main article image
(NASA/JPL-Caltech/M. Seibert (Carnegie Institution for Science)/K. Hoadley (Caltech)/GALEX Team)

The Mystery of This Strange, Blue Nebula May Have Finally Been Solved

20 NOVEMBER 2020

In 2004, astronomers found something really weird. Around 6,200 light-years away, a star was found surrounded by a ring-shaped nebula glowing in invisible ultraviolet light.

There's nothing else quite like it in the Milky Way galaxy, making it difficult to figure out how and why the object, named the Blue Ring Nebula, got that way.


We now, 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 TYC 2597-735-1 was once a binary system; less than 5,000 years ago, the two stars mooshed together to become one. The nebula comprises gas and debris ejected during the violent event.

It is, astronomers say, one of the youngest such merged binaries we have found yet - constituting a sort of Goldilocks-style 'missing link' in the story of stellar binary mergers.

"The merging of two stars is fairly common, but they quickly become obscured by lots of dust as the ejecta from them expands and cools in space, which means we can't see what has actually happened," said astronomer Keri Hoadley of Caltech, lead author on the team's paper.

"We think this object represents a late stage of these transient events, when the dust finally clears and we have a good view. But we also caught the process before it was too far along; after time, the nebula will dissolve into the interstellar medium, and we would not be able to tell anything happened 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 trinary or quaternary systems.

Evidence suggests that all stars start their lives with binary companions (and the Sun could have a lost twin somewhere out there), which means that the potential number of binary systems that have either separated or merged is, well, astronomical.

It's not unexpected. For any two stars on a mutual orbit, there's a strong possibility that, as their orbit loses energy, it decays, causing them to spiral towards each other and eventually collide.

But we have only seen one such local merger in action. In the Milky Way, the most recent stellar merger was observed in 2008… but it was the first such event in known recorded history.

The Blue Ring Nebula could be the next youngest.

Theoretical models, designed by astrophysicist Brian Metzger from Columbia University, show that its strange shape, ultraviolet glow and complex ring structures are most consistent with a pair of cones of material, blasting outwards from the object in the centre, 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 he saw it," said Hoadley.

"He'd say, 'If this is a stellar merger, then you should see X,' and it was like, 'Yes! We see that!'"

The most likely scenario, as described by those 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 neared the end of its life, it started to puff up, eventually getting so close to the companion that the companion accreted some of the larger star's mass.

Incapable of holding this extra mass, the smaller star spilled out material into the system's second Lagrange point (L2), which spun out into a disc around the two stars.

Meanwhile, the smaller star 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, restricting and shaping the material into two cones blasting away.

Each of these cones would be too faint to see by itself. But, because of our viewing angle - looking almost straight into one of these cones - the two cones overlap.

This is the ultraviolet ring, visualised as a blue glow as hydrogen collides and is energised by the interstellar medium, that we see in the Blue Ring Nebula. We can also see the red glow of energised hydrogen at the shock fronts of the emission cones as two overlapping rings.


As for the star TYC 2597-735-1 itself, it's currently between about 1.1 and 2 solar masses, and has likely evolved off the main sequence, no longer fusing hydrogen in its core. It's probably on its way to becoming a white dwarf star - the evolutionary 'dead' stage of stars that start out around the same mass as the Sun.

Finally figuring out where TYC 2597-735-1 and the Blue Ring Nebula fit on the star evolutionary tree might help us figure out how often these stellar collisions take place in our galaxy.

"We see plenty of two-star systems that might merge some 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," Metzger said.

"We think there are probably plenty of young remnants of stellar mergers in our galaxy, and the Blue Ring Nebula might show us what they look like so we can identify more of them."

The research has been published in Nature.