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In 2004, astronomers found something really strange. About 6,200 light-years away, a star was found surrounded by a ring-shaped nebula that glowed with invisible ultraviolet light.
There’s nothing quite like it in the Milky Way galaxy, making it difficult to figure out how and why the object, called the Blue Ring Nebula, became like this.
Now finally we have an answer that works. The complex structures around the star, called 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 came together to become one. The nebula comprises gas and debris expelled during the violent event.
It is, astronomers say, one of the youngest merged binaries we have found so far, constituting a kind of Goldilocks-style “missing link” in the history of stellar binary mergers.
“The merging of two stars is quite common, but they are quickly obscured by a large amount of dust as the ejection from them expands and cools in space, which means that we cannot see what really happened,” he said astronomer Keri Hoadley of Caltech. lead author of the team article.
“We think this object represents a late stage of these transient events, when the dust finally clears up and we have a good view. But we also caught the process before it was too advanced; after a while, the nebula will dissolve in between. interstellar, and we couldn’t say anything at all. “
Binary systems are extremely common throughout the Milky Way.
Up to 85 percent of all the stars in the galaxy could be in binary pairs, or even in trinary or quaternary systems.
Evidence suggests that all stars begin their lives with binary partners (and the Sun could have a missing twin somewhere), meaning that the potential number of binary systems that have separated or merged is, well, astronomical.
It is not unexpected. For any two stars in a mutual orbit, there is a strong possibility that as their orbit loses energy, it will break down, causing them to spiral toward each other and eventually collide.
But we’ve only seen a local fusion 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 could 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, shooting outward from the object in the center, of an event that took place. less than 5,000 years ago.
“It wasn’t just that Brian was able to explain the data we were looking at; he was essentially predicting what we had observed before he saw it,” Hoadley said.
“He was like, ‘If this is stellar fusion, then you should see X’, and it was like, ‘Yeah! We see that!'”
The most likely scenario, as those models describe it, begins with two stars, one about the mass of the Sun and a smaller companion about one-tenth its mass.
As the Sun-like star approached the end of its life, it began to swell and eventually got so close to the companion that the companion accumulated some of the larger star’s mass.
Unable to contain this extra mass, the smaller star spilled material at the second Lagrange point of the system (L2), which spun in a disk around the two stars.
Meanwhile, the smaller star approached the larger star, starting the uncontrolled fusion process.
When a layer of gas was expelled from the merged stars, the disk acted as a kind of collar, restraining and shaping the material into two cones that shot up.
Each of these cones would be too dim to see on its own. But, due to our angle of view, looking almost directly at one of these cones, the two cones overlap.
This is the ultraviolet ring, visualized 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 emission cones as two overlapping rings.
As for the star TYC 2597-735-1 itself, it currently has between about 1.1 and 2 solar masses, and is likely to have evolved from the main sequence, no longer fusing hydrogen at its core. It is probably on its way to becoming a white dwarf star, the “dead” evolutionary stage of stars that start around the same mass as the Sun.
Finally, figuring out where TYC 2597-735-1 and the Blue Ring Nebula fit into the evolutionary tree of stars could help us figure out how often these stellar collisions take place in our galaxy.
“We see many two-star systems that might one day merge, and we think we have identified stars that merged perhaps millions of years ago. But we have almost no data on what happens 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 look like so we can identify more.”
The research has been published in Nature.
