In 2004, astronomers found something really strange. About 6,200 light-years away, a star was found surrounded by an annular nebula glowing in invisible ultraviolet light.
There is nothing else like it in the Milky Way galaxy, which makes it difficult to figure out how and why the object called the Blue Nebula became such.
Now we finally have an answer that works. The complex structures around the star, named TYC 2597-735-1, are the result of two stars merging.
It turns out that TYC 2597-735-1 was once a binary system; less than 5,000 years ago, the two stars merged to become one. The fog consists of gas and debris being ejected during the violent event.
It is, say astronomers, one of the youngest such merged binaries we have found yet ̵
“The fusion of two stars is quite common, but they are quickly hidden by lots of dust when the draft from them expands and cools down in space, which means we can not see what actually happened,” said the astronomer Keri Hoadley from Caltech, lead author on the team paper.
“We believe that this object represents a late phase of these transient events when the dust finally clears and we have a good view. But we also captured the process before it was too long; after time the fog will dissolve in the interstellar medium, and we could not tell at all that anything happened. “
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 trinear or quaternary systems.
Evidence suggests that all stars start their lives with binary companions (and the sun may have a lost twin somewhere out there), meaning that the potential number of binary systems that have either separated or merged is, well, astronomical .
This is not unexpected. For two stars on a mutual orbit, there is a strong possibility that when their orbit loses energy, it decays, causing them to spiral toward each other and eventually collide.
But we have only seen such a local merger in action. In the Milky Way, the most recent star fusion was observed in 2008 … but it was the first such event in known recorded history.
The blue ring fog may be the next youngest.
Theoretical models, designed by astrophysicist Brian Metzger of Columbia University, show that its strange shape, ultraviolet glow, and complex ring structures are most consistent with a pair of cones of material bursting outward from the object in the center of an event that took place for less than 5,000 years ago.
“It was not just that Brian could explain the data we saw; he was essentially predicting what we had observed before he saw it,” Hoadley said.
“He would say, ‘If this is a star fusion, then you should see X,’ and it was like, ‘Yeah! We see it! “”
The most likely scenario, as described by these models, begins with two stars, one about the mass of the sun and a smaller companion about one-tenth of its mass.
As the sun-like star approached the end of its life, it began to inflate and eventually come so close to the companion that the companion accredited some of the larger star’s mass.
The smaller star was unable to hold this extra mass and spilled material into the system’s second Lagrange point (L2), which spread out in a disk around the two stars.
Meanwhile, the smaller star moved closer to the larger star and started the ongoing fusion process.
When a gas shell was pushed out of the fusing stars, the disk acted as a kind of collar that bounded and shaped the material into two cones that burst away.
Each of these cones would be too weak to see by itself. But because of our point of view – looking almost straight into one of these cones – the two cones overlap.
This is the ultraviolet ring, visualized as a blue glow when hydrogen collides and gets energy from the interstellar medium that we see in the blue ring nebula. We can also see the red glow of energy with hydrogen at the impact fronts on the emission cones as two overlapping rings.
As for the star TYC 2597-735-1 itself, it is currently between approx. 1.1 and 2 solar masses and has probably evolved from the main sequence and no longer melts hydrogen in its core. It is probably on its way to becoming a white dwarf star – the evolutionary ‘dead’ stage of stars starting around the same mass as the sun.
Finally, finding out where TYC 2597-735-1 and Blue Ring Nebula fit on the star’s evolutionary tree can help us figure out how often these star collisions occur in our galaxy.
“We see lots of two-star systems possibly merging one day, and we think we’ve identified stars that merged maybe millions of years ago. But we have almost no data on what happens in between, said Metzger.
“We think there are probably lots of young remnants of star clusters in our galaxy, and the Blue Ring Nebula can show us what they look like so we can identify more of them.”
The research is published in Nature.