In 1181, a dying star left a trail in the night sky for six months.
The striking object appeared as bright as Saturn near the constellation Cassiopeia, and historical chronicles from China and Japan recorded it as a "guest star."
Chinese astronomers used this term to denote 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. It's a question that has puzzled astronomers for centuries.
Now, a new study has for the first time described SN 1181 in detail by creating a computer model of the supernova's evolution from the time it first erupted until the present day. The research team 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.
The researchers said the analysis strongly suggested that SN 1181 belongs to a rare class of supernovae called Type Iax where the thermonuclear burst could have been the result of not one but two white dwarfs that collided violently but did not fully explode, 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. "But this is the only one we know of in our own galaxy." Ko is a doctoral student in astronomy at the University of Tokyo.
What's more, the study also found that, inexplicably, the fast stellar wind detected in previous studies only began blowing from the zombie star's surface 20 years ago, contributing 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 planetary formation, experts say.
It took astronomers 840 years to solve the first major mystery of SN 1181: pinpointing 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 in England, traced it to a nebula in the constellation Cassiopeia in 2021.
Amateur astronomer Dana Patchick discovered the nebula in 2013 while searching through the archives of NASA's Wide-Field Infrared Survey Explorer, or 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 afternoon and was sitting at home," Zijlstra said. "I linked the supernova to the nebula using data from old Chinese catalogs. I think that's now generally accepted - a lot of people have looked at it and agree that it seems to be right. 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 property is unusual for a supernova remnant, because the explosion should have destroyed the white dwarf.
Zijlstra and his co-authors wrote a study about the discovery in September 2021. The paper 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 created when a sun-like star exhausts its fuel begins to gather material from another nearby star. Many stars exist in pairs, or binary systems, unlike the sun. The white dwarf gathers 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 where this explosion is stopped for some reason. "One possibility is that Type Iax is not so much an explosion, but a merger of two white dwarfs," Zijlstra said. "The two come together, collide with 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 could explain another curious aspect of the zombie star SN 1181. It contains no hydrogen or helium, which is highly unusual in space, Zijlstra said.
"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 up 10,000 atoms before you find one that's not 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 knowledge of where to look for SN 1181 and the suggestion that it might be a remnant of Type Iax, Ko and his colleagues set to work to unravel its remaining secrets.
"By carefully monitoring 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 previous 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 reached interstellar space. An inner one, which is more recent, is harder to explain.
The research shows that this inner shock region could be a sign that the star has started burning again centuries after the explosion. That could lead to a surprising discovery, says Ko. It seems that a fast stellar wind started blowing from the surface of the star only 20 to 30 years ago.
Normally, this rapid stream of particles, which astronomers call "stellar wind," would blow away from the white dwarf as a byproduct of the star's rapid rotation immediately after the supernova explosion.
"We don't fully understand why the star reignited and the stellar wind started so recently," Ko said. "We hypothesize 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 gravity of the central white dwarf. This material could eventually have been gravitationally drawn onto the white dwarf, causing it to reignite."
Zijlstra notes, however, that this theory conflicts with observations showing that the star's brightness has decreased over the past century.
"It's not clear how that relates to the wind that's kicking in," he said. "I would have expected the star to have gotten brighter rather than dimmer."
Ko and his colleagues are aware of this problem. They said they believe there is a relationship between the wind and the eclipse, and that they are investigating it.
The researchers are preparing further 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.
According to Ko, these studies will contribute to scientists' knowledge about all supernovae.
"Type Ia supernovae 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 important challenges in modern astronomy."
By studying SN 1181 and its incomplete explosion, scientists can gain insight into the mechanism of Type Ia supernovae, he added.
Because objects like SN 1181 are important for the production of so many of the elements that make up humans, studying them is a great opportunity, Zijlstra said.
"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 an event like this from 1,000 years ago, where we can still see the ejected materials, and maybe in the future we can see exactly what elements were created in the event."
This knowledge would help scientists understand how the Earth formed and how these elements were acquired, Zijlstra added.
Historically, ancient observations of supernovae have been vital to modern astrophysics, said Bradley Schaefer, a 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 supernova-to-remnant connections. The object is important as the only possible case for obtaining good observations of the elusive Type Iax.
"It has come to be realized that Type Iax supernovae make up about 20% of the supernovae in every galaxy, including our own Milky Way, and that they may make up most of the mysterious dust in the early universe," Schaefer said in an email.
He added that astrophysicists won't get a better observed case for a Type Iax event in our lifetime, so researchers must work hard to understand SN 1181.
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