The Blue Ring Nebula, which stumped scientists when discovered in 2004 for its rarity, appears to be the youngest known example of two stars fused into one.

16 years ago, scientists on NASA’s Galaxy Evolution Explorer mission detected a different object everyone they had seen in the Milky Way: a large, faint blob of gas that seemed to have a star at its center. At the ultraviolet wavelengths used by the satellite, the blob appeared blue, though it doesn’t actually emit light visible to the human eye, and careful observations identified two thick rings within it, which is why the team dubbed it the Blue Ring Nebula. . Since then, they studied it with multiple ground and space telescopesBut the more they learned about him, the more mysterious he seemed.

A team of scientists has now combined ground-based observations with detailed theoretical models to investigate the properties of the object. The article describing their findings appears in Nature.

“We were in the middle of an observation one night, with a new spectrograph that we had recently built, when we received a message from our colleagues on a peculiar object composed of a nebulous gas that was expanding rapidly from a central star, “Stefansson said in a statement.” How did it form? What are the properties of the central star? We were immediately excited for help solve the mystery!”.

Most of the stars in the Milky Way are in binary systems – pairs of stars orbiting each other. If close enough, such systems can find their demise in a stellar fusion event: As the stars evolve, they expand, and if they are close enough, one of the stars can engulf its orbiting companion, causing the companion to spiral inward, until the two stars collide. As the partner loses its orbital energy, it can eject material at high speeds.

Could that explain the mysterious Blue Ring Nebula?

More evidence in support of this hypothesis came from observations with two different spectrographs in large telescopes on the ground: the HIRES optical spectrograph on the 10-meter Keck Telescope on top of Maunakea in Hawaii, and the Near-Infrared Habitable Zone Planet Finder on the Observatory’s 10-meter Hobby-Eberly Telescope McDonald’s in Texas, a new near-infrared spectrograph that Stefánsson helped design, build, and commission to detect planets around nearby stars.

“The spectroscopic observations they were key to allowing us to better understand the object, from which we see that the central star is inflated and we see accretion signatures probably from a surrounding disk of debris, “said Stefansson.

“In fact, spectroscopic data coupled with theoretical modeling shows that the Blue Ring Nebula is consistent with the image of a binary star system in fusion, suggesting that the inward-spiraling companion was likely a low-mass star, ” Keri Hoadley assured, postdoctoral fellow at Caltech and lead author of the article.

Although the relics of some binary fusion events of this type have been observed before, all of these objects have been enveloped by clouds and opaque dust, obstructing the view of the properties of the central star remnant. The Blue Ring Nebula is the only object that allows an unobstructed view of the central stellar remnant, offering a clear window to its properties and giving clues about the fusion process.

“The Blue Ring Nebula is weird,” said Hoadley. “As such, it is really exciting that we could find it, and we are excited about the possibility of finding more objects of this type in the future. If so, that would allow us to learn more about the stellar fusion remnants and the processes that govern them “.