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In 2004, astronomers found something really bizarre. About 6,200 light years away, a star was found surrounded by a ring-shaped nebula glowing in invisible ultraviolet light.
There is nothing else like it in the Milky Way galaxy, which makes it difficult to understand how and why the object, named the Blue Ring Nebula, arrived this 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 fusion 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 includes gas and debris ejected during the violent event.
It is, say astronomers, one of the youngest merged binaries we have found to date – constituting a sort of Goldilocks-like “ missing link ” in the history of stellar binary mergers.
“Merging two stars is quite common, but they quickly become obscured by a lot of dust as the ejecta expand and cool in space, which means we can’t see what really happened. “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 and we have good eyesight. But we also caught the process before it got too far along; after time, the nebula will dissolve into the interstellar medium., and we couldn’t say anything at all. “
Binary systems are extremely common throughout the Milky Way.
Up to 85% of all stars in the galaxy could be in binary pairs, or even in trinary or quaternary systems.
Evidence suggests that all stars start their lives with binary companions (and the Sun might have a lost twin somewhere), which means that the potential number of binary systems that have separated or merged is, well, astronomical. .
It is not unexpected. For two stars in a mutual orbit, there is a strong possibility that as their orbit loses energy, it will decay, causing them to spiral towards each other and eventually collide.
But we only saw one local merger in action. In the Milky Way, the most recent stellar fusion was observed in 2008… but it was the first such event in known history.
The Blue Ring Nebula could be the next youngest.
Theoretical models, devised by Columbia University astrophysicist Brian Metzger, show that its odd shape, ultraviolet glow, and complex ring structures are more consistent with a pair of cones of material, exploding outward from the l object in the center, from an event that has taken place. less than 5,000 years ago.
“It wasn’t just that Brian could explain the data we were seeing; he was basically predicting what we observed before we saw it,” Hoadley said.
“He was like, ‘If it’s a stellar fusion, then you should see X’, and it was like ‘Yes! We see this! “”
The most likely scenario, as depicted by these models, begins with two stars, one on the Sun’s mass and a smaller companion about a tenth of its mass.
As the Sun-like star neared the end of its life, it began to swell, eventually getting so close to the mate that the mate accreted part of the mass of the larger star.
Unable to contain this extra mass, the smaller star dumped matter into the system’s second Lagrange point (L2), which spun around the two stars in a disc.
During this time, the smaller star moved closer to the larger star, triggering the process of runaway fusion.
When a shell of gas was ejected from the merging stars, the disc acted like a sort of collar, restricting and shaping the material into two explosive cones.
Each of these cones would be too weak to be seen on their own. But, due to our angle of view – looking almost directly into one of these cones – the two cones overlap.
It is the ultraviolet ring, visualized as a blue glow when hydrogen collides and is fueled by the interstellar medium, that we see in the Blue Ring Nebula. We can also see the red glow of the energized hydrogen on the shock fronts of the emission cones as two overlapping rings.
As for the star TYC 2597-735-1 itself, it is currently between around 1.1 and 2 solar masses and has probably evolved out of the main sequence, no longer fusing hydrogen in its nucleus. It is probably on its way to becoming a white dwarf star – the evolutionary “dead” stage of stars that begin around the same mass as the Sun.
Finally, figuring out where TYC 2597-735-1 and the Blue Ring Nebula fit in the star’s evolutionary tree could help us understand how often these stellar collisions take place in our galaxy.
“We see a lot of two-star systems that might 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 a lot of young stellar fusion remnants in our galaxy, and the Blue Ring Nebula could show us what they look like so that we can identify more.”
The research was published in Nature.
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