17th-Century Astronomers May Have Watched Stars Collide

A historic explosion, seen in the sky across Europe in the 17th century, has turned out to be a case of mistaken identity.

For decades, scientists thought the explosion was a classical nova, a stellar outburst that’s less catastrophic than a supernova. But a new look at the remnant left behind by the explosion reveals a more violent history, astronomers report Monday in Nature. Now called CK Vulpeculae, the explosion was what’s called a red transient, produced when two stars violently collide.

Red transients are thought to be a relatively rare type of stellar smashup; based on the chemical elements produced in the collision, this one appears to have been particularly violent, possibly even head-on, says study author Tomasz Kaminski, of the European Southern Observatory.

Swan Star 

In 1670, a new star appeared in the sky. Located near the head of Cygnus the Swan, the guest star was first spotted in June. It stuck around through the summer but faded in the fall.

In March 1671, the star reappeared—and over that summer, it shone brighter than ever before. Astronomers of the time, including lunar cartographer Hevelius and Giovanni Domenico Cassini (the namesake of NASA’s Saturn-exploring Cassini spacecraft), kept track of the star’s light until it vanished that October. It made one more feeble appearance in 1672 before disappearing for good.

At the time, astronomers didn’t know what they were looking at. “There would still have been controversy as to whether the nova was in the starry realms or in the Earth's atmosphere,” says Owen Gingerich, professor emeritus of astronomy and the history of science at the Harvard-Smithsonian Center for Astrophysics.

Because new stars and comets were clearly unstable, Gingerich says, astronomers at the time believed they could not be made of the incorruptible celestial ether from which Aristotle said the distant stars were fashioned. 

That view would soon change. By the next century, when Halley’s comet reappeared as predicted, “finally a majority of educated people accepted that the Earth was moving around a fixed sun, and novae were seen as distant stars,” Gingerich says.

Mystery Star

For more than three centuries, the true nature of the explosion would remain a mystery. During that time, CK Vulpeculae was thought to be the oldest classical nova on record.

But the region of the sky where the star appeared held frustratingly few clues about what had taken place. Whatever was there was almost too dim to be seen—until the 1980s, when Michael Shara of the American Museum of Natural History, in New York City, and his colleagues took a close look at the spot and discovered a dim, dumbbell-shaped nebula at the exact coordinates.

In 2014, Kaminski and his colleagues pointed three telescopes at CK Vulpeculae. After a few months of observation, they had a perplexing set of data in hand. “I was sure that whatever CK Vulpeculae turned out to be, it would undoubtedly be a very special object,” Kaminski says.

Observations with the APEX Telescope indicated that the CK Vulpeculae remnant had a strange chemical fingerprint: Most significantly, it held a surprisingly large amount of heavy nitrogen, which is an atomic form, or isotope, with an extra neutron.

There were also electrically charged molecules that don’t normally live in nova remnants, and which suggested the region was “hostile to molecules,” the authors wrote in Nature.

The team realized it wasn’t looking at a classical nova at all. Instead, it seemed the fingerprint—and the historical observations—best matched a rare type of violent stellar explosion known as a red transient.

Shara isn’t convinced. He says that while the observations are excellent, the isotopic ratios don’t definitively rule out a slow classical nova, the type of explosion he and his colleagues identified in the 1980s. “CK Vulpeculae might have been a stellar merger, or a very slow nova,” says Shara. “The Nature paper makes a good case for a merger, but is by no means definitive.”

Rare Collision

Red transients are formed when two stars collide, exploding with a brightness that lies somewhere between a nova and a supernova. It’s not clear what kind of star is produced in the merger, but the remnant quickly cools and fades.

Though CK Vulpeculae may be a red transient, Kaminski says, some of the charged molecules observed near the remnant don’t quite match more recently observed explosions.

The discrepancy could be due to the remnant’s age. Other red transients observed have been young, born within the last few decades, while 1670 CK Vulpeculae has had more than three centuries to evolve.

Red transients are thought to be a rare type of stellar explosion, but several have appeared in the Milky Way in the past decade: notably, V838 Mon in 2002—which is associated with a spectacular light echo—and V1309 Sco, which Kaminski and his colleagues used as a type of Rosetta stone for translating the molecular species produced in these explosions.    

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