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In 2004, astronomers discovered something truly strange. About 6,200 light-years away, a star was found surrounded by a ring-shaped nebula that glowed with invisible ultraviolet light.
There is nothing else like it in the Milky Way galaxy, making it difficult to understand how and why the object, called the Blue Ring Nebula, arrived that way.
Now, finally, we have an answer that works. The complex structures around the star, named TYC 2597-735-1, are the result of the merger of two stars.
It turns out that TYC 2597-735-1 was once a binary system; less than 5,000 years ago, the two stars mixed together to become one. The nebula includes gas and debris ejected during the violent event.
It is, astronomers say, one of the youngest such merged binaries we have ever found, forming a Goldilocks-style “missing link” in the history of stellar binary mergers.
“The merger of two stars is quite common, but they are quickly obscured by a lot of dust as ejection from them expands and cools into space, which means we can’t see what really happened,” said astronomer Keri Hoadley of Caltech, lead author of the team paper.
“We think this object represents an advanced stage of these transient events, when the dust finally clears and we have a good view. But we also detected the process before it was too long; after time, the nebula will dissolve into the interstellar medium and we wouldn’t be able to say that anything happened. “
Binary systems are extremely common throughout the Milky Way.
Up to 85 percent of all stars in the galaxy could be in binary pairs, or even trinary or quaternary systems.
Evidence suggests that all stars begin their lives with binary companions (and the Sun may have a lost twin somewhere out there), meaning the potential number of binary systems that have separated or joined is, well, astronomical.
It is not unexpected. For any two stars on a reciprocal orbit, there is a strong possibility that as their orbit loses energy, it decays, spiraling them towards each other and eventually colliding.
But we’ve only seen one of these local mergers in action. In the Milky Way, the most recent stellar merger was observed in 2008 … but it was the first such event in recorded history.
The Blue Ring Nebula may be the next youngest.
Theoretical models, designed by Columbia University astrophysicist Brian Metzger, show that its strange shape, ultraviolet glow, and complex ring structures are more consistent with a pair of material cones, which explode outward from the object. in the center, from an event that took place less than 5,000 years ago.
“It wasn’t just that Brian could explain the data we were seeing; he was essentially predicting what we had observed before we saw it,” Hoadley said.
“It said, ‘If this is a stellar merger, you should see X’, and it was like, ‘Yeah! We see it!'”
The most likely scenario, as described by those models, begins with two stars, one of the mass of the Sun and a companion smaller than about one-tenth of its mass.
As the Sun-like star neared the end of its life, it began to swell, getting so close to its companion that the companion accumulated some of the mass of the larger star.
Unable to hold this extra mass, the smaller star poured material into the system’s second Lagrange point (L2), which turned into a disk around the two stars.
Meanwhile, the smaller star has moved closer to the larger star, kicking off the runaway merger process.
When a shell of gas was ejected from the merging stars, the disc acted as a sort of collar, shrinking and shaping the material into two exploding cones.
Each of these cones would be too faint to be seen on their own. But, due to our viewing angle – looking almost straight into one of these cones – the two cones overlap.
This is the ultraviolet ring, displayed as a blue glow when hydrogen collides and is energized by the interstellar medium, which we see in the Blue Ring Nebula. We can also see the red glow of energized hydrogen on the shock fronts of the cones of emission as two superimposed rings.
As for the star TYC 2597-735-1 itself, it is currently between about 1.1 and 2 solar masses and probably evolved from the main sequence, no longer fusing hydrogen in its core. It is likely to become a white dwarf, the “dead” evolutionary stage of stars that begin around the same mass as the Sun.
Finally, understanding where TYC 2597-735-1 and the Blue Ring Nebula fit into the star’s evolutionary tree could help us understand how often these stellar collisions occur in our galaxy.
“We see a lot of two-star systems that could merge someday, and we think we’ve identified stars that merged maybe millions of years ago. But we have almost no data on what’s going on in between,” Metzger said.
“We think there are probably many young remnants of stellar mergers in our galaxy, and the Blue Ring Nebula could show us what they are so we can identify more of them.”
The research was published in Nature.
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