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 constellation Cassiopeia, 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 has puzzled 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 shortly after the initial explosion to today. The research team compared the model with archived 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 supernovae called Type Iax, in which the thermonuclear explosion may be the result of not one, but two two white dwarfs that violently collided but 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 the Astrophysical Journal and a PhD candidate in astronomy at the University of Tokyo. “But this is the only one we know of in our own galaxy.”
Additionally, the study also found that a high-speed stellar wind, detected in previous studies, inexplicably began blowing from the surface of the zombie star just 20 years ago, adding to SN 1181’s mysterious aura. Unraveling the mechanism behind this supernova event could help astronomers better understand the life and death of stars and how they contribute to planet formation, experts say.
Failed supernova detonation
It took astronomers 840 years to solve SN 1181’s first big puzzle — pinpointing its position in the Milky Way. The 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 to a nebula in the constellation Cassiopeia.
Amateur astronomer Dana Patchick discovered the nebula in 2013 while searching through NASA’s Wide-Field Infrared Survey Explorer (WISE) archive. 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,” Zijlstra said. “I linked the supernova to the nebula using records from old Chinese catalogues. I think that’s 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 rapidly spinning, Earth-sized object called a white dwarf — a dense, dead star that has exhausted its nuclear fuel. This is unusual for a supernova remnant, as the explosion should have destroyed such a 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 category of Type Iax supernovae 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 to collect material from another nearby star. Many of these exist in pairs, or in a binary system, unlike the star itself.
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 rarer Type Iax supernova is a scenario in which this explosion is interrupted for some reason. “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 supernova to suddenly brighten.”
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 percent of the universe is hydrogen, and the rest is almost exclusively helium. Everything else is quite rare,” he said. “You have to look at 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 happened to (the zombie star).”
Unexplained 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 set to work to uncover its remaining secrets.
“By accurately tracking the temporal evolution of the remnant, we were able to obtain detailed properties of the SN 1181 explosion for the first time. We confirmed that the obtained data are consistent with a type Iax supernova,” Ko said, 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 composed of two distinct shock regions. An outer one formed when material ejected by the supernova explosion encountered interstellar space. The inner, more recent one is more difficult to explain.
The study suggests that this inner shock region could be a sign that the star began to burn again centuries after the explosion, leading to a surprising discovery, Ko added: The high-speed stellar wind appears to have started blowing off the star’s surface just 20 to 30 years ago.
Normally, this fast-moving stream of particles that astronomers call the 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 that show the star has dimmed over the past century. “It’s not clear how this relates to the onset of the 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 it. The 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 scientists expand their understanding of all supernovae. “Type Ia supernovae were crucial to the discovery of 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 insight into the mechanism of Type Ia supernovae.
Great opportunity
Because 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’s valuable to have an example of this that occurred 1,000 years ago, where we can still see the ejecta, and perhaps in the future we’ll be able to 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 percent of supernovae in any galaxy, including our own 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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Source: CNN Brasil
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