Astronomers have determined that VVV CL001, a metal-poor globular cluster near the center of the Milky Way, is about 13.4 billion years old, making it one of the oldest known star clusters in our galaxy. The cluster lies behind thick clouds of interstellar dust, which made its distance and age difficult to establish. The paper, outlining the methodology for estimating its age and distance, was posted to the arXiv preprint server on Oct. 20. It has been accepted for publication by Astronomy & Astrophysics.
Galactic fossils
Globular clusters are some of the oldest members of the Milky Way and hold clues to how our galaxy first formed. These star clusters are home to stars that are 10 billion to 13 billion years old, which preserve fossil records of the conditions under which our galaxy first assembled.
A cluster’s age and chemical properties can be used to put a limit on our galaxy’s age. Because older clusters generally have low metal content, astronomers study metal-poor globular clusters to determine their precise ages.
One such metal-poor globular cluster, called VVV CL001, was discovered by the VISTA Variables in the Vía Láctea Survey (VVV) in 2010. “It was observed as a low-mass, low-luminosity stellar group in the inner bulge region…,” researchers write in the paper. It was confirmed as a cluster in 2019.
Interestingly, this star cluster appears very close to the galaxy’s center. But researchers found that it is moving toward us at a very different speed than other stars in that area and thought that it might not really belong there.
To find out where it came from, researchers examined its path around the galaxy. Since its orbit depends on how far away the cluster is, different studies assumed different distances and got different paths. The main culprit turned out to be the uncertainty in measuring the actual distance.
Measuring the cluster’s distance is tricky because it sits in a crowded region where dust dims starlight. The cluster is also small and faint, and earlier images were not deep enough to show enough of its stars. On top of that, RR Lyrae stars, which astronomers normally use to measure distance, haven’t been found in it.
Hints hidden in the stages
In this new study led by J. Olivares Carvajal of the INAF Capodimonte Astronomical Observatory, astronomers used deep and high-quality near-infrared observations from the FourStar Infrared Camera on the Magellan Baade 6.5m Telescope to refine the cluster’s distance. They used Hubble Space Telescope pictures to estimate the cluster’s age.
Carefully tracking how the stars move across the sky and using a machine learning algorithm, they picked out stars that truly belonged to the cluster. The team found around 155 stars, including those in the horizontal branch (HB) and main-sequence turnoff (TO) stages. The turnoff is the point where a star finishes burning hydrogen steadily in its core.
After this, it swells into a red giant and settles into a calmer stage where it burns helium in its core. Such a star belongs to the horizontal branch in the color-magnitude diagram. Previous studies had not revealed these two stages in this cluster.
“We found a significant number of new cluster members, including those in the HB and TO evolutionary stages, bringing the total to 155 stars,” the team writes. Because the turnoff point depends on age, it is the best marker for working out how old the cluster is. HB stars have a steady brightness, so they can be used as reference points for measuring distance.
Over the limit?
By comparing the cluster’s stars with a theoretical stellar evolution model, they found a distance of about 7.34 kiloparsecs (roughly 24,000 light-years). This places the cluster inside the galactic bulge. “Our result also has the smallest distance error reported in the literature to date,” the researchers write. “Furthermore, we will receive new optical observations to improve this result in future work.”
Next, using Hubble Space Telescope pictures, which are less affected by dust, they measured the age of the cluster as about 13.4 billion years. Additionally, they used the TO cluster stars and found an age in a similar range. “Both methods estimate the cluster age at 13.4±0.4 Gyr, suggesting that VVV CL001 could be the oldest GC in the bulge and perhaps the galaxy…,” the team explains.
Thus, belonging to the crowded bulge of the Milky Way, it may represent an ancient stellar population that formed in place. Researchers are planning a high-resolution spectroscopic follow-up of the cluster to confirm the results using the cluster’s chemical makeup.
Because the age of the universe imposes an upper limit of roughly 13.8 billion years, any age estimate for the cluster must fall below this value within its uncertainty. The team says that if it was born here and is really this old, they may need to rethink the age of the galaxy, and possibly even of the universe.
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Publication details
J. Olivares Carvajal et al., VVV CL001: A strong candidate for the most ancient star cluster in the Galaxy, arXiv (2026). DOI: 10.48550/arxiv.2610.27969
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Dust-shrouded Milky Way star cluster may be nearly as old as the universe (2026, November 2)
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