detected in past studies, started to blow from the surface of the zombie star as recently as 20 years ago, adding to SN 1181's mysterious aura. Unlocking the mechanism behind this supernova event could help astronomers come to a better understanding of the life and death of stars and how they contribute to planetary formation, experts say.

Failed detonation of a supernova

It took astronomers 840 years to solve SN 1181's first great riddle - pinpointing its location in the Milky Way. The dying star was the last pre-telescopic supernova without a confirmed remnant, until in 2021 Albert Zijlstra, a professor of astrophysics at the University of Manchester in England, traced it back to a nebula in the constellation Cassiopeia.

Amateur astronomer Dana Patchick discovered the nebula in 2013 when searching the archive of NASA's Wide-Field Infrared Survey Explorer, or WISE. But Zijlstra, who was not involved with the new study, was the first to make the connection to SN 1181.

"During (the height of) Covid, I had a quiet afternoon and was sitting at home," Zijlstra said. "I matched the supernova to the nebula using records from ancient Chinese catalogs. I think that has been now generally accepted - a lot of people have looked at it and they have agreed that it seems to be correct. This is the remnant of that supernova."

The nebula is about 7,000 light-years away from Earth, and at its center there is a fast-spinning, Earth-size object called a white dwarf - a dense, dead star that has depleted its nuclear fuel. The feature is unusual for a supernova remnant because the explosion should have obliterated the white dwarf.

Zijlstra and his coauthors wrote a September 2021 study about the discovery. The report suggested that SN 1181 might belong to the elusive Type Iax category of supernova due to the presence of this "zombie" white dwarf.

In the more common Type Ia supernova, a white dwarf that forms when a sunlike star has exhausted its fuel begins to accumulate material from another nearby star. Many stars exist in pairs, or a binary system, unlike the sun. The white dwarf accumulates material until it collapses under its own gravity, reigniting nuclear fusion with a massive explosion that creates one of the brightest objects in the universe.

The rarer Type Iax is a scenario in which this explosion, for some reason, is halted. "One possibility is that the Type Iax is not so much an explosion, but a merger of two white dwarfs," Zijlstra said. "The two come together, hitting each other at full speed, and that can generate a lot of energy. That energy causes the sudden brightness of the supernova."

That massive collision might explain another curious aspect of the SN 1181 zombie star. It contains no hydrogen or helium, which is highly unusual in space, Zijlstra said.

"About 90% of the universe consists of hydrogen and the rest is almost exclusively helium. Everything else is pretty rare," he said. "You need to look up 10,000 atoms before you find one that isn't hydrogen or helium. But our star (the sun at the center of our solar system) only has (primarily) those. So, clearly, something extreme has happened to (the zombie star)."

Unexplained stellar wind

Armed with the knowledge of where to look for SN 1181, and the suggestion that it could be a Type Iax remnant, Ko and his colleagues got to work to uncover its remaining secrets.

"By accurately tracking the time evolution of the remnant, we were able to obtain detailed properties of the SN 1181 explosion for the first time. We confirmed that these detailed properties are consistent with a Type Iax supernova," Ko said, adding that the computer model in the study is consistent with past observations of the remnant from telescopes, including the European Space Agency's XMM-Newton space telescope and NASA's Chandra X-ray Observatory.

Ko's analysis shows that two distinct shock regions make up the remnant of SN 1181. An outer one formed when material was ejected by the supernova explosion and met interstellar space. An inner one that's more recent is more difficult to explain.

The study suggests that this inner shock region might be a sign that the star has started to burn again, centuries after the explosion, leading to a surprising find, Ko added: High-speed stellar wind seems to have started blowing from the surface of the star just 20 to 30 years ago.

Typically, this rapid stream of particles astronomers call stellar wind should blow off from the white dwarf as a by-product of the star's fast spinning right after the supernova explosion.

"We do not fully understand why the star reignited and the stellar wind started so recently," Ko said. "We theorize that the star reignited because SN 1181 was a Type Iax supernova, which is an incomplete explosion. As a result, the material ejected by the explosion did not escape completely and remained within the gravitational influence of the central white dwarf. This material could eventually have accreted onto the white dwarf due to its gravity, causing it to reignite."

However, Zijlstra noted, that theory is in contrast with observations that show the star's brightness has dimmed over the past century.

"It's not clear how that relates to the wind turning on," he said. "I would have expected the star to have brightened rather than faded."

Ko and his colleagues are aware of this problem. They said they believe there is some relationship between the wind and the dimming, and are investigating it.

The researchers are preparing further observations of SN 1181 with two instruments they haven't used: the Very Large Array of radio telescopes in New Mexico and the Subaru Telescope in Hawaii.

These studies, Ko said, will help inform scientists' knowledge of all supernovas.

"Type Ia supernovas have been crucial in discovering the accelerating expansion of the universe," he said. "But despite their importance, their explosion mechanism remains unknown, making it one of the most significant challenges in modern astronomy."

By studying SN 1181 and its incomplete explosion, he added, scientists can gain insights into the mechanism of Type Ia supernovas.

Opportunity of a lifetime

Because objects such as SN 1181 are important for making so many of the elements that humans are also made of, studying them is a great opportunity, according to Zijlstra.

"These very energetic events can build up elements that are heavier than iron, like rare earths," he said. "It's valuable to have an example of such an event from 1,000 years ago where we can still see the ejected materials, and maybe in the future we could see exactly which elements were created in the event."

This knowledge would help scientists understand how Earth formed and obtained these elements, Zijlstra added.

Historically, ancient observations of supernovas have been of supreme importance for modern astrophysics, said Bradley Schaefer, a professor emeritus of astrophysics and astronomy at Louisiana State University, who was not involved with the latest study.

Schaefer added that SN 1181 represents one of the few reliable connections from supernova to supernova remnant. The object is important as the only possible case for getting good observations of the elusive Type Iax.

"The realization has been that Type Iax supernova constitute roughly 20% of the supernovae in any galaxy, including our own Milky Way, and they might form most of the mysterious dust in the early Universe," Schaefer said in an email.

In our lifetime, he added, astrophysicists will not get any better observed case for a Type Iax event, so researchers should push hard for understanding SN 1181.

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