Astronomers believe they have solved the 16-year-old mystery of a star surrounded by an enigmatic ring of ultraviolet light. If they’re right, the “Blue Ring Nebula” offers a glimpse of a fleeting phenomenon: a star still reeling from its birth via the merger of two other stars.
“Finding a bona fide merger event will be very helpful in developing our understanding of stellar mergers,” says astronomer Boris Gaensicke of the University of Warwick, who was not involved in the study. And because many, if not most, stars originate as binaries, mergers could drive the births of countless stars, says astronomer Morgan Fraser at University College Dublin, also not involved with the work. “There’s a lot we don't know about how stellar mergers work.”
The star, known as TYC 2597-735-1, and its ring were first discovered by the Galaxy Evolution Explorer (GALEX), a NASA mission that ended in 2013. At the time, GALEX Principal Investigator Chris Martin of the California Institute of Technology thought it was an interesting object that would get researchers a quick paper once they’d figured it out. But the more he and his colleagues looked at it, the more puzzled they became.
The ring turned out to be two cone-shaped blasts of material: one moving toward Earth at 400 kilometers per second, the other away from us at the same speed. The star seemed old because it was lacking in hydrogen fuel. But it also emitted a lot of infrared light, suggesting the presence of a disk of glowing hot dust around it—the sign of a young star. It was like “a Sherlock Holmes mystery,” Martin told reporters at a teleconference yesterday.
In 2017, astronomer Keri Hoadley joined Martin’s team and took up the case. “It was my second day on the job and I was immediately hooked,” she said at the teleconference. One suspicion was the star was tearing apart a planet that had drifted too close. But with data from a planet-finding instrument on the Hobby-Eberly Telescope in Texas, Hoadley was able to rule out that scenario.
With so much conflicting information, the scientists brought in theorist Brian Metzger of Columbia University to help make sense of it. He suggested the team was witnessing events just several thousand years after a stellar merger.
Typically, blast debris from a merger obscures the star so observers can’t see what’s going on. In this case, the GALEX team caught it just as the debris clouds had thinned out enough to reveal the merged star, Metzger theorized, but were not so dispersed as to become invisible. Because observers had never seen a merged star at that stage before, the team hadn’t recognized what it was. “All the paradoxical data sets fell into place,” Hoadley said.
The researchers then collaborated with Ken Shen of the University of California, Berkeley, to produce a stellar evolution model that fit their data. Today in Nature, they describe what they believe happened. A Sun-like star, having burned up all its hydrogen fuel, began to swell up, and a smaller star in orbit with it started to siphon off some of its outer material, which settled into a disk around the smaller star. The smaller star spiraled in and merged with its swelling partner, producing a blast of material. But the disk around the smaller star blocked some of the blast, channeling material into two debris cones, one directed toward Earth, the other away. When those cones struck clouds of gas that exist in the space between stars, the shock heated hydrogen molecules in debris causing them to fluoresce with an ultraviolet glow.
“It’s kind of unique—one of a kind right now,” team member Mark Seibert of the Carnegie Institution for Science said at the teleconference. He believes the finding will help astronomers understand the transition from a merged pair of stars to the weird stars seen millions of years later. “It’s the Rosetta Stone of that process.”