As the constellation Cygnus glides gracefully along the Milky Way every late spring and summer, the cosmic bird’s left wing houses a faint binary star called KIC 9832227. The two stars spinning around one another are merging, on a path to an explosion that will result in a red nova, said Molnar and his colleagues.
For KIC 9832227, the orbital period is currently just under 11 hours, he said, and “as that period gets shorter, we infer that the separation between the stars is getting smaller. Hence they are spiraling in together.”
The astronomers first presented this star’s red nova prognostication at the January 2015 American Astronomical Society meeting, but the predictions teemed with unknowns. “The core of this [new] scientific presentation is that we have done two strong tests and that our hypothesis [from 2015] is holding up,” he said. “We have eliminated the alternative interpretations and we have also refined the predicted time to 2022, plus or minus one year.”
To refine the prediction, the astronomers examined a recent red nova — a star called V1309 Scorpii, discovered in September 2008. Using V1309 Scorpii research (conducted by Polish astronomer Romuald Tylenda) as a sort of cosmological blueprint, Molnar and colleagues found similar characteristics — from before the earlier explosion occurred.
Like the current candidate, V1309 Scorpii was a contact binary star and — similarly — its orbiting period (the time it took for the two stars in the binary system to spin around one another) decreased. Scientists had noted a changing light curve, all evidence of impending eruption.
This quest to comprehend red novas began when Molnar and then-Calvin College student Daniel Van Noord attended a presentation by astronomer Karen Kinemuchi of Apache Point Observatory in New Mexico, that asked if KIC 9832227 was a pulsing or binary star. Van Noord, who has since graduated, began dedicated observations at the Calvin observatory, where he found the orbital period shrinking.
The two spinning stars share a communal atmosphere “like two peanuts sharing a common shell,” Molnar said.
Beyond what humans can see with the unaided eye, KIC 9832227 is currently a 12th magnitude object, which means it's pretty dim. When it explodes sometime around 2022, “we expect it will reach second magnitude at the brightest,” Molnar said, where it will be among the brighter stars in the sky.
“We truly favor the merger hypothesis. … Now is the time to broaden our work and study the system more fully to … know what leads to a stellar explosion,” Molnar said. “Like many science stories, this one is gradually unfolding.”