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In 2004, scientists from NASA’s Galaxy Evolution Explorer spotted an unusual object in our Milky Way. The object appears as a large, faint gas spot with a star in the center.
In the ultraviolet wavelengths used by the satellite, the blob appeared blue. Here’s the thing: despite having a blue color, the blob does not emit light.
Careful observations reveal that the blob has two large blue rings inside it. Hence, astronomers dubbed it the Blue Ring Nebula.
For 16 years, the nebula was studied using multiple ground and space telescopes, but the more they learned about it, the more mysterious it seemed.
A team of scientists, including Guðmundur Stefánsson of Princeton University, Henry Norris Russell Postdoctoral Fellow in Astrophysical Sciences, combined terrestrial observations with detailed theoretical models to study the properties of the object.
The nebula has two opposite sides, suggesting a bipolar outflow of material from its star TYC 2597-735-1. The spectrum of TYC 2597-735-1 and its proximity to the galactic plane suggests it is an old star, but it has abnormally low surface gravity and long-term detectable brightness decay, which is unusual for its evolutionary stage.
Its ring is made up of hydrogen gas. The properties of this system suggest that it is the remnant of two stars meeting their ultimate end: an inward orbital dance that led to the merger of the two stars. The result offers a new window into the fate of many closely orbiting binary star systems.
Stefánsson said, “We were in the middle of observing one night, with a new spectrograph we had recently built when we received a message from our colleagues about a particular object made up of a hazy gas rapidly expanding away from a central star. How was it formed? What are the properties of the central star? We were immediately thrilled to help solve the mystery! “
Most of the stars in the Milky Way are found in binary systems – pairs of stars orbiting one another. If they are close enough to each other, such systems can meet their end in a stellar fusion event: as stars evolve, they extend. If they are close enough to each other, one of the stars can engulf its orbiting companion, causing it to rotate inward until the two stars collide. When the companion loses its orbital energy, it can eject material at high speed.
Further evidence to support this hypothesis came from observations with two different spectrographs on large ground-based telescopes: the HIRES optical spectrograph on the 10-meter Keck telescope atop Maunakea in Hawaii, and the near-infrared habitable zone Planet Finder on the 10-meter Hobby-Eberly telescope at the McDonald Observatory in Texas, a new near-infrared spectrograph that Stefánsson helped design, build and commission to detect planets around nearby stars.
Stefánsson said, “The spectroscopic observations were instrumental in allowing us to further understand the object, from which we see that the central star is inflated, and we see signs of accretion probably from a surrounding debris disk.”
Keri Hoadley, postdoctoral fellow at Caltech and lead author of the paper, said: “Indeed, the spectroscopic data associated with the theoretical modeling shows that the Blue Ring Nebula is consistent with the image of a merging binary star system, suggesting that the inward spiraling companion was likely a low-mass star.” .
“The Blue Ring Nebula is the only object that allows an unobstructed view of the central stellar remnant, offering a clear window into its properties and providing clues to the merging process.”
“The Blue Ring Nebula is rare. It is fascinating that we were able to find it and are excited about the possibility of finding more similar items in the future. If so, this would allow us to gain more insight into the remnants of stellar mergers and the processes that govern them. “
Journal reference:
- Hoadley, K., Martin, DC, Metzger, BD et al. A blue ring nebula from a stellar merger several thousand years ago. Nature 587, 387–391 (2020). DOI: 10.1038 / s41586-020-2893-5
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