Go outside on the next clear night and look to the north, and you’ll see the famous W-shaped constellation Cassiopeia. Once you’ve seen that there are five clear stars in a familiar shape, you’ll never be able to see it, not only because it’s so recognizable, but because it’s a circumpolar constellation—it orbits the North Star—so it’s almost always on it northern hemisphere is the night sky. If a sixth bright star suddenly appeared, you would notice, and so would every stargazer on the planet.
That’s exactly what happened in the year 1181, when astronomers in China and Japan recorded a so-called guest star that appeared in Cassiopeia for 185 days, after which it disappeared forever. “The guest star was bright enough that three separate groups in China observed it within a couple of days of each other, and it was also seen in Japan,” said Robert Fesen, professor of physics and astronomy at Dartmouth College in the UK and leader. author of an article soon to be published in the journal Astrophysical Journal Letters. “To the ancients, their television set was the sky, so they would have easily noticed and certainly recorded the sudden appearance of a bright new star in the sky.”
What was that? It was almost certainly a supernova, an exploding star, or a collision between two stars, which is incredibly rare to observe. In fact, only nine such guest stars have ever been observed throughout human history, most of them in ancient times.
Finally, it seems that the source of the guest star of 842 years ago has been found. Announced and shown by Fesen at last week’s 241st meeting in American Astronomical Society was a new telescope image showing a fireworks-like display of filaments radiating from a very strange star at the center of an object called Pa 30 in—you guessed it—the constellation Cassiopeia.
“I’ve never seen any object — and certainly no supernova remnant in the Milky Way — that looks quite like this, and neither have any of my colleagues,” Fesen said. “To the ancient astronomers, the new star would have been nearly as bright as or brighter than Vega, the fifth brightest star in the sky as seen from Earth.
Pa 30 is a fog and has been known for years. In fact, it was first observed in 2013 by amateur astronomer and co-author Dana Patchik, but only ever imaged before as a very faint and diffuse object. It is a dense area with illuminated gas, dust and other material.
The research confirms that Pa 30 is indeed the source of the 1181 Far East supernova scene.
Currently expanding at about 2.4 million mph, scientists were able to rewind the cosmic clock to confirm that yes, the source would have exploded in the 12th century. “Our new observations put a much tighter constraint on the object having an expansion age of about 850 years, which is perfect for it to be the remnants of the 1181 guest star,” Fesen said.
Equally important, this new research reveals that Pa 30 is rich in sulfur and organics and contains virtually no hydrogen or helium. It was revealed using new sulfur-sensitive filters attached to the 2.4-meter Hiltner telescope at the MDM Observatory adjacent to Kitt Peak National Observatory, Arizona, from which three new 2000-second exposures of Pa 30 were taken .
Pa 30 has been the subject of several papers over the past decade, but the revelation of its main elements is the smoking gun that it is actually likely the result of a collision between two white dwarf stars.
A white dwarf star is what our own sun will one day end up as. Faint, extremely dense stars no larger than our planet but containing the mass of the Sun-like star, collisions of white dwarf stars are thought to sometimes cause supernova explosions.
“Our deeper images show that Pa 30 is not only beautiful, but now that we can see the true structure of the nebula, we can examine its chemical composition and how the central star generated its remarkable appearance, and then compare these properties with predictions from specific models of rare white dwarf mergers, Fesen said. “This remnant will allow astronomers to study a particularly interesting type of supernova that until now they could only investigate from theoretical models and examples in distant galaxies.”
Wishing you clear skies and big eyes.