Solved the mysterious 16-year-old cosmic mystery by revealing a stellar missing link

The Blue Ring Nebula, which has puzzled scientists for over a decade, appears to be the youngest known example of two stars merged into one.

In 2004, scientists with NASAThe space-based Galaxy Evolution Explorer (GALEX) spotted an object unlike anything else seen before in ours Milky Way galaxy: a large, faint drop of gas with a star in the center. In the GALEX images, the blob appeared blue – although it doesn’t actually emit light visible to the human eye – and subsequent observations revealed a thick ring-like structure within it. So the team dubbed it the Blue Ring Nebula. Over the next 16 years they studied it with more ground and space telescopes, but the more they learned, the more mysterious it seemed.

A new study published online November 18 in the journal Nature may have solved the case. Applying cutting-edge theoretical models to the slew of data collected on this object, the authors hypothesize that the nebula – a cloud of gas in space – is likely composed of debris from two stars that collided and merged into a single star. .

While fused star systems are believed to be quite common, it is nearly impossible to study them immediately after they form because they are obscured by debris that the collision begins. Once the debris has been cleared, at least hundreds of thousands of years later, it is difficult to identify because it resembles unfused stars. The Blue Ring Nebula appears to be the missing link: Astronomers are only seeing the star system a few thousand years after the merger, when evidence of the union is still abundant. It appears to be the earliest known example of a merged star system at this stage.

Operated between 2003 and 2013 and operated by NASA’s Jet Propulsion Laboratory in Southern California, GALEX was designed to help study the history of star formation in most of the universe by taking a census of young star populations in other galaxies. . To do this, the mission observed both near UV light (wavelengths slightly shorter than visible light) and far UV. Most of the objects seen by GALEX radiated both near (represented as yellow in GALEX images) and far (represented as blue) UV, but the Blue Ring Nebula stood out because it emitted only distant UV light.

The size of the object was similar to that of a supernova remnant, which forms when a massive star runs out of fuel and explodes, or a planetary nebula, the swollen remnants of a star the size of our Sun. But the Blue Ring Nebula it had a living star in the center. Additionally, supernova remnants and planetary nebulae radiate in multiple wavelengths of light outside the UV range, while further research has shown that the Blue Ring Nebula did not.

Phantom Planet

In 2006, the GALEX team examined the nebula with the 200-inch (5.1-meter) Hale telescope at the Palomar Observatory in San Diego County, California, and then with even more powerful 10-meter telescopes (33 feet) at the WM Keck Observatory in Hawaii. They found evidence of a shock wave in the nebula, suggesting that the gas that makes up the Blue Ring Nebula had actually been ejected by some sort of violent event around the central star. Keck’s data also suggested that the star was dragging a large amount of material across its surface. But where did the material come from?

“For a long time we thought that maybe there was a planet several times the mass of Jupiter being torn apart by the star, and that was throwing all that gas out of the system, “said Mark Seibert, an astrophysicist at the Carnegie Institution for Science and a member of Caltech’s GALEX team, which manages JPL.

But the team wanted more data. In 2012, using the first full-sky survey of NASA’s Wide-field Infrared Survey Explorer (WISE), a space telescope that studied the sky in infrared light, the GALEX team identified a disk of dust orbiting closely around the star. (WISE was reactivated in 2013 as a NEOWISE mission for asteroid hunting.) Archival data from three other infrared observatories, including NASA’s Spitzer Space Telescope, also located the disk. The discovery didn’t rule out the possibility that a planet was also orbiting the star, but eventually the team would prove that the disc and material ejected into space came from something bigger than even a giant planet. Then, in 2017, the Habitable Zone Planet Finder on the Hobby-Eberly telescope in Texas confirmed that there was no compact object orbiting the star.

More than a decade after discovering the Blue Ring Nebula, the team had collected data on the system from four space telescopes, four ground-based telescopes, historical observations of the star dating back to 1895 (in order to look for changes in its brightness over time) and with the help of citizen scientists through the American Association of Variable Star Observers (AAVSO). But an explanation for what had created the nebula still eluded them.

The Blue Ring Nebula is made up of two expanding debris cones. The base of a cone is moving towards the Earth. Both bases are outlined in magenta, revealing the shockwaves created as debris runs through space. Blue represents the material behind the shock wave and is visible only where the cones overlap. Credit: NASA / JPL-Caltech / R. Male

Stellar survey

By the time Keri Hoadley started working with the GALEX science team in 2017, “the group had hit a wall” with the Blue Ring Nebula, she said. But Hoadley, an astrophysicist at Caltech, was fascinated by the object and its bizarre features, so she took on the challenge of trying to solve the mystery. It seemed likely that the solution would come not from multiple observations of the system, but from cutting-edge theories that could make sense of the existing data. So Chris Martin, principal investigator for GALEX at Caltech, contacted Brian Metzger of Columbia University for help.

As a theoretical astrophysicist, Metzger creates mathematical and computational models of cosmic phenomena, which can be used to predict how these phenomena will look and behave. He specializes in cosmic mergers: collisions between a variety of objects, be they planets and stars or two black holes. With Metzger on board and Hoadley leading the work, things progressed rapidly.

“It wasn’t just that Brian could explain the data we were seeing; it essentially predicted what we had observed before we saw it, “Hoadley said.” It said, ‘If this is a stellar merger, you should see X’, and it was like, ‘Yes! We see it!’ “

The team concluded that the nebula was the product of a relatively recent stellar merger that probably occurred between a star similar to our Sun and another star only about one-tenth that size (or about 100 times the mass of Jupiter). . Towards the end of its life, the sun-like star began to swell, crawling closer to its mate. Eventually, the smaller star fell in a downward spiral towards its larger companion. Along the way, the larger star tore apart the smaller star, wrapping itself in a ring of debris before completely engulfing the smaller star.

This was the violent event that led to the formation of the Blue Ring Nebula. The merger launched a cloud of hot debris into space that was cut in two by the gas disk. This created two cone-shaped clouds of debris, their bases moving away from the star in opposite directions and widening as they travel outward. The base of one cone is coming almost directly to Earth and the other almost directly away. They are too faint to see on their own, but the area where the cones overlap (as seen from Earth) forms the central blue ring observed by GALEX.

Millennia have passed. The expanding cloud of debris has cooled and formed molecules and dust, including hydrogen molecules that collided with the interstellar medium, the scattered collection of atoms and energetic particles that fill the space between stars. The collisions excited the hydrogen molecules, causing them to radiate in a specific wavelength of distant UV light. Over time, the glow became bright enough for GALEX to see.

Stellar mergers can also occur once every 10 years in our Milky Way galaxy, which means it’s possible that a sizable population of stars we see in the sky was once twice.

“We see many two-star systems that could merge someday and we think we have identified stars that merged perhaps millions of years ago. But we have almost no data on what happens in between, “Metzger said.” We think there are likely many young stellar merger remnants in our galaxy, and the Blue Ring Nebula could show us what they are like so we can identify more of them. “.

While this is likely the conclusion to a 16-year mystery, it could also be the start of a new chapter in the study of stellar mergers.

“It’s amazing that GALEX was able to find this very faint object that we weren’t looking for, but it turned out to be something really interesting for astronomers,” Seibert said. “It just reiterates that when you look at the universe in a new wavelength or in a new way, you find things you never imagined you would find.”

JPL, a division of Caltech, managed the GALEX mission for NASA’s Science Mission Directorate. The mission was developed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland as part of the Explorers program. JPL also managed the Spitzer and WISE missions and manages the NEOWISE mission.