A radio structure thought for 40 years to tower over the Milky Way’s centre is actually a nearby stellar bubble — not the fossil scar of a black hole eruption

In 1984, radio astronomers reported a loop of emission apparently rising from the centre of the Milky Way. Its position made an eruption from the galaxy’s central black hole, Sagittarius A*, one possible explanation.

New observations now place the so-called Galactic Center Lobe much closer to Earth. Rather than a fossil outflow from the galactic nucleus, it appears to be a closed shell of ionised hydrogen about 6,500 light-years away and roughly 115 light-years across.

This is one study, not settled consensus. It does, however, reinforce independent radio and infrared work published in 2024 that had already placed the object in the foreground.

A name built around its position on the sky

Yoshiaki Sofue and Toshihiro Handa introduced the radio lobe in Nature in 1984 after finding it in a 10-gigahertz survey. The feature sits north of the Galactic plane and extends across nearly one degree of the sky.

From Earth, that direction is crowded. Nearby clouds, star-forming regions and structures near the galactic nucleus overlap along the same line of sight. The lobe consequently accumulated explanations involving a magnetic tube, a galactic wind, star formation and past activity from Sagittarius A*.

Even under the old Galactic-centre interpretation, the loop would have been hundreds of light-years tall, not taller than the Milky Way is wide. Its apparent importance came from the possibility that it traced an energetic event at the nucleus, not from exceeding the galaxy’s diameter.

The black hole interpretation was not unreasonable. The Milky Way really does contain much larger structures centred on its nucleus, including the two gamma-ray-emitting Fermi bubbles announced by NASA in 2010. But an object appearing in the same direction is not necessarily at the same distance.

A new map revealed a closed loop

Kathryn Kreckel and colleagues used the Sloan Digital Sky Survey’s Local Volume Mapper to examine the lobe with optical integral-field spectroscopy. Their paper in Astronomy & Astrophysics maps the spectrum of the gas across the structure rather than treating it as one radio outline.

A sulphur emission line at 953.2 nanometres, less obstructed by dust than shorter-wavelength visible light, exposed ionised gas around a closed outer loop. The interior was not filled with the same emission.

Nitrogen-line measurements also showed a relatively uniform velocity pattern across the object. That supported the conclusion that its apparently separate arcs belong to one coherent bubble.

Dust placed it about 6,500 light-years away

The decisive test concerned distance. Dust reddens light by absorbing more blue light than red light. The team estimated that reddening from the relative strengths of hydrogen emission lines, then compared the result with three-dimensional maps showing how dust accumulates along the sightline.

The match put the lobe at about two kiloparsecs, or 6,520 light-years, from Earth. The galactic centre is roughly four times farther away.

At that foreground distance, its span of about one degree corresponds to roughly 35 parsecs, or 115 light-years. That is large by the standards of a stellar bubble, but nowhere near the Milky Way’s roughly 100,000-light-year diameter.

The scale correction also agrees with a 2024 study led by L. D. Anderson, which found that the object’s radio and mid-infrared properties resemble those of ordinary Galactic H II regions. Associated low-frequency radio absorption had already indicated that it must sit in front of the galactic centre.

Hot young stars are a better explanation

An H II region is a cloud in which ultraviolet radiation from hot stars has stripped electrons from hydrogen atoms. Across the lobe, the optical line ratios are consistent with this process, known as photoionisation, rather than the shock excitation expected from a violent nuclear outflow.

The closest visual analogue may be Barnard’s Loop in Orion, another enormous arc of ionised gas. Kreckel’s team suggests that a young cluster offset from the bubble’s centre could supply the radiation now lighting its shell.

That part of the explanation remains incomplete.

The researchers have not yet identified the stellar population responsible, and the shell may preserve more than one generation of massive-star activity. The new observations determine the lobe’s location and excitation more securely than they reconstruct its entire history.

The Milky Way still carries scars from its centre

Moving this loop into the foreground does not erase evidence that the Milky Way’s nucleus has undergone energetic episodes. The Fermi bubbles extend tens of thousands of light-years above and below the plane, while X-ray and radio structures trace activity on other scales.

The result instead illustrates a basic difficulty of observing our own galaxy from inside it. On the crowded route towards the centre, unrelated structures can overlap so neatly that angular alignment looks like physical connection.

The authors propose keeping the initials GCL while changing their meaning from Galactic Center Lobe to “Greatly Confused Loop”. The joke is useful shorthand for the scientific correction: after four decades, the object has become smaller, closer and more ordinary, but also more accurately placed.