In 2004, scientists from NASA’s Galaxy Evolution Explorer (GALEX) spacecraft spotted an object different from any they had ever seen in our Milky Way galaxy: a large, faint drop of gas with a star in the center. Although it does not actually emit light visible to the human eye, GALEX captures a spot in ultraviolet (UV) light and thus appears blue in images; subsequent observations also revealed a thick ring structure in it. That’s what the team called the Blue Ring Nebula. For the next 16 years, they studied it with a number of Earth-based and space-based telescopes, including the WM Keck Observatory in Maunakeya, Hawaii, but the more they learn, the more mysterious it seems.
A new study published online on November 1
While unified star systems are thought to be quite common, they are almost impossible to study as soon as they form because they are obscured by debris fired by the collision. Once the debris is cleared – at least hundreds of thousands of years later – they are difficult to identify because they look like unconnected stars. The Blue Ring Nebula appears to be the missing link: astronomers see the star system only a few thousand years after the merger, when evidence of unification is still plentiful. This seems to be the first known example of a unified star system at this stage.
Operated between 2003 and 2013 and operated by NASA’s Southern California Jet Propulsion Laboratory, GALEX was created to help study the history of star formation by observing young stellar populations in UV light. Most objects observed by GALEX emit both ultraviolet rays (represented as yellow in GALEX images) and distant UV rays (represented as blue), but the Blue Ring Nebula stands out because it emits only far UV light.
The size of the object is similar to that of the supernova remnant that forms when a massive star runs out of fuel and explodes or a planetary nebula, the inflated remnants of a star the size of our Sun. But the Blue Ring Nebula had a living star at its center. In addition, supernova remnants and planetary nebulae emit multiple wavelengths of light outside the UV range, while the Blue Ring Nebula does 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 Keck Observatory telescopes. They found evidence of a shock wave in the nebula using the Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS), which suggests that the gas that makes up the Blue Ring Nebula was indeed expelled by some violent event around the central star.
“Keck’s LRIS spectra on the strike front were invaluable in nailing the way the Blue Ring Nebula appeared,” said Kerry Howdley, an astrophysicist at Caltech and lead author of the study. “Its speed was moving too fast, for a typical planetary nebula, but too slow to be supernova. This unusual speed between them gave us a strong idea that something else must have happened to create the nebula.”
Data from the Echelle spectrometer (HIRES) of the Keck Observatory also suggest that the star pulls a large amount of material on its surface. But where does the material come from?
“HIRES observations at Keck gave us the first evidence that the system is accumulating material,” said co-author Mark Seibert, an astrophysicist at the Carnegie Institution of Science and a member of the GALEX team at Caltech that runs JPL. “For a long time, we thought there might be a planet several times the mass of Jupiter that was torn by the star and ejected all that gas from the system. Although the HIRES data seems to support this theory, she also told us. to be cautious about this interpretation, assuming that the rise may have something to do with movements in the central star’s atmosphere. “
To gather more data, in 2012 the GALEX team used NASA’s Broadband Infrared Explorer (WISE), a space telescope that examines the sky in infrared light, and identified a disk of dust orbiting the star. Archive data from three other infrared observatories also spotted the disk. The discovery did not rule out the possibility that a planet would also orbit the star, but in the end, the team would show that the disk and material ejected into space came from something larger than even a giant planet. Then in 2017, the Hobby-Eberly Telescope in Texas confirmed that there was no compact object orbiting the star.
More than a decade after the discovery of the Blue Ring Nebula, the team has collected data on a system of four space telescopes, four terrestrial telescopes, historical observations of the star from 1895 (to look for changes in its brightness over time) and the help of civilian scientists. through the American Association of Variable Star Observers (AAVSO). But the explanation for what created the nebula is still slipping away.
When Howley began working with the GALEX research team in 2017, the group had “hit a wall” with the Blue Ring Nebula, she said. But Howley was fascinated by the hitherto inexplicable object and its bizarre features, so she accepted the challenge of trying to solve the mystery. It seemed likely that the solution did not come from more observations of the system, but from avant-garde theories that could make sense of existing data. So Chris Martin, chief researcher at GALEX at Caltech, turned to Columbia University’s Brian Metzger for help.
As a theoretical astrophysicist, Metzger makes mathematical and computational models of cosmic phenomena that can be used to predict what these phenomena will look like and behave. He specializes in cosmic mergers – collisions between different objects, whether they are planets and stars or two black holes.
“Not only was Brian able to explain the data we saw; he was essentially predicting what we observed before he saw it,” Howley said. He would say, “If this is a stellar merger, then you should see X” and it was like “Yes! We see this! “
The team concluded that the nebula was the product of a relatively fresh stellar fusion that probably occurred between a star similar to our Sun and another about a tenth of that size (or about 100 times the mass of Jupiter). Towards the end of his life, the Sun-like star began to swell, crawling closer to his moon. Eventually, the smaller star spirals down to its larger satellite. Along the way, the larger star tore the smaller one, wrapping itself in a ring of debris before swallowing the smaller one completely.
This was the violent event that led to the formation of the Blue Ring Nebula. The merger fired a cloud of hot debris into space, which was cut in two by the gas disk. This created two cone-shaped clouds of debris, the bases of which move away from the star in opposite directions and expand as they travel outward. The base of one cone comes almost directly to Earth and the other almost directly. They are too weak to see for themselves, but the area where the cones overlap (as seen from Earth) forms the central blue GALEX ring observed.
Millennia passed, and the expanding cloud of debris cooled to form molecules and dust, including hydrogen molecules that collided with the interstellar medium, the scarce collection of atoms, and energy particles that filled the space between the stars. The collisions excited the hydrogen molecules, causing them to emit a certain wavelength of far UV light. Over time, the glow became bright enough for GALEX to see.
Star fusion can occur as often as once every 10 years in our Milky Way galaxy, which means that it is possible that a significant population of the stars we see in the sky may have been once.
“We see a lot of two-star systems that could merge one day, and we think we’ve identified stars that may have merged maybe millions of years ago. But we have almost no data on what’s going on between them,” Metzger said. . “We think there are probably a lot of young remnants of stellar mergers in our galaxy, and the Blue Ring Nebula can show us what they look like so we can identify more of them.”
While this is probably the conclusion of a 16-year-old mystery, it could also be the beginning of a new chapter in the study of stellar mergers.
“It’s amazing that GALEX was able to find this really weak object that we weren’t looking for, but it turns out to be something really interesting for astronomers,” Seibert said. “It just repeats that when you look at the universe at a new wavelength or in a new way, you find things you never imagined you would do.”