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The Oldest Record Nova Wasn't a Star Exploding, It Was Two Stars Colliding

The only thing more exciting than one star exploding? Two stars exploding.
Remnant of Nova Vul 1670. Image: Tomasz Kaminski

In 1670, a bright nova burst into existence between the constellations Cygnus and Vulpecula. Skygazers across the world marveled at the brilliant outburst, known as CK Vulpeculae, and recorded its odd behavior over the next two years until it finally dimmed to invisibility. The blast has since racked up major historical cred as both the oldest and faintest recorded nova in history.

This chart of the position of a nova (marked in red) that appeared in the year 1670 was recorded by the famous astronomer Hevelius. Credit: Royal Society

But there's a twist: CK Vulpeculae wasn't actually a classical nova—an event driven by the explosion of white dwarf stars, producing a supernova-lite. The outburst appears to have been fueled by the only cosmic event more exciting than one star exploding—two stars colliding, and then exploding. Called red transient outbursts or red novae, these cataclysmic mergers are very rare and are much brighter than classical novae.

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"In our galaxy, there are five objects that we are sure are red novae," astrophysicist Tomasz Kaminski told me over Skype. Kaminski is the lead author of a new paper about the discovery, which was published Monday in Nature.

These explosive collisions were only formally recognized as a new class of stellar explosion in 2007, but astronomers are already rapidly closing in on their mysteries. Every instance observed so far has included a similar pattern in the intensity of the light, defined by about three peaks in luminosity over an extended period of months or years.

According to Kaminski, these extremely bright periods represent the "grazing collisions" two stars undergo as they spiral ever closer to each other. "Our working hypothesis that this is very much related to the way the spiraling-in process works," he said.

The 17th century astronomers that described CK Vulpeculae noted a very similar light curve, supporting the notion that this was no ordinary nova. But the real smoking gun revealed by Kaminski's team was the isotopic composition of the explosion's fallout, which was parsed in unprecedented detail by the APEX radio telescope and Submillimeter Array in Chile.

"The advantage of using submillimeter telescopes is that different isotopes give you a separate signature in the spectrum, while in the optical and the infrared they all blend to one feature," Kaminski told me. "You can study this, and derive the isotopic ratios, which are very important."

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The researchers were especially primed to recognizing these contrasting signatures of novae, having just completed in-depth observations of 17 galactic novae using APEX. CK Vulpeculae simply didn't fit the profile of a solo explosion.

The remnant of Nova Vul 1670 is surrounded by a dusty torus (yellow), cool molecular gas (red) and hot atomic gas (blue). Credit: Tomasz Kaminski

"When we looked at the first spectra from APEX, it was very clear that we were looking at something unusual, even more unusual than the other red novae," said Kaminski. "If you compare the values we got from observations to those predicted by theory, it is very clear that [CK Vulpeculae] couldn't be a classical nova."

The isotopic ratios of carbon, nitrogen, and oxygen indicated something much that something much more catastrophic than a regular nova had produced the nebula surrounding the star today. In fact, the molecules seemed to indicate that the merger had dramatically spewed stellar guts all over the place.

"They fit very well to what is expected of a stellar interior," Kaminski said of the molecules detected in the nebula. "That's why we think the material that once was in the center of the star and took part in nuclear reactions was, in the merger, somehow dredged up or just splashed around and that's why we see it now in CK Vulpeculae."

Moreover, the fact that so much dust and gas had developed in the 345 years since the event's light reached Earth suggests that the nebula must be fairly cold. That's another calling card left behind by stellar mashups, as opposed to the high temperatures associated with solo explosions.

"The characteristic thing about red novae is that after the explosion they become very cool, in an astronomical sense," Kaminski said. "They reach temperatures of about [2000] Kelvin and the surrounding material is of even lower temperatures. So discovering molecules around CK Vulpeculae gives you more evidence that this object had to cool down at some stage and develop all the molecules and dust."

CK Vulpeculae's reclassification as a red nova only enhances its intrigue. Now, astronomers know what kind of long-term cosmic wreckage these collisions leave behind, in comparison to the five recent mergers discovered in the last two decades.

"Twenty years ago, no one really cared about stellar mergers," Kaminski told me. "They were predicted to happen, but no one believed you could observe them in real time. This is an achievement of the last few years—that we realize that we can actually observe these things happen in real time."

Correction: This story originally quoted a source as saying red novae reach temperatures of 200 degrees Kelvin, which was the correct quote but incorrect stat; the source later clarified that red novae reach temperatures of 2000 degrees Kelvin.