For six months in 1181, a dying star left its mark on the night sky. The impressive object appeared as bright as Saturn in the vicinity of the Cassiopeia constellation, and historical chronicles from China and Japan recorded it 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 catastrophic 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 has intrigued astronomers for centuries.
A new study has described the celestial body in detail for the first time, creating a computer model of its evolution from just after the initial explosion to today. The research team compared the model with archived telescope observations of its nebula — the gigantic cloud of gas and dust, visible today, that is the remnant of the monumental event.
The researchers said the analysis strongly suggested 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 violently collidedbut failed to detonate completely, leaving behind a “zombie star”.
“There are 20 or 30 candidates for type Iax supernovae,” said Takatoshi Ko, lead author of the study published July 5 in Astrophysical Journal and a doctoral candidate in astronomy at the University of Tokyo. “But this is the only one we know of in our own galaxy.”
Furthermore, the study also found that, inexplicably, a high-speed stellar wind, detected in previous studies, began blowing from the surface of the zombie star just 20 years ago, adding to the mysterious aura of SN 1181. Unraveling the mechanism behind this A supernova event could help astronomers better understand the life and death of stars and how they contribute to planetary formation, experts say.
Failure to detonate a supernova
It took astronomers 840 years to solve SN 1181’s first big puzzle — locating its position in the Milky Way. The star was the last pre-telescopic supernova without a confirmed remnant, until, in 2021, Albert Zijlstra, professor of astrophysics at the University of Manchester in England, tracked it to a nebula in the constellation Cassiopeia.
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 [o auge da] Covid, I had a quiet afternoon and was at home,” said Zijlstra. “I related the supernova to the nebula using records from ancient Chinese catalogs. I think this has been accepted — a lot of people have looked at it and 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 is a fast-spinning, Earth-sized object called a white dwarf — a dense, dead star that has exhausted its nuclear fuel. This feature is unusual for a supernova remnant, as the explosion should have destroyed this body.
Zijlstra and his co-authors wrote a study in September 2021 about the discovery. The report suggested that SN 1181 may belong to the elusive Type Iax supernova category due to the presence of this “zombie white dwarf.”
In the most common supernova, type Ia, a white dwarf forms when a Sun-like star runs out of fuel and begins accreting material from another nearby star. Many of these exist in pairs, or in a binary system, different from the star.
The white dwarf accumulates material until it collapses under its own gravity, restarting nuclear fusion with a massive explosion that creates one of the brightest objects in the universe.
The rarest type Iax supernova is a scenario in which this explosion, for some reason, is interrupted. “One possibility is that Type Iax is not so much an explosion as a merger of two white dwarfs,” Zijlstra said. “The two come together, colliding with each other at full speed, and this can generate a lot of energy. This energy causes the sudden supernova glow.”
This massive collision could explain another curious aspect of the zombie star SN 1181. It contains neither hydrogen nor helium, which is extremely unusual in space, according to Zijlstra.
“About 90% of the universe consists of hydrogen and the rest is almost exclusively helium. Everything else is quite rare,” he said. “You have to look for 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) these. So clearly something extreme happened to (the zombie star).”
Inexplicable stellar wind
With the knowledge of where to look for SN 1181 and the suggestion that it could be an Iax-type remnant, Ko and his colleagues began working to uncover its remaining secrets.
“By precisely tracking the temporal evolution of the remnant, we were able to obtain detailed properties of the SN 1181 explosion for the first time. We confirm that the data obtained is consistent with a type Iax supernova,” said Ko, adding that the study’s computational model is consistent with previous observations of the remnant made by telescopes including the European Space Agency’s (ESA) XMM-Newton space telescope. and NASA’s Chandra X-ray Observatory.
Ko’s analysis reveals 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 more recent internal one is more difficult to explain.
The study suggests that this internal shock region could be a sign that the star began to burn again centuries after the explosion, leading to a surprising discovery, Ko added: high-speed stellar wind appears to have started blowing from the star’s surface. just 20 to 30 years ago.
Normally, this rapid stream of particles that astronomers call stellar wind should be expelled from the white dwarf as a byproduct of the body’s rapid rotation shortly after the supernova explosion.
“We don’t 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 ejected material did not escape completely and remained under the gravitational influence of the central white dwarf. This material may have eventually accumulated due to its gravity, causing it to reignite.”
However, Zijlstra noted that this theory contrasts with observations showing that the star’s brightness has decreased over the last century. “It’s unclear how this relates to the onset of wind,” he said. “I would expect the star to have brightened rather than dimmed.”
Ko and his colleagues are aware of this problem. They said they believe there is some relationship between the stellar wind and the star’s dimming, and they are investigating this. Researchers are preparing new observations of SN 1181 with two instruments they have not yet used: the Very Large Array of radio telescopes in New Mexico and the Subaru Telescope in Hawaii.
These studies, Ko said, will help expand scientists’ knowledge of all supernovae. “Type Ia supernovae were crucial to the discovery of the accelerating expansion of the universe,” he said. “But despite their importance, the mechanism behind their explosion 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 insight into the mechanism of type Ia supernovae.
Great opportunity
As objects like SN 1181 are important for the formation of many of the elements that make up human beings, studying them is a great opportunity, according to Zijlstra.
“These very energetic events can form elements heavier than iron, such as rare earths,” he said. “It is valuable to have an example of this type from 1,000 years ago where we can still see the ejecta, and perhaps in the future we can see exactly what elements were created in the event.”
This knowledge would help scientists understand how Earth formed and how it acquired 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 study.
Schaefer added that SN 1181 represents one of the few reliable connections between a supernova and its remnant. The object is important as the only possible case for obtaining good observations of the rare type.
“The finding is that type Iax supernovae make up approximately 20% of the supernovae in any galaxy, including our Milky Way, and may have formed most of the mysterious dust in the early Universe,” Schaefer said in an email.
He added that in our lifetime, astrophysicists will not have a better observed case of a Type Iax event, so researchers must do their best to understand SN 1181.
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