18th November 2020
The Blue Ring Nebula consists of two expanding cones of gas that are ejected into space by a star fusion. As the gas cools, it forms hydrogen molecules that collide with particles in interstellar space, causing them to emit distant ultraviolet light. Invisible to the human eye, it is shown here as blue. Credits: NASA / JPL-Caltech / M. Seibert (Carnegie Institution for Science) / K. Hoadley (Caltech) / GALEX-Team
The Blue Ring Nebula, which puzzled scientists for over a decade, appears to be the latest known example of two stars fused into one.
In 2004, scientists from NASA's space-based Galaxy Evolution Explorer (GALEX) discovered an object they had never seen in our Milky Way galaxy: a large, faint lump of gas with a star in its center. In the GALEX images, the blob appeared blue – although it does not emit any light visible to the human eye – and subsequent observations revealed a thick ring structure in it. So the team called it the Blue Ring Nebula. For the next 16 years, they studied it with several Earth- and space-based telescopes, but the more they learned, the more mysterious it seemed.
A new study published online November 18 in the journal Nature may have cracked the case. By applying cutting-edge theoretical models to the amount of data collected for this object, the authors determine that the nebula – a gas cloud in space – is likely made up of debris from two stars that collided and merged into a single star.
The Blue Ring Nebula consists of two hollow, cone-shaped debris clouds that move in opposite directions away from the central star. The base of a cone moves almost directly to the earth. As a result, astronomers looking at the nebula will see two circles that partially overlap. Credits: Mark Seibert
While merged star systems are believed to be quite common, it is next to impossible to study them immediately after they are formed, as they are obscured by debris that sets off the collision. Once the debris is gone – at least hundreds of thousands of years later – it is difficult to identify because it resembles non-merged stars. The Blue Ring Nebula seems to be the missing link: astronomers see the star system only a few thousand years after the merger, when there is still ample evidence of union. It appears to be the first known example of a merged star system at this point in time.
GALEX operated between 2003 and 2013 and was administered by NASA's Jet Propulsion Laboratory in Southern California. It was developed to study the history of star formation in most of the universe by counting young star populations in other galaxies. For this purpose, the mission observed both near UV light (wavelengths slightly shorter than visible light) and far UV. Most of the objects seen by GALEX emitted both near UV (shown as yellow in GALEX images) and far UV (shown as blue), but the Blue Ring Nebula stood out because it only emitted far UV light.
The size of the object was similar to that of a supernova remnant that forms when a massive star runs out of fuel and explodes, or a planetary nebula, the inflated remains of a star the size of our sun. But the Blue Ring Nebula had a living star in the middle. Additionally, supernova remnants and planetary nebulae emit multiple wavelengths of light outside of the UV range, while further research showed that the Blue Ring Nebula did not.
In 2006, the GALEX team examined the nebula with the 5.1-meter Hale telescope at the Palomar Observatory in San Diego County, California, and then with the even more powerful 10-meter telescopes 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 was actually ejected by a violent event around the central star. Keck data also indicated that the star was pulling a large amount of material onto its surface. But where did the material come from?
"For a long time we thought that there might be a planet where the mass of Jupiter is being torn apart many times over from the star and throwing all the gas out of the system," said Mark Seibert, astrophysicist at the Carnegie Institution for Science and Member of Caltech's GALEX team that manages JPL.
But the team wanted more data. In 2012, the GALEX team used the first full sky survey by NASA's WISE (Wide Field Infrared Survey Explorer), a space telescope that examined the sky in infrared light, a disk of dust that orbits tightly around the star. (WISE was reactivated in 2013 as a NEOWISE asteroid hunting mission.) Archival data from three other infrared observatories, including NASA's Spitzer Space Telescope, also discovered the disk. The result didn't rule out a planet orbiting the star as well, but eventually the team would show that the disk and material ejected into space came from something larger than itself a giant planet. In 2017, the Habitable Zone Planet Finder of the Hobby Eberly Telescope in Texas confirmed that no compact object orbits the star.
More than a decade after the Blue Ring Nebula was discovered, the team had collected data on the system from four space telescopes, four ground-based telescopes, and historical observations of the star dating back to 1895 (to look for changes in its brightness) and with the help of citizen scientists by the American Association of Variable Star Observers (AAVSO). But an explanation for what had created the mist still escaped them.
When Keri Hoadley was working with the GALEX science team in 2017, "the group hit some kind of wall with the Blue Ring Nebula," she said. But Hoadley, an astrophysicist at Caltech, was intrigued by the object and its bizarre features, and so she accepted the challenge of solving the puzzle. It seemed likely that the solution would come not from further observations of the system, but from cutting edge theories that might make sense from the data available. Chris Martin, Principal Investigator for GALEX at Caltech, asked Brian Metzger of Columbia University for help.
As a theoretical astrophysicist, Metzger creates mathematical and computational models of cosmic phenomena that can be used to predict how these phenomena will look and behave. He specializes in cosmic fusions – collisions between different objects, whether planets and stars or two black holes. With Metzger on board and Hoadley in charge of the work, things went quickly.
"It wasn't just that Brian could explain the data we were seeing. He was essentially predicting what we saw before he saw it," said Hoadley. "He would say," If this is a stellar merger then you should see X "and it was like," Yeah! We see that! & # 39; "
The team concluded that the nebula was the product of a relatively fresh stellar fusion that likely 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 ) scam. Towards the end of its life, the sun-like star swelled and crept closer to its companion. Eventually the smaller star fell in a downward spiral towards its larger companion. On the way, the larger star tore the smaller star apart and wrapped itself in a ring of debris before completely swallowing the smaller star.
This was the violent event that led to the formation of the Blue Ring Nebula. The merger hurled a cloud of hot debris into space, which was split in two by the gas disk. This created two cone-shaped debris clouds, the bases of which moved away from the star in opposite directions and broadened on their way out. The base of one cone comes almost straight to the earth and the other almost straight away. They are too faint to be seen on their own, but the area where the cones overlap (when viewed from Earth) forms the central blue ring GALEX observed.
Millennia passed. The expanding cloud of debris cooled, forming molecules and dust, including hydrogen molecules, that collided with the interstellar medium, the sparse collection of atoms and energetic particles that fill the space between stars. The collisions excited the hydrogen molecules and made them radiate in a certain wavelength of distant UV light. Over time, the glow became just bright enough for GALEX to see.
Star fusions can occur in our Milky Way Galaxy every 10 years, meaning that a sizable population of the stars we see in the sky may have been two.
"We see a lot of two-star systems that could one day merge, and we believe we have identified stars that may have merged millions of years ago. However, we have almost no data on what happens in between," said Metzger. "We believe our galaxy is likely to have many young remnants of star fusions, and the Blue Ring Nebula could show us what they look like so we can identify more of them."
While this is likely the conclusion of a 16 year old riddle, it could also mark the beginning of a new chapter in the study of star fusions.
"It's amazing that GALEX was able to find this really faint object that we weren't looking for, but that turns out to be something really interesting to astronomers," said Seibert. "It just repeats that when you look at the universe in a new wavelength or in a new way, you find things that you never imagined."
JPL, a division of Caltech, led 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 directed the Spitzer and WISE missions as well as the NEOWISE mission. Further information on the GALEX mission can be found at: