The sharpest image yet of the Pa 30 nebula shows its firework streaks are chains of gas knots, each big enough to hold our planets' orbits ten times over.
Astronomers using the Gemini North telescope in Hawaii have captured the most detailed image yet of Pa 30, the remains of a supernova that observers in China, Japan and the Arab world recorded in 1181. The new view shows about ten times more detail than before, and the nebula's firework-like streaks turn out to be chains of gas knots, like pearls on a string.
Pa 30 came from a rare, incomplete Type Iax explosion, possibly two white dwarfs colliding, and it is the only known example in our galaxy. Its neat structure could help scientists understand how such stars die and find similar remnants.
In 1181, astronomers in China and Japan noticed a new star that hung in the sky for months. Nearly 850 years later, the Gemini North telescope has taken the sharpest picture yet of what that explosion left behind, and its wispy streaks turn out to be strings of giant pearls.
Why should you care about a cloud of gas 7,500 light-years away? Pa 30 is one of the few places where a medieval sky-watcher’s notes line up with a modern image of the same event. It also holds a “zombie star” that survived its own explosion, something astronomers still cannot fully explain. The new high-definition view gives them fresh clues.
Earlier pictures showed Pa 30 as a ball of long, thin streaks spraying outward like fireworks or a dandelion seed head. The new observations, made with Gemini North on Mauna Kea in Hawaii, reveal about ten times as many of those features as any previous image.
At this sharper resolution, the streaks are not smooth lines at all. They break into chains of glowing gas knots, like beads on a string, and the knots are remarkably similar in size.
To build the picture, the team tracked light from two kinds of charged gas: sulfur, which glows red, and oxygen, which shows up in blue-green. The sulfur light was what exposed the knotted chains. The study was published in The Astrophysical Journal.
Location helped too. Gemini North sits more than 4,200 meters up on Mauna Kea, above much of the air that blurs starlight, which is one reason the team could get the best-resolution images and analysis of Pa 30 so far.
Each knot is about four light-days across. A light-day is the distance light covers in a whole day, and light circles Earth more than seven times every second, so these are huge.
Study co-leader Ilaria Caiazzo, an assistant professor at the Institute of Science and Technology Austria, put it in solar system terms: the region where our planets orbit could fit inside each knot around ten times, with space left over. She called the knots strikingly uniform and said the team is eager to model how they formed.
That uniformity is the puzzle. The knots are probably clumps of material blasted out in the explosion, but supernova debris usually forms messy, irregular blobs. Neat, evenly sized knots lined up along filaments are something new, and explaining them could reveal how this kind of star dies.
Most thermonuclear supernovae destroy their star completely. Pa 30 did not. It was a rare, incomplete blast called a Type Iax supernova, and part of the star survived. That leftover is the zombie star.
Researchers think Pa 30 may have formed when two white dwarfs collided. White dwarfs are the dense, Earth-sized cores left after stars like the sun burn out. Pa 30 is the only known object of this kind in the Milky Way, and the only Type Iax remnant anywhere where telescopes can see the surviving star.
The new data held another surprise. The zombie star sits almost exactly in the middle of the nebula. Lopsided explosions usually kick a surviving star off to one side. Here, the measured sideways speed was so small that, as Caiazzo explained, the star barely appears to be moving away at all. That suggests the explosion was close to symmetrical.
Pa 30 was first spotted in 2013 by a citizen scientist, an amateur combing through survey images. Researchers later linked it to the “guest star” recorded independently in Chinese, Japanese and Arabic sources in 1181.
That makes it one of only five supernovae recorded before telescopes existed whose remnants had long gone unidentified. The medieval observers had no idea they were watching a star tear itself apart, but their notes helped modern scientists date the event.
Co-leader Tim Cunningham, now an assistant professor at the University of Warwick in the UK, said reliable records of exploding stars go back only about a thousand years, so similar remnants may still be hiding unnoticed in our galaxy.
Pa 30 is about 7,500 light-years from Earth, close by cosmic standards and much nearer than our galaxy’s center, about 26,000 light-years away. That closeness is why Gemini North can see so much detail. In distant galaxies, Type Iax supernovae appear only as points of light.
By learning what one of these remnants looks like up close, astronomers can build better tools to spot others in the huge data sets new telescopes produce. Cunningham said the findings will help researchers screen that flood of data. For the rest of us, it is a striking reminder that something people saw with their own eyes in the Middle Ages is still unfolding today.
Gemini North is half of the International Gemini Observatory, along with its twin, Gemini South, in Chile. It is partly funded by the US National Science Foundation and run by NSF NOIRLab.
It is the remnant of a rare Type Iax supernova seen from Earth in 1181, about 7,500 light-years away, with a surviving zombie star at its center.
Chains of gas knots along its filaments, each about four light-days wide and unusually uniform in size.
The Gemini North telescope on Mauna Kea in Hawaii.
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