How the nearby supernova left its mark on life on Earth

For six months in 1181, a dying star left a mark in the night sky. The spectacular object appeared as bright as Saturn in the vicinity of the constellation Cassiopeia, and historical records from China and Japan recorded it as a "guest star". Chinese astronomers used the term to refer to 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. Now, a new study has first described SN 1181 in detail by creating a computer model of the evolution of the supernova from immediately after the initial explosion to today. The research team compared the model to the archival observations of the telescope of its nebula – the giant cloud of gas and dust, visible to this day, which is the remains of the huge event. The researchers said the analysis strongly suggests that SN 1181 belongs to a rare class of supernova called Type IAX where thermonuclear ignition could be the result of not just one but two white dwarfs who collided violently but failed to explode completely, leaving behind "zombie stars."

Super Nova bombing failed

It took astronomers 840 years to solve the first big mystery of SN 1181, which was to locate it in the Milky Way. The dying star was the last supernova before the telescope without confirmed remnants, until 2021, when Albert Zelsstra, professor of astrophysics at the University of Manchester in England, followed it to a nebula in the constellation That Chair. Amateur astronomer Dana Bachek discovered the nebula in 2013 when searching the archives of NASA's Vast Field Infrared Scanning Explorer, or WISE. But Zelzstra, who was not involved in the new study, was the first to associate it with the supernova 1181. "During Covid's (peak), I spent a quiet afternoon sitting at home," Zielstra said. "I matched the supernova to the nebula using logs from ancient Chinese catalogs. I think this is generally accepted now - a lot of people have looked at it and agreed that it seems right. The nebula is about 7,000 light-years from Earth, and at its center is an Earth-sized rapidly rotating object called the White Dwarf – a dense dead star that has exhausted its nuclear fuel. This feature is unusual for the remnants of the supernova because the explosion should have wiped out the white dwarf. Zelstra and colleagues wrote a study in September 2021 about the discovery. The report suggested that supernova 1181 may belong to the supernova class of the evasive type IAX due to the presence of this white dwarf "zombie". In the more common Type I A supernova, the white dwarf that forms when a sun-like star exhausts its fuel begins to accumulate material from another nearby star. Many stars are found in pairs, or binary systems, unlike the Sun. The white dwarf collects material until it collapses under the influence of its own gravity, reignite nuclear fusion with a massive explosion that creates one of the brightest objects in the universe.

Unexplained stellar wind

Thanks to the knowledge we gained about where to search for the supernova 1181, and the suggestion that it may be an IAX-type remnant, Coe and his colleagues began working to uncover its remaining secrets. "By carefully tracking the evolution of the remnants over time, we were able to obtain detailed characteristics of the supernova explosion 1181 for the first time. We confirmed that these detailed characteristics are consistent with an IAX supernova," he said, adding that the computer model in the study is consistent with previous observations of remnants from telescopes, including the European Space Agency's XMM-Newton Space Telescope and NASA's Chandra X-ray Observatory. Coe's analysis shows that two distinct shock zones form the remnants of the 1181 supernova. An outer region formed when matter was taken out of the supernova explosion and met interstellar space. The newer interior is more difficult to interpret. The study suggests that this internal shock zone may be a sign that the star is starting to burn again, centuries after the explosion, leading to a surprising discovery. This fast stream of particles that astronomers call stellar wind usually explodes from the white dwarf as a by-product of the star's rapid rotation immediately after the supernova explodes.

The opportunity of a lifetime

Because objects like SN 1181 are important for making so many of the elements that make up humans as well, studying them represents a great opportunity, according to Zelsstra. "These highly active events can lead to the formation of elements heavier than iron, such as rare earth elements," he said. "It is important to have an example of such an event from 1,000 years ago where we can still see the projectile material, and maybe in the future we can see exactly the elements created in the event." Zigelstra added that this knowledge will help scientists understand how the Earth formed and obtain these elements. Bradley Schaefer, professor emeritus of astrophysics and astronomy at Louisiana State University, who was not involved in the latest study, said ancient observations of supernova have been of paramount importance to historically modern astrophysics. Schaefer added that SN 1181 represents one of the few reliable links between the supernova and the supernova remnant. This goal is important as the only possible case for obtaining good feedback for the dodgy IAX type.