Missing-Link Blue Ring Nebula Could Hold Key To Mystery Merging Stars

A head-scratching astronomical anomaly – and a thorn in the side to scientists trying to understand how the universe evolved – could have been explained, with a new theory identifying a missing link in one of the weirdest objects in the known galaxy. The Blue Ring Nebula shouldn't really exist, scientists concluded when it was spotted back in 2004, with its bizarre double-cone structure and a star at its center.

Caught by the Galaxy Evolution Explorer, or GALEX, the object was completely new to astronomers. Showing up as blue to the instrument, and with a thick ring structure inside, it was named the Blue Ring Nebula; details beyond that, such as how it formed, were in short supply however.

Now, though, a new study published this week could explain its unusual form. It suggests that the Blue Ring Nebula is actually the result of two stars that collided, merging into a single star in the process and leaving two vast, conical clouds of debris as the aftermath. Making the theory particularly special is that, while merged star systems aren't particularly rare themselves, observations of them are.

According to NASA, "they are nearly impossible to study immediately after they form because they're obscured by debris the collision kicks up. Once the debris has cleared – at least hundreds of thousands of years later – they're challenging to identify because they resemble non-merged stars."

In contrast, the Blue Ring Nebula is an anomaly among merged stars. The theory suggests that it's only been a few thousand years since the two stars collided, one a similar size to our own Sun, and the other about a tenth of that. The former began to swell, so the explanation goes, and then drew in the smaller star, tearing it apart and finally consuming it completely.

That merger caused an eruption of hot debris, which was then sliced into two by the gas disc. Each half then flowed out from the merged star, spreading into the distinctive cone structure as they went. Though collisions between the cooling hydrogen molecules and the preexisting interstellar medium, far-UV light was generated, and that's what GALEX first spotted.

To actually figure that out, though, new scientific methods were required. Astrophysicist Keri Hoadley worked with theoretical astrophysicist Brian Metzger to apply computational models of cosmic mergers to what the various instruments trained on the Blue Ring Nebula had recorded.

The hope is that, by validating those models, astronomers will now have a better idea of what to look out for in order to identify signs of previous merged stars. With some estimates suggesting stellar mergers could occur as frequently as every decade in the Milky Way, that's a lot of potential candidates for stars that look solitary today, but which started out as two.