The cause of a light as bright as Saturn spotted in the night sky by 12th century astronomers has finally been discovered.
The light spotted 840 years ago by Chinese and Japanese observers was the result of a supernova triggered by the collision of two extremely dense stars in the Milky Way, according to a study.
The finding provides evidence of a rare form of collision between white dwarfs—the smoldering remnants of exhausted stars—and Type Iax supernova, marking only the second time such a phenomenon has been discovered in our galaxy.
A Type Iax supernova arises from the collision of two degenerate stellar objects like white dwarfs. They are dimmer than Type Ia supernovas which occur in binary systems with a normal star and a white dwarf.
An international team of astronomers discovered that the collision created the star known as Parker's Star and its nebula Pa 30, the shell of gas and dust that surrounds it.
The event was visible for 85 days from August 6, 1181 AD to February 6, 1182 AD and had until now been unexplained. The astronomers compared the light to Saturn in manuscripts, which the authors of the study said likely referred to its brightness.
Unaware of the existence of supernovas—massive stellar explosions commonly triggered when a star reaches the end of its lifetime—Chinese astronomers believed that such flashes of light in the sky were stars that appeared briefly then disappeared. They named these fleeting but brilliant flashes of light "guest stars."
Most of these guest stars have been identified and linked with their supernova cause, but the origins of the Chinese Guest Star of AD 1181, or SN 1181 to give it its modern name, have remained a mystery. In fact, until this research, it was the only "historical supernova" of the last 1,000 years not linked to a real event.
The authors of the study published in The Astrophysical Journal Letters wrote that another nebula, 3C 58, had previously been considered the remnant of the supernova behind AD 1181 due to its close proximity, but it was ruled out because of its age and close observations. The team said that this left "no other viable candidate known for the remnant."
That was until the team, which included researchers from China, Japan, the U.K., Spain, France, and Hungary, considered the nebula Pa 30. They found the gas and dust that comprise this nebula was spreading at 684 miles per second. This told them that the nebula was created around 1,000 years ago. Considering this and the nebula's distance from Earth puts it in the correct time frame to be spotted by early astronomers in 1181.
"When I realized we had an excellent candidate for this historic SNR of 1181 AD myself and my team, especially my postdoc Andreas Ritter who I had got to do most of the data reduction, were very, very excited," Quentin Parker, director of the Laboratory for Space Research at Hong University, who led the research, told Newsweek via email. "So we had nailed the REAL current counterpart to the old Chinese recorded SN 1181 'Guest star' explosion.
"This is a major deal as far as we are concerned with strong historic and scientific interest."
Pa 30 had previously been considered to be created as the result of a collision between two dense stars called white dwarfs. These degenerate stars form when smaller stars burn up their nuclear fuel and begin to dim towards the end of their lives. This results in the loss of their outer layers and a remaining core of dense material.
This type of collision results in a form of stellar explosion called Type Iax supernova, which is notable because it is dimmer than more common Type Ia supernovas.
If that is indeed the cause of this nebula and SN 1181, it marks only the second time such a collision has been discovered in our galaxy.
"Pa 30 now provides an excellent, viable candidate for the SN 1181 eruption that fits the location, the age, the brightness, and even the visible duration given its likely Type Iax nature," the authors said. "[SN 1181] is also the first recorded supernova Type Iax event in the Galaxy of which there are now perhaps fully two cases known."
For the team, future investigations could focus on the unique nature of the stars at the center of the nebula Pa 30, its nature, and its origins.
Parker said: "Much more work needs to be done on Parker's star itself. It has a unique spectrum."
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