In 2004, astronomers spotted a unique celestial feature that, when observed in ultraviolet, bore the appearance of a glowing eye staring directly back at Earth. Unbeknownst to them, it would take 16 years to figure out what caused the Blue Ring Nebula to form.
The Blue Ring Nebula is the remnant of two merging stars, according to new research published today in Nature.
The bright yellow dot at the centre of the feature is the surviving stellar remnant (the merger of two stars); the blue ring is an expanding cloud of debris interacting with the stellar medium; and the magenta outline is the leading edge of the shockwave. To be clear, the blue and magenta colours seen in the nebula represent ultraviolet light, and they can’t actually be seen with the naked eye.
Colliding stars are spotted all the time, so this discovery might not seem very special or exciting. What is special about all of this, however, is the timing of our observations, as astronomers are witnessing this stellar merger some 5,000 years after it happened (excluding the time it took the light to reach Earth). Typically, colliding stars are seen immediately after their smash-up.
“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,” explained Keri Hoadley, the lead author of the new study and a physicist at the California Institute of Technology, in a statement. “We think this object represents a late stage of these transient events, when the dust finally clears and we have a good view.”
To which she added: “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.”
The Blue Ring Nebula, therefore, is not something we’ve ever seen before, and it’s providing some new science. The expanding blue ring, for example, is actually one of a pair. We can’t really see it from our perspective on Earth, but this nebula is actually shaped like two funnels connected together at their points, with the merged star at centre. These rings are moving away from each other in opposite directions, at speeds reaching 402 km per second. The team calls this feature a “biconical symmetrical outflow,” and it’s providing some important clues as to what happened during the fatal encounter.
Speaking at a press conference on Tuesday, Mark Seibert, a co-author and astrophysicist with the Carnegie Institution for Science, said that “everything we’ve learned is interesting” and that the merged star “is one of a kind right now.” Indeed, this stellar object is somewhat of an oddball, in that it’s surrounded by a disk — the material from which is pouring directly into the star.
This story started in 2004 when Seibert, while participating in NASA’s Galaxy Evolution Explorer mission (GALEX), was one of the first to gaze upon the Blue Ring Nebula. At the time, Chris Martin, a co-author of the new study and a physicist at Caltech, thought “it was a really interesting object” and that his team “should come up with a nice paper within the year to explain it all,” as he told reporters yesterday.
This paper “within the year,” however, was not to be, as the nature of the object remained elusive. Data gathered by Caltech’s Hale Telescope at Palomar Observatory and the W. M. Keck Observatory in Hawaii revealed the presence of a shockwave around the star, which hinted at something violent and dramatic. An early hypothesis was that a hot Jupiter-like planet was caught in a death spiral around the star, and we were witnessing its destruction in the form of the nebula. But the researchers could not be certain, and “ambiguities remained about it throughout the study,” said Martin.
The scientists also looked into the status of the central star, known as TYC 2597-735-1, finding that it was quite old and no longer burning hydrogen in its core. Many other aspects of the object did not match their expectations for stars. Calling it a “Sherlock Holmes mystery,” Martin said it “became impossible to come up with a scenario to explain all of these observations,” so “after a few years of study, we went on to do other things, and the project laid dormant for a while.”
Things changed in 2017 when Hoadley joined Martin’s group as a postdoctoral fellow, and she appeared eager to take the lead on this hibernating project.
“I heard about it on the second day of the job and was immediately hooked,” she said at the virtual press conference.
The team had collected lots of data, but her challenge was to “figure out how to put all the pieces together” and to especially determine how it was that the nebula was glowing in the first place.
As work progressed, however, the team began to realise they weren’t dealing with a planet and that the likely scenario involved a stellar collision. The mass of material being ejected by the star, for example, was too much for a planet, as Hoadley explained. What’s more, data collected by the Habitable Zone Planet Finder on the Hobby-Eberly Telescope in Texas found no evidence of a planet within this system.
Things got weirder when the team took a look at archival data collected by NASA’s Spitzer Space Telescope and the Wide-field Survey Explorer (WISE), along with other infrared observatories. This data pointed to the presence of an accretion disk around the star. Normally, these rings of dust are seen around young stars, but TYC 2597-735-1 is actually quite old.
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To help make sense of all this data, the team recruited astrophysicist Brian Metzger from Columbia University, an expert in cosmic mergers. Excitingly, Metzger’s mathematical and computational models meshed well with observations made of the Blue Ring Nebula.
“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,” explained Hoadley in a Caltech press release. “He’d say, ‘If this is a stellar merger, then you should see X,’ and it was like, ‘Yes! We see that!’”
So here’s the story of the Blue Ring Nebula, as revealed in the new paper.
Thousands of years ago, a small star was orbiting a larger star with roughly the same mass as our Sun. As the larger star got older, however, it became bloated, expanding to reach very close to its smaller companion. The smaller star — around one-tenth the size of our Sun — fell into a downward spiral that produced a gaseous disk. The big star eventually subsumed the smaller star, producing an expanding cloud of debris that was sliced in half by the disk. This merger resulted in the two cone-shaped debris clouds we see today.
In the thousands of years that followed, the expanding cloud of debris cooled, forming hydrogen molecules that interacted with the stellar medium. Today, we see these collisions as bright ultraviolet emissions. The cloud is now “dissolving into the interstellar medium,” and “we’re just happening to catch it as all the exciting particles are present,” Hoadley told reporters.
Hoadley expects the Blue Ring Nebula to last for another thousand to a few tens of thousands of years, after which time the feature will disappear completely. It’s a veritable blink of the eye in cosmological terms, and a wonderful opportunity to do some fascinating science.