Astronomers Crack the Case of the Blue Ring Nebula

The source of the dazzling display puzzled scientists for more than a decade

A photo of the blue ring nebula in space
The Blue Ring Nebula is invisible to the human eye, but telescopes can measure the ultraviolet light, which is shown here in blue. NASA/JPL-Caltech/M. Seibert (Carnegie Institution for Science)/K. Hoadley (Caltech)/GALEX Team

About 6,200 lightyears from Earth, a mysterious star appears surrounded by a doughnut of foggy blue light. It’s an old-looking star sitting in a young star’s dust cloud, and scientists have been trying to figure out how it formed since they first spotted it in 2004. Now, a team of astronomers says they’ve cracked the case, Monica Young reports for Sky & Telescope.

In a paper published on November 18 in the journal Nature, the research team explains how a collision of two stars several thousand years ago would create the structure observed today. It’s currently the only known example of a two-star collision that’s in the middle of transitioning from its debris-strewn initial stage to the late stage when the debris would become invisible.

“It’s kind of unique—one of a kind right now,” said Carnegie Institution for Science astrophysicist Mark Seibert of the Carnegie Institution for Science at a teleconference, Daniel Clery reports for Science magazine. The new understanding of the blue ring nebula may help astronomers understand other merged stars, he says, adding, “it’s the Rosetta Stone of that process.”

The strange star is not surrounded by a blue ring after all. Instead, it's flanked by a pair of cones that face outward, like megaphones pointing in opposite directions. Each cone is too faint to be observed on its own, but because one cone is lined up behind the other from Earth’s point of view, telescopes like NASA’s Galaxy Evolution Explorer (GALEX) could make out the shape of a blue ring.

Astronomers initially thought that the structure may have formed when a planet many times the size of Jupiter fell into a star and got torn apart. But in 2012, researchers discovered a disk of dust orbiting around the central star. The disk blasted the tips off of the cones and launched them outward; one is hurtling toward Earth at about 250 miles per second. By the disk’s size, astronomers knew it came from something much larger than a planet, reports Inverse’s Passant Rabie.

California Institute of Technology astrophysicist Chris Martin described the problem as “a Sherlock Holmes mystery,” at the teleconference, per Science magazine. In 2017, the researchers contacted Columbia University theorist and astronomer Brian Metzger for help.

Metzger suggested that the blue ring nebula, with its strange cones and dust disk, could represent the events just a few thousand years after the collision of two stars.

As described in the Nature paper, the evidence suggests that one of the stars was about the size of Earth’s Sun, and it began to expand as it reached the end of its lifecycle. As it grew, it got closer to a star about one-tenth its size. The large star’s gravity pulled in the smaller neighbor, and the two collided, throwing up a cloud of debris and a blast that cut the dust cloud in half, per Sky & Telescope.

"It wasn't just that [Metzger] could explain the data we were seeing; he was essentially predicting what we had observed before he saw it," says CalTech astrophysicist Keri Hoadley in a NASA statement. "He'd say, 'If this is a stellar merger, then you should see X,' and it was like, 'Yes! We see that!'"

As thousands of years passed, the debris cloud around the merged star cooled into molecules, including hydrogen molecules, which became energized over time until they began to radiate ultraviolet light. As that glow grew brighter, and because the two ones overlapped, GALEX could observe them. And because most stars originate as binaries that collide, the blue ring nebula fills in an important midpoint of the timeline from massive debris cloud to clear, shining star.

University of Warwick astronomer Boris Gaensicke, who wasn’t involved in the study, tells Science magazine, “Finding a bona fide merger event will be very helpful in developing our understanding of stellar mergers,” which make up most of the stars in the Milky Way.