In 2004, astronomers using NASA’s Galaxy Evolution Explorer discovered an unusual, ring-shaped ‘blue’ nebula and the star at its center, TYC 2597-735-1. New observations of the object suggest that TYC 2597-735-1 merged with its lower-mass stellar companion less than 5,000 years ago and created a bipolar outflow of material.

The Blue Ring Nebula is located 6,197 light-years away in the constellation of Hercules. The nebula is 13 light years wide and is composed of 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. Image credit: NASA / JPL-Caltech / M. Seibert, Carnegie Institution for Science / K. Hoadley, Caltech / GALEX Team.

The Blue Ring Nebula is located 6,197 light-years away in the constellation of Hercules. The nebula is 13 light years wide and is composed of 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. Image credit: NASA / JPL-Caltech / M. Seibert, Carnegie Institution for Science / K. Hoadley, Caltech / GALEX Team.

“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 co-author Dr. Guðmundur Stefánsson, a postdoctoral researcher in the Department of Astrophysical Sciences at Princeton University.

“How did it form? What are the properties of the central star? We were immediately excited to help solve the mystery!”

Most stars in the Milky Way are in binary stellar systems — pairs of stars orbiting each other. If they are close enough together, such systems can meet their demise in a stellar merging event.

To test this hypothesis, Dr. Stefánsson and colleagues observed the Blue Ring Nebula with two different spectrographs on large ground-based telescopes: the HIRES optical spectrograph on the 10-m Keck Telescope and the near-infrared Habitable-zone Planet Finder on the 10-m Hobby-Eberly Telescope at McDonald Observatory.

“The spectroscopic observations were key in allowing us to understand the object further, from which we see that the central star is inflated, and we see signatures of accretion likely from a surrounding disk of debris,” Dr. Stefánsson said.

“Indeed, the spectroscopic data coupled with theoretical modeling shows that the Blue Ring Nebula is consistent with the picture of a merging binary star system, suggesting that the inwards spiraling companion was likely a low-mass star,” said lead author Dr. Keri Hoadley, a postdoctoral researcher in the Cahill Center for Astrophysics at Caltech.

Although the relics of a few such binary merging events have been observed before, all such objects have been enshrouded by opaque dust and clouds, obstructing the view of the properties of the central stellar remnant.

The Blue Ring Nebula is the only object allowing an unobstructed view of the central stellar remnant, offering a clear window into its properties and yielding clues about the merging process.

“The Blue Ring Nebula is rare. As such, it is really exciting that we were able to find it, and we are excited about the possibility of finding more such objects in the future,” Dr. Hoadley said.

“If so, that would allow us to gain further insights into the remnants of stellar mergers and the processes that govern them.”

“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 co-author Dr. Mark Seibert, an astrophysicist at the Carnegie Institution for Science and a member of the GALEX team at Caltech.

“We see plenty of two-star systems that might merge someday, and we think we’ve identified stars that merged maybe millions of years ago,” said co-author Dr. Brian Metzger, a researcher in the Department of Physics at Columbia University and the Center for Computational Astrophysics at Flatiron Institute.

“But we have almost no data on what happens in between.”

“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.”

“The merger of two old stars explains the seemingly conflicting observations about the central star’s youthful luminosity and aged chemical makeup,” said co-author Dr. Andrew McWilliam, an astrophysicist at the Carnegie Institution for Science.

The team’s work was published in the November 19, 2020 edition of the journal Nature.

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K. Hoadley et al. 2020. A blue ring nebula from a stellar merger several thousand years ago. Nature 587, 387-391; doi: 10.1038/s41586-020-2893-5

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