For four decades, astronomers have puzzled over a gigantic loop that appears to be expanding outward from the center of the Milky Way. Known as the Galactic Center Lobe (GCL), this structure has been attributed to a wide range of causes—from the aftermath of a supernova to an ancient eruption from the galaxy’s core. So many competing explanations exist that one group of researchers described it as a “Rorschach test for galactic astrophysics.” Now, a verdict has finally been reached.
According to a paper led by astrophysicist Katherine Kreckel from Heidelberg University in Germany, the Galactic Center Lobe is neither located at the galaxy’s core nor is it a lobe at all. Instead, it is a closed loop situated much closer to Earth, at a distance of approximately 6,520 light-years. The findings were published in the journal Astronomy & Astrophysics.
This distance means it is far smaller than it would be if it were 26,000 light-years away at the galactic center—rather than being a massive remnant from a supermassive black hole explosion millions of years ago, it is a bubble of material that may have been shaped and ionized by stellar activity. Kreckel and her colleagues propose renaming it the “heavily entangled loop.”
It is simply astonishing. This object is one of the most recognizable features in radio images of the galactic center. It looks exactly like a giant blade erupting from the turbulent chaos at the Milky Way’s core, seemingly towering thousands of light-years above it.
And all this time, we have only seen part of the picture.
As the researchers write, solving this mystery became a “40-year struggle to separate genuine nuclear structures from the rest of the galactic disk.” The challenges linked to the GCL are multifaceted.
First, measuring distances to objects in space is an extremely difficult task. Second, one must observe the center of the galaxy itself—the most densely populated region of the galaxy, filled with stars, dense clouds of molecular gas, dust, and other objects lying along our line of sight.
Then, the lower half of the GCL is positioned against the galactic plane. In radio images, this lower part of the loop blends with the surrounding light, making the object appear as an open lobe extending from the galactic center, rather than a closed bubble in front of it.
Kreckel and her colleagues took a fresh look at this object. They used data from the SDSS-V Local Volume Mapper survey, which creates detailed maps of glowing gas in the Milky Way by measuring light emitted in the optical and infrared spectra by various types of gas.
One particularly useful source of emission turned out to be ionized sulfur, which has a longer, redder wavelength capable of penetrating dust more effectively than shorter-wavelength radiation.
This meant that the ionized sulfur from the lower part of the loop could shine through, finally revealing that the GCL is a much more extensive structure than radio observations had suggested.
Observations of the sulfur content 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 in the Milky Way, they concluded that it must be located just 6,520 light-years from Earth.
The bubble itself is a vast cloud of hydrogen gas, glowing under the influence of intense ultraviolet radiation.
Although the researchers have yet to identify the stars responsible for forming the bubble, they believe it was likely shaped by an earlier generation of massive stars born in the same stellar nursery, similar to an object known as Barnard’s Loop.
Such nurseries often contain clusters of extremely massive stars that live very short lives and end in supernova explosions. These explosions can carve out a cavity in the dusty regions where the stars died, creating shock waves 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 vantage point, the edge of the bubble appears brightest, so it looks like a loop rather than a three-dimensional volume of space.
According to the researchers, the GCL has a diameter of about 115 light-years, which is smaller than the famous Barnard’s Loop in the constellation Orion, but close enough in scale to suggest that both structures may have been created by the same process.
This is a fine example of detective work, also demonstrating how effectively the Milky Way can conceal its features.
Even in a region as well-studied as the center of the Milky Way, what looks like a lobe and seems to be a lobe can turn out to be a false lead.
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