(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 Cassiopeia constellation, and historical chronicles of China and Japan described it. registered 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 baffled 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 immediately after the initial burst appeared to the present day. The team of researchers compared the model with archival observations made with telescopes of its nebula, the gigantic 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 violently collided but failed to detonate. at all, leaving behind a “zombie star”.
“There are 20 or 30 candidates for type Iax supernovae,” 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 blowing from the surface of the zombie star just 20 years ago, adding to the mysterious aura of SN 1181. Unraveling the mechanism of this supernova could help astronomers better understand life and science, experts say. death of stars and their contribution to the formation of planets.
It took astronomers 840 years to solve the first big puzzle of SN 1181: determining its location in the Milky Way. The dying star was the last pre-telescopic supernova without a confirmed remnant, until in 2021 Albert Zijlstra, professor of astrophysics at the University of Manchester, England, located it in a nebula in the Cassiopeia constellation.
Amateur astronomer Dana Patchick discovered the nebula in 2013 while searching 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 (the height of) Covid, I was having a quiet afternoon and sitting at home,” Zijlstra said. “I matched the supernova with the nebula using records from old Chinese catalogues. I think it’s become generally accepted now – a lot of people looked at it and agreed that it looks right. “This is the remnant of that supernova.”
The nebula is 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, as the explosion should have destroyed the white dwarf.
Zijlstra and his co-authors wrote a study on the discovery in September 2021. The 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 runs out of fuel begins to accumulate material from another nearby star. Many stars exist in pairs, or in 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, stops. “One possibility is that type Iax is not so much an explosion as a merger of two white dwarfs,” explains Zijlstra. “The two come together, collide with each other at full speed, and that can generate a lot of energy. “That energy causes the sudden glow of the supernova.”
That massive collision could explain another curious aspect of the zombie star SN 1181. It does not contain hydrogen or helium, something very unusual in space, according to Zijlstra.
“About 90% of the universe is made up of hydrogen and the rest is almost exclusively helium. Everything else is pretty weird,” she said. “You have to look for 10,000 atoms before you find one that is 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).”
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 set to work to unlock its secrets.
“By accurately monitoring 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 match those of a type Iax supernova,” says Ko, who adds that the model computer science of the study coincides with previous observations of the remnant made with telescopes such as the XMM-Newton from 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 interior, more recent, is more difficult to explain.
The study suggests that this inner shock region could be a sign that the star has started burning again, centuries after the explosion, leading to a surprising finding, Ko added: the high-speed stellar wind appears to have started blowing from the star’s surface only 20 or 30 years ago.
Normally, this rapid stream of particles that astronomers call the 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 flared up again and the stellar wind started so recently,” Ko explains. “Our theory is that the star flared up again because SN 1181 was a type Iax supernova, which is an incomplete explosion. . As a result, the material ejected by the explosion did not completely escape and remained within the gravitational influence of the central white dwarf. “Over time, this material could have accumulated on the white dwarf due to its gravity, causing it to reignite.”
However, Zijlstra noted, that theory contrasts with observations that show the star’s brightness has dimmed over the last century.
“It’s not clear how that relates to wind ignition,” he said. “I would have expected the star to have brightened instead of dimmed.”
Ko and his colleagues are aware of this problem.
They said they believe there is some relationship between the wind and the darkening, and they are investigating it.
Researchers are preparing new observations of SN 1181 with two 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 discovering the accelerated expansion of the universe,” he said. “But despite its importance, its 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, the study of objects like SN 1181 represents a great opportunity, since they are important for the formation of many of the elements that human beings are also made of.
“These highly energetic phenomena 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 materials ejected, and perhaps 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 obtained these elements, Zijlstra added.
Historically, ancient observations of supernovae have been of utmost 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.
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Schaefer added that SN 1181 represents one of the few reliable connections between supernova and supernova remnant. The object is important as the only possible case to obtain good observations of the elusive Type Iax.
“It has been concluded that type Iax supernovae constitute approximately 20% of the supernovae in any galaxy, including our Milky Way, and could form most of the mysterious dust in the early Universe,” Schaefer explained in an email. electronic.
He added that astrophysicists will not have a better observed case of a type Iax event in our lifetime, so researchers should work to understand SN 1181.
