Scientists have just discovered the Milky Way’s equivalent of a giant fake mustache.
For four decades, astronomers have puzzled over a giant loop apparently ballooning out of the center of the Milky Way.
Known as the Galactic center lobe (GCL), the structure has been blamed on everything from the aftermath of a supernova to an ancient eruption from the Milky Way’s core – so many competing explanations that one team described it as “a Rorschach test for Galactic astrophysics.”
Now, the verdict is finally in.
According to a paper led by astrophysicist Kathryn Kreckel of Heidelberg University in Germany, the Galactic center lobe is not in the galactic center, nor is it a lobe. Instead, it’s a closed loop much closer to Earth, around 6,520 light-years away.
This distance means it’s much smaller than the size it would be at a galactic center distance of 26,000 light-years away – not the towering remnant of a supermassive black hole tantrum millions of years ago, but a bubble of material that may have been carved and ionized by stellar activity.
Kreckel and her colleagues propose renaming it the “greatly confused loop”.
It’s kind of mind-blowing. The object is one of the most recognizable features in radio images of the galactic center. It looks exactly like a gigantic lobe erupting from the roiling chaos at the Milky Way’s core, apparently towering thousands of light-years above it.
And all this time, we were only seeing part of it.
Untangling the mystery, the researchers write, has been a “40-year struggle to separate genuine nuclear features from the foreground galactic disk.”
The difficulty posed by the GCL is multifold.
First, measuring the distances to objects in space is notoriously difficult. Then there’s peering towards the galactic center itself – the most densely populated region of the galaxy, filled with stars, dense clouds of molecular gas, dust, and other objects, overlapping along our line of sight.
Then, the bottom half of the GCL sits against the backdrop of the galactic plane. In radio images, that lower portion of the loop blends into the surrounding light, making the object look like an open lobe from the galactic center rather than a closed bubble in front of it.
Kreckel and her colleagues looked at the object differently.
They used data from the SDSS-V Local Volume Mapper survey, which produces detailed maps of glowing gas within the Milky Way by measuring light emitted across the optical and infrared spectrum by many different kinds of gas.
One emission that proved particularly useful is ionized sulfur, which has a longer, red wavelength that can penetrate dust more effectively than shorter wavelengths.
This means that ionized sulfur from the bottom part of the loop was able to shine through, finally revealing that there was a lot more to the GCL than radio observations suggested.
The sulfur observations also helped the researchers estimate the bubble’s distance. By comparing the amount of dust dimming the bubble’s light with detailed three-dimensional maps of dust across the Milky Way, the researchers concluded it must lie only about 6,520 light-years from Earth.
The bubble itself is a vast cloud of hydrogen gas glowing under intense ultraviolet radiation.
Although the researchers have yet to identify the stars responsible, they think the bubble was probably carved out by an earlier generation of massive stars born in the same stellar nursery, like a similar object named Barnard’s Loop.
Such nurseries often host groups of very massive stars, which live very short lives that end in supernova explosions. These explosions can blast away a cavity in the dusty regions wherein the stars died, creating shocks at the boundary that then trigger a new wave of star formation.
This new generation of stars then ionizes the gas, causing it to glow. From our point of view, the edge of the bubble appears brightest, so it looks like a loop instead of a three-dimensional volume of space.
Related: Astronomers May Have Solved The Mystery of The Bubbles Towering Over The Milky Way
At around 115 light-years across, the GCL is smaller than Orion’s famous Barnard’s Loop, but close enough in scale that the two structures may have been created by the same process, the researchers say.
It’s a lovely piece of detective work that also shows how effectively the Milky Way can camouflage its features.
And even in a region as well-studied as the Milky Way’s center, sometimes what looks like a lobe and quacks like a lobe can still turn out to be a red herring.
The findings have been published in Astronomy & Astrophysics.
This article was fact-checked by Michael Irving and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
