Astronomers have discovered a stellar cluster hidden in the crowded disk of the Milky Way. The object, named Garro 04, was first identified as an unusual collection of stars in observations from the VVVX survey and was later confirmed using infrared spectroscopy. A paper outlining the properties of this seemingly mysterious stellar system was posted to the arXiv preprint server on September 21.
Stars as fossils
The inner regions of the Milky Way galaxy, especially the Galactic plane and the Galactic bulge, are crowded with stars and rich in star formation activity. They are also notable for dense dust lanes that block optical light wavelengths. Groups of stars in these regions are also tracers of the Milky Way’s chemical history and evolution. Examining these regions can reveal the galaxy’s past mergers—and other events that may have significantly affected its growth.
Infrared wavelengths can peer through the thick dust and reveal the hidden stellar clusters in those regions. Recently, many such previously hidden open and globular clusters have been spotted by near-infrared surveys like the VISTA Variables in the Via Láctea and its extension (VVVX).
An infrared scan
In this work led by Elisa R. Garro of the European Southern Observatory, researchers report the discovery of a new stellar system. They first discovered the new cluster, Garro 04, by visually spotting a prominent clump of red giant stars in VVVX infrared imaging. “We identified this star cluster as a clear overdensity of red giant stars,” the team writes in the paper.
For further analysis, they combined multiple survey datasets such as the near-infrared survey VVVX, Gaia Data Release 3 for precise positions and motions, and 2MASS for supplementary infrared photometry into one combined catalog. Their analysis reveals that Garro 04 is located around 32,800 light-years from the sun.
Since cluster member stars should all move together through space in a similar direction or speed, they used this shared motion to statistically separate true members from unrelated foreground/background stars through something called proper-motion selection. This retained only 613 of the 2,145 stars (around 29%) in the field as likely genuine members of the cluster. Density mapping confirmed “the reality of the cluster”—meaning it shows a genuine, smoothly peaked concentration of stars at the cluster’s center.
Neither here nor there
Researchers estimated that Garro 04 is about 5 billion years old and has a metallicity lower than that of the sun. The cluster moves on a nearly circular, low-tilt path around the galaxy, staying close to the galactic disk—confirming it’s a genuine disk cluster, not a halo or bulge object.
At the moment, its age and chemical makeup suggest it may belong to a genuinely ambiguous class of object. Garro 04 is too old to be a typical “young” open cluster, but not old or metal-poor enough to be a classic globular cluster.
“We will dedicate a follow-up paper to perform the chemical analysis and better disentangle the real nature of this cluster, and classify it as GC (globular cluster]), OC (open cluster), or intermediate-class object,” the researchers write. As a result, it joins a small but growing list of similarly puzzling “in-between” clusters found in other recent studies.
Researchers suspect that many small and faint star clusters may remain hidden in the Milky Way’s dusty central regions. “… this further discovery suggests that many other small, low-luminosity clusters are still missing from our compilation, both in the Galactic bulge and disk,” they conclude.
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Publication details
Elisa R. Garro et al, A Newly Discovered VVVX Star Cluster Surfing the Galactic Disk, arXiv (2026). DOI: 10.48550/arxiv.2609.24811
Journal information:
arXiv
Key concepts
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Hidden star cluster discovered in Milky Way’s crowded disk defies easy classification (2026, September 30)
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