The Case of the Blue Ring Nebula

Stars collide with each other all the time. We’ve seen it happen in real-time, catching the sudden burst of light — and then the clouds of dust and gas that cover up the end result. For decades, the particulars of these commonplace events have remained hidden.

At the same time, we’ve seen enough oddball stars to know that mergers, common as they are, also have a profound effect on stellar evolution. Betelgeuse, for example, is thought to have subsumed another star hundreds of thousands of years ago, which would explain the giant’s surprisingly fast rotation.

Now, astronomers have for the first time caught a stellar merger just a couple thousand years after it happened, after the smoke of the collision has cleared. The singleton that remains will act as a Rosetta Stone for astronomers seeking to understand these events and their role in stellar evolution.

The Blue Ring Nebula and a Red Herring

The story begins in 2004, when astronomers working with NASA’s GALEX mission discovered a star surrounded by a fuzzy ring radiating far-ultraviolet light. The researchers dubbed it the Blue Ring Nebula.

“We said this is a really interesting object, let’s do a little investigation and come up with a nice paper we can publish at the end of the year to explain it all,” says GALEX principal investigator Christopher Martin (Caltech).

Little did he know — only after 16 years of further observations and investigations are the astronomers, led by postdoctoral fellow Keri Hoadley (Caltech), finally publishing their story in the November 18th Nature.

The GALEX observations of the ring initially resembled a shock wave traveling outward from a star gone supernova. But a closer look at the data showed that the ultraviolet ring (colored blue in the image) came from fluorescent hydrogen molecules — in other words, not what you’d see if it were a supernova. Additional observations using the Keck telescope atop Mauna Kea, Hawai‘i, showed additional rings of ionized hydrogen offset and moving outward from the central star at 400 kilometers per second (900,000 mph).

These observations were already somewhat unusual but then an investigation of archival infrared data revealed an unexpected piece of the puzzle: the star appeared to be emitting extra infrared radiation, a glow that likely comes from a surrounding disk of dust and gas.

Such a disk would be expected around a young star still collecting gas from its surroundings. But this star, dubbed TYC 2597-735-1, isn’t young. In fact, both its location and its spectrum mark it as part of the Milky Way’s older population of stars. Seeing this star with a disk was like catching grandpa riding a skateboard.

The central star at first glance appears relatively normal: at most twice as massive as the Sun and more than 100 times as bright, it’s a giant that has run out of hydrogen to fuse in its core. Yet even for its advanced age, it has a surprisingly low surface gravity — in other words, it’s puffy.

---

---

The team thought they might have found a broader picture in which to place all these puzzle pieces when high-resolution spectroscopy from the Keck telescope showed hints of a hot Jupiter orbiting the star. Maybe interactions between the star and planet had set something off. But the hint was only a red herring — later independent measurements proved there was no planet.

“It gave us a little bit of a push,” Hoadley says. The team was back to the drawing board, but even as many of the researchers moved on to other projects, all ideas were back on the table.

When Hoadley joined the group, she was hooked on the mystery the Blue Ring Nebula presented, and ultimately, she connected with theoretician Brian Metzger (Columbia University), who specializes in simulating stellar deaths and afterlives. He suggested that the collision was actually between two stars.

“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,” Hoadley explains. “He'd say, ‘If this is a stellar merger, then you should see X,’ and it was like, ‘Yes! We see that!’”

“Suddenly, all of these paradoxical data sets came together to tell this beautiful story.”

A Tale of Two Stars

Several thousand years ago, two stars came together 6,200 light-years from Earth. One star was about ten times as massive as the other, and its girth only expanded as it aged. After a brief play at gravitational tug-of-war, the more massive star consumed its smaller companion, an explosive event that ejected a cloud of debris. A disk formed around the remaining star, slicing the debris cloud in two to create a double-cone-shape outflow.

We’re lucky to see this object now, Metzger says: Not right after its merger , when there’s too much debris flying around to make out what’s happening, but also not tens or hundreds of thousands of years from now, when we wouldn’t be able to be certain a stellar merger happened. The Blue Ring Nebula fills in that gap in the timeline.

“It's like catching sight of a baby when it first walks,” says study coauthor and GALEX team member Don Neill (Caltech). “If you blink, you might miss it.”

How a single star goes from childhood to old age and beyond is downright predictable, but many stars do unpredictable things. With the Blue Ring Nebula, astronomers now have a key tool with which to understand how mergers can throw off stellar evolution.