A supernova shone brightly in the night sky 1,000 years ago. Now, astronomers have found the remains of its “zombie star.”

(CNN) – For six months in 1181, a dying star left its mark on 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 described it. recorded as a “guest star.”

Chinese astronomers used this term to designate 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 the few supernovae documented before the invention of telescopes, and it puzzled astronomers for centuries.

Now, a new study has described SN 1181 in detail for the first time by creating a computer model of the supernova's evolution from just after the initial outburst to the present day. The team of researchers compared the model to 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.

According to the researchers, the analysis suggests that SN 1181 belongs to a rare class of supernovae called type Iax, in which the thermonuclear explosion could be the result of not one, but two white dwarfs that have collided violently but failed to fully detonate, leaving behind a “zombie star“.

“There are 20 or 30 Type Iax supernova candidates,” says Takatoshi Ko, lead author of the study published July 5 in The astrophysical journal“But this is the only one we know of in our own galaxy.” Ko is a PhD student in astronomy at the University of Tokyo.

What's more, the study also found that, inexplicably, the high-speed stellar wind, detected in Previous studiesbegan to blow from the surface of the zombie star just 20 years ago, adding to SN 1181's mysterious aura. Unraveling the mechanism of this supernova could help astronomers better understand the life and death of stars and their contribution to planet formation, experts say.

It took astronomers 840 years to solve the first big mystery of SN 1181: determining its location in the Milky Way. The dying star was the last pre-telescopic supernova without a confirmed remnant, until Albert Zijlstra, a professor of astrophysics at the University of Manchester, England, located it in a nebula in the constellation Cassiopeia, in 2021.

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

“During the height of Covid, I had a quiet evening and was sitting at home,” Zijlstra said. “I matched the supernova to the nebula using records from old Chinese catalogues. I think it’s now generally accepted – a lot of people looked at it and agreed that it looks right. This is the remnant of that supernova.”

The nebula is located about 7,000 light-years from Earth, and at its center is a rapidly spinning Earth-sized object called a white dwarf, a dense, dead star that has exhausted its nuclear fuel. This is an unusual feature for a supernova remnant, since the explosion should have destroyed the white dwarf.

Zijlstra and his co-authors wrote a paper on the discovery in September 2021The report suggested that SN 1181 could belong to the elusive category of Type Iax supernovae due to the presence of this “zombie” white dwarf.

In the more common Type Ia supernova, a white dwarf that forms when a Sun-like star exhausts its fuel begins accreting material from another nearby star. Many stars exist in pairs, or a binary system, unlike the Sun. The white dwarf accretes material until it collapses under its own gravity, reigniting nuclear fusion in 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, stops. “One possibility is that Type Iax is not so much an explosion as a merger of two white dwarfs,” Zijlstra explains. “The two come together, crash into each other at high speed, and that can generate a lot of energy. That energy causes the sudden brightness of the supernova.”

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

“About 90% of the universe is made up of hydrogen and the rest is almost exclusively helium. Everything else is pretty rare,” he said. “You have to search 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 (mostly) those. So clearly something extreme has happened to (the zombie star).”

Armed with the knowledge of where to look for SN 1181 and the suggestion that it might be an Iax-type remnant, Ko and his colleagues set about unlocking its secrets.

“By accurately tracking the temporal evolution of the remnant, we were able to obtain detailed properties of the explosion of SN 1181 for the first time. We confirmed that these detailed properties are consistent with those of a Type Iax supernova,” says Ko, adding that the computer model of the study is consistent with previous observations of the remnant made with telescopes such as the XMM-Newton of the European Space Agency and the Chandra X-ray Observatory from NASA.

Ko's analysis shows that the remnant of SN 1181 is made up of two distinct shock regions. An outer one formed when material was ejected by the supernova explosion and encountered interstellar space. The younger inner one is harder to explain.

The study suggests that this inner shock region could be a sign that the star has started burning up again, centuries after the explosion, leading to a surprising finding, Ko added: The high-speed stellar wind appears to have only started blowing off the star's surface 20 to 30 years ago.

Normally, this fast-moving stream of particles that astronomers call a stellar wind should be blown away from the white dwarf as a byproduct of the star's rapid spin just after the supernova explosion.

“We don’t fully understand why the star reignited and the stellar wind started so recently,” Ko explains. “Our theory is 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 fully escape and remained within the gravitational influence of the central white dwarf. Over time, this material might have accumulated on the white dwarf due to its gravity, causing it to reignite.”

However, Zijlstra noted, that theory is at odds with observations showing the star's brightness has dimmed over the past century.

“It’s not clear how that relates to the wind flare,” he said. “I would have expected the star to have brightened rather than dimmed.”

Ko and his colleagues are aware of this problem.

They said they believe there is some connection between the wind and the darkening, and that they are investigating it.

Researchers are preparing new observations of SN 1181 with two previously unused instruments: the Very Large Array of radio telescopes in New Mexico and the Subaru Telescope in Hawaii.

According to Ko, these studies will help scientists better understand all supernovae.

“Type Ia supernovae have been crucial to uncovering the accelerated expansion of the universe,” he said. “But despite their importance, their explosion mechanism remains unknown, making it one of the most important challenges of modern astronomy.”

By studying SN 1181 and its incomplete explosion, he added, scientists can better understand the mechanism of Type Ia supernovae.

According to Zijlstra, studying objects like SN 1181 represents a great opportunity, as they are important for the formation of many of the elements that humans are also made of.

“These highly energetic events can accumulate elements heavier than iron, such as rare earth elements,” he explains. “It is very valuable to have an example of such an event from 1,000 years ago where we can still see the ejected materials, and perhaps in the future we will be able to see exactly what elements were created in the event.”

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

Historically, ancient observations of supernovae have been of paramount importance to modern astrophysics, said Bradley Schaefer, professor emeritus of astrophysics and astronomy at Louisiana State University, who was not involved in the latest study.

Schaefer added that SN 1181 represents one of the few reliable connections between supernova and supernova remnants. The object is important as the only possible case for obtaining good observations of the elusive Type Iax.

“Type Iax supernovae have been found to account for about 20 percent of supernovae in any galaxy, including our own Milky Way, and may form the bulk of the mysterious dust in the early universe,” Schaefer said in an email.

He added that astrophysicists will not have a better observed case of a Type Iax event in our lifetime, so researchers should strive to understand SN 1181.