“We were in the middle of observing one night, with a new spectrograph that we had recently built, when we received a message from our colleagues about a peculiar object composed of a nebulous gas expanding rapidly away from a central star,” said Princeton University astronomer Guðmundur Stefánsson, a member of the team that discovered a mysterious object in 2004 using NASA’s space-based Galaxy Evolution Explorer (GALEX) . “How did it form? What are the properties of the central star? We were immediately excited to help solve the mystery!”
A 16-Year-Old Mystery
NASA announced today that it has solved the 16-year-old mystery of the object –similar in size that of a supernova remnant–unlike any they’d seen before in our Milky Way galaxy: a large, faint blue cloud of gas in space with a living star at its center. Subsequent observations revealed a thick ring structure within it, leading to the object being named the Blue Ring Nebula.
In a new study, published in the journal Nature, a team of scientists at California Institute of Technology and Columbia University offers an explanation for the formation of the blue ring around the star, named TYC 2597-735-1.
Over the next 16 years, says NASA ,they studied it with multiple Earth- and space-based telescopes, but the more they learned, the more mysterious it seemed until a new study, published online on Nov. 18 in the journal Nature suggests that using cutting-edge theoretical models the nebula is likely composed of debris from two stars that collided and merged into a single star. consists of two hollow, cone-shaped clouds of debris moving in opposite directions away from the central star. The base of one cone is traveling almost directly toward Earth. As a result, says NASA, astronomers looking at the nebula see two circles that partially overlap.
The Missing Link
While merged star systems are thought to be fairly common, they are nearly impossible to study immediately after they form because they’re obscured by debris the collision kicks up. Once the debris has cleared – at least hundreds of thousands of years later – they’re challenging to identify because they resemble non-merged stars. The Blue Ring Nebula appears to be the missing link: astronomers are seeing the star system only a few thousand years after the merger, when evidence of the union is still plentiful vs hundreds of thousands of years later when they because they resemble non-merged stars.. It appears to be the first known example of a merged star system during its early stage.
GALEX , Operational between 2003 and 2013 and managed by NASA’s Jet Propulsion Laboratory, was designed to help study the history of star formation throughout most of the universe by taking a census of young star populations in other galaxies in both near-UV light (wavelengths slightly shorter than visible light) and far-UV. Most objects seen by GALEX radiated both near-UV (represented as yellow in GALEX images) and far-UV (represented as blue). The Blue Ring Nebula appeared unique because it emitted only far-UV light.
The object’s size was similar to a planetary nebula, the puffed-up remains of a star the size of our Sun. But planetary nebulas like supernova remnants radiate in multiple light wavelengths outside the UV range, while further research showed that the Blue Ring Nebula did not.
In 2006, reveals NASA, “the GALEX team looked at the nebula with the 200-inch (5.1-meter) Hale telescope at the Palomar Observatory in San Diego County, California, and then with the even more powerful 10-meter (33-foot) telescopes at the W.M. Keck Observatory in Hawaii. They found evidence of a shockwave in the nebula, suggesting the gas composing the Blue Ring Nebula had indeed been expelled by some kind of violent event around the central star. Keck data also suggested the star was pulling a large amount of material onto its surface. But where was the material coming from?”
Giant Phantom Planet?
“For quite a long time we thought that maybe there was a planet several times the mass of Jupiter being torn apart by the star, and that was throwing all that gas out of the system,” said Mark Seibert, an astrophysicist with the Carnegie Institution for Science and a member of the GALEX team at Caltech, which manages JPL.
In 2012, using the first full-sky survey from NASA’s Wide-field Infrared Survey Explorer (WISE), a space telescope that studied the sky in infrared light, the GALEX team identified a disk of dust orbiting closely around the star. Archival data from three other infrared observatories, including NASA’s Spitzer Space Telescope, also spotted the disk. The finding didn’t rule out the possibility that a planet was also orbiting the star, but eventually the team would show that the disk and the material expelled into space came from something larger than even a giant planet.
Then in 2017, says the JPL, the Habitable Zone Planet Finder on the Hobby-Eberly Telescope in Texas confirmed there was no compact object orbiting the star..
After studying 10 years of data collected from four space telescopes, four ground-based telescopes, historical observations of the star going back to 1895 to look for changes in its brightness over time, and with the help of citizen scientists through the American Association of Variable Star Observers (AAVSO), The Blue Ring Nebula is thought to be the product of two stars merging into one. The collision of the bodies ejected a cloud of hot debris into space. A disk of gas orbiting the larger star cut the cloud in half, creating two cones that are moving away from the star in opposite directions.
By the time Keri Hoadley, an astrophysicist at Caltech, began working with the GALEX science team in 2017, “the group had kind of hit a wall” with the Blue Ring Nebula, she said. But Hoadley, was fascinated by the object and its bizarre features, so she accepted the challenge of trying to solve the mystery. It seemed likely, she says that the solution would not come from more observations of the system, but from cutting-edge theories that could make sense of the existing data.
Metzger, who mathematical and computational models of cosmic phenomena, which can be used to predict how those phenomena will look and behave, specializes in cosmic mergers – collisions between a variety of objects, whether they be planets and stars or two black holes.
With Metzger on board and Hoadley shepherding the work, things progressed quickly.
“It wasn’t just that Brian could explain the data we were seeing; he was essentially predicting what we had observed before he saw it,” said Hoadley. “He’d say, ‘If this is a stellar merger, then you should see X,’ and it was like, ‘Yes! We see that!'”
“Stellar mergers are a relatively common occurrence in our Galaxy,” said Metzger, a theoretical astrophysicist and professor in the Department of Physics at Columbia, who is a lead author of the Nature paper. “But this is the first time we’ve discovered such a merger at this critical, revealing stage in its evolution.”
The team, says NASA, concluded that “the nebula was the product of a relatively fresh stellar merger that likely occurred between a star similar to our Sun and another star only about one-tenth that size (or about 100 times the mass of Jupiter). Nearing the end of its life, the Sun-like star began to swell, creeping closer to its companion. Eventually, the smaller star fell into a downward spiral toward its larger companion. Along the way, the larger star tore the smaller star apart, wrapping itself in a ring of debris before swallowing the smaller star entirely.
“This was the violent event that led to the formation of the Blue Ring Nebula. The merger launched a cloud of hot debris into space that was sliced in two by the gas disk. This created two cone-shaped debris clouds, their bases moving away from the star in opposite directions and getting wider as they travel outward. The base of one cone is coming almost directly toward Earth and the other almost directly away. They are too faint to see alone, but the area where the cones overlap (as seen from Earth) forms the central blue ring GALEX observed.
Millennia passed. The expanding debris cloud cooled and formed molecules and dust, including hydrogen molecules that collided with the interstellar medium, the sparse collection of atoms and energetic particles that fill the space between stars. The collisions excited the hydrogen molecules, causing them to radiate in a specific wavelength of far-UV light. Over time, the glow became just bright enough for GALEX to see.
Stellar mergers may occur as often as once every 10 years in our Milky Way galaxy, meaning it’s possible that a sizeable population of the stars we see in the sky were once two.
“We see plenty of two-star systems that might merge some day, and we think we’ve identified stars that merged maybe millions of years ago. But we have almost no data on what happens in between,” said Metzger. “We think there are probably plenty of young remnants of stellar mergers in our galaxy, and the Blue Ring Nebula might show us what they look like so we can identify more of them.”
“It’s amazing that GALEX was able to find this really faint object that we weren’t looking for but that turns out to be something really interesting to astronomers,” said Seibert. “It just reiterates that when you look at the universe in a new wavelength or in a new way, you find things you never imagined you would.”
Source: “A blue ring nebula from a stellar merger several thousand years old,” by Keri Hoadley, Christopher Martin, Brian Metzger, Mark Seibert, Andrew McWilliam, Ken Shen, James Neill, Guðmundur Stefánsson, Andrew Monson and Bradley Schaefer, appears in the Nov. 19 issue of Nature.
Image credit top of page: composed of expanding hydrogen gas (blue) expanding from a central star, which is the remnant core of a stellar merger. Red filaments are shockwave filaments from the merging event. Credit: NASA/JPL-Caltech/M. Seibert (Carnegie Institution for Science)/K. Hoadley (Caltech)/