In June of 1670 - almost exactly 350 years ago - French astronomer Voituret Anthelme recorded a 'new' star in the sky. In the Northern constellation of Vulpecula, a pinpoint of light flared into brightness before gradually fading from view with the naked eye over a year later.
The event was classified as a nova, a cataclysmic stellar event, and in recent years, astronomers have been working to uncover what caused it. But a brand new discovery has thrown a spanner into the works.
According to observations taken with the Gemini Observatory, the star, CK Vulpeculae, is actually five times farther away than astronomers thought. This means that the explosion was much more powerful than astronomers thought, too - up to 25 times more energetic. And the cloud of glowing ejected material surrounding it is also expanding into space at a much faster rate.
Since 2015, astronomers have been exploring the possibility that the CK Vulpeculae nova was the result of two stars colliding. The first evidence, led by Tomasz Kamiński of the ESO and the Max Planck Institute for Radio Astronomy in Germany, was the unusually high mass and chemical composition of the gas surrounding the star, which scientists found was consistent with a cataclysmic merger of two main-sequence stars.
In 2018, two follow-up studies emerged. Kamiński and his team found a radioactive isotope of aluminium that had long been sought in interstellar space, linking its production to stellar mergers.
And another team, led by astronomer Stewart Eyres of the University of New South Wales in Australia, looked at the chemical composition of the CK Vulpeculae nebula and found molecules that couldn't have survived a collision between two main-sequence stars. This, they said, suggested a different event - a collision between a white dwarf star and a brown dwarf.
That aluminium isotope warranted closer inspection, so an international team of astronomers, co-led by Dipankar Banerjee of the Physical Research Laboratory in India, Tom Geballe of Gemini Observatory and Nye Evans of Keele University in the UK deployed the Gemini North's near-infrared spectroscope to get a better look, imaging the entire nebula in infrared.
That's when things started to look a bit peculiar. Specifically, the outer edges of the CK Vulpeculae nebula, a bi-lobed gaseous structure blasting away from the faint radio source (the star, or what's left of it) inside it. The iron atoms in these edges displayed redshift and blueshift, the apparently lengthening or shortening of wavelengths of light as it travels away from or towards the viewer.
These shifts were much more pronounced than expected, suggesting that the cloud of material was expanding much faster than previous measurements.
"We did not suspect that this is what we would find," Banerjee said. "It was exciting when we found some gas traveling at the unexpectedly high speed of about 7 million kilometres (4.3 million miles) per hour. This hinted at a different story about CK Vulpeculae than what had been theorised."
If the material is moving faster than thought, that means that the object should also be larger than we thought, so the team got to work. They carefully studied the nebula's velocity and rate of expansion over 10-year baseline, and position in the sky, and determined that the object is around 10,000 light-years away.
That's a lot farther than previous calculations that placed it around 1,630 light-years away. And if the object is a lot farther, it would have to have been a lot more energetic to produce light visible from Earth in 1670; around 25 times more energetic than previous estimates, according to the team's calculations.
That's more energy than a nova is capable of producing. Which means it's back to the drawing board to figure out what produced the explosion.
"In terms of energy released, our finding places CK Vulpeculae roughly midway between a nova and a supernova," Evans said. "It is one of a very few such objects in the Milky Way and the cause - or causes - of the outbursts of this intermediate class of objects remain unknown. I think we all know what CK Vulpeculae isn't, but no one knows what it is."
These rare objects are known as Intermediate Luminosity Optical Transients (ILOTs), and it's unclear what causes them. Several papers have suggested that they are produced by binaries, where at least one of the stars is a giant, but that hypothesis has not been absolutely confirmed.
The researchers have not delved into the possible origins of CK Vulpeculae. That will require further study - but doing so may help solve the mystery of ILOTs into the bargain.
"It is difficult at this stage to offer a definitive or compelling explanation for the origin of the 1670 eruption of CK Vulpeculae," Banerjee said. "Even 350 years after Voituret's discovery, the nature of the explosion remains a mystery."
The research has been accepted into The Astrophysical Journal Letters, and is available on arXiv.