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

Astronomers may have finally solved the case of the unusual gas blob that has bewildered researchers for more than 16 years. An international team that includes scientists from Penn State has applied cutting-edge theoretical models to the smattering of data collected on this object, dubbed the Blue Ring Nebula, and speculates that the nebula – a cloud of gas in space – is likely composed of debris of two stars that collided and merged into one star.

The nebula was spotted in 2004 by scientists with NASA’s Galaxy Evolution Explorer (GALEX) and was unlike anything else they had seen in our 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 careful observations identified two thick rings within it. Over the next 16 years, astronomers studied the Blue Ring Nebula with multiple ground and space telescopes, but the more they learned about it, the more mysterious it seemed.

“In 2018 we observed the Blue Ring Nebula with the Habitable Zone Planet Finder (HPF) on the 10m Hobby-Eberly telescope to uncover its mysteries: How was the gaseous material ejected? What were the properties of the central star? “said Andrew Monson, associate research professor of astronomy and astrophysics at Penn State and author of the paper.” The spectroscopic data from HPF played a key role in helping us understand this exciting and unusual object. “

With insights from spectroscopic observations coupled with detailed theoretical models, the researchers discovered a possible explanation: a stellar fusion. A paper describing the potential merger appears online Nov.18 in the journal Nature, and a publicly accessible discussion of the results is available on the HPF blog.

The Blue Ring Nebula is made up of two cone-shaped hollow debris clouds moving in opposite directions from the central star. The base of a cone travels almost directly to Earth. As a result, astronomers observing the nebula see two circles that partially overlap.

Hoadley et al. 2020 (Nature)

Although fused star systems are most likely not uncommon, they are nearly impossible to study immediately after they form because they are obscured by the debris raised by the collision. Once the debris has been completely cleared away – at least hundreds of thousands of years later – it is difficult to positively identify it as it mostly looks like unbelievable 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 merging is still abundant, and is the earliest known example of a star system merged in this one. phase.

Mysterious observations

GALEX, which operated between 2003 and 2013 and was operated by NASA’s Jet Propulsion Laboratory in Southern California, observed a wide range of ultraviolet (UV) light to identify young stars in other galaxies. Most of the objects seen by GALEX radiated both near UV light (wavelengths only slightly shorter than visible light) and far UV, but the Blue Ring Nebula stood out because it emitted only distant UV light.

The size of the object indicated that it may have been a supernova remnant, which forms when a massive star runs out of fuel and explodes, or perhaps a planetary nebula, the swollen remnants of a star the size of our sun. But the Blue Ring Nebula still had a living star in the center.

The GALEX team examined the nebula with two separate telescopes in 2006 and found evidence of a shock wave in the nebula, suggesting that the gas cloud had actually been ejected into space by some sort of violent event around the central star. The data also suggested that that star was dragging a large amount of material across its surface, creating visible changes in the star’s brightness. But where did the material come from?

“For some time, astronomers thought that perhaps a large planet was being torn apart by the star, emitting this material,” said Gudmundur Stefansson, a former graduate student at Penn State who is currently a postdoctoral fellow at Princeton University. “But with the data obtained from HPF, we helped confirm that there was no such planet orbiting the star.”

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

Stellar investigation

While the team appeared to have hit a wall with the Blue Ring Nebula, the insights of cosmic fusion specialist Brian Metzger of Columbia University and Keri Hoadley, a postdoctoral researcher at the California Institute of Technology reignited the project. They suggested that the Blue Ring Nebula could be the result of a stellar merger, and the theoretical expectations of such an event well explained the observed data.

The team concluded that the nebula was a relatively cool stellar merger that likely occurred between a star similar to our Sun and a smaller one only about 100 times the mass of Jupiter. Towards the end of its life, the sun-like star began to swell. As the swollen outer layers of the larger star came a little closer to the companion star, the smaller object began to siphon that material away, surrounding itself with a disk of gas from the smaller star.

Once the smaller star was pulled completely into its larger companion, the merger launched a cloud of hot debris that was cut in two by the gas disk. This created two cone-shaped clouds of debris – their bases move away from the star in opposite directions, becoming wider as they move. The base of one cone is coming almost directly towards the Earth and the other almost directly away, forming the two “rings” seen by GALEX.

The Blue Ring Nebula is made up of two cone-shaped hollow debris clouds moving in opposite directions from the central star. The base of a cone travels almost directly to Earth. As a result, astronomers observing the nebula see two circles that partially overlap.

As expanding debris has traveled through space for millennia, it has cooled and formed 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. . Those collisions destroyed the dust and most of the other molecules that originally formed in the ejected material, but excited the hydrogen molecules and caused them to radiate in a specific wavelength of distant UV light. Over time, the glow became bright enough for GALEX to see.

“This was not the kind of object I ever expected to observe with HPF,” Monson said. “It is exciting to know that a tool we built at Penn State can contribute in ways we never imagined to explore and discover new things about the cosmos.”