A star cluster near the Milky Way’s core is 13.4 billion years old

A faint knot of stars hidden behind thick dust near the center of the Milky Way formed when the universe was only a few hundred million years old. A team of astronomers based mostly in Chile has measured the age of this globular cluster for the first time with stars that can actually serve as a clock, and the answer places it among the oldest objects in our galaxy.

That matters well beyond one cluster. Nothing can be older than the universe, so the oldest stars we can date set a floor under its age, and this cluster now presses hard against that floor.

The cluster, catalogued as VVV CL001, is 13.4 billion years old by the new estimate, give or take 0.4 billion years. The universe is about 13.8 billion years old, which means the cluster’s stars were born roughly 400 million years after the Big Bang. Earlier studies had put VVV CL001 at around 12 billion years, but none of them could see deep enough into the cluster to read its age directly.

The work was led by Julio Olivares Carvajal, of Italy’s INAF Capodimonte Observatory in Naples and the Pontifical Catholic University of Chile, and has been accepted by the journal Astronomy & Astrophysics.

How to date a star cluster you can barely see

VVV CL001 sits in the constellation Sagittarius, in the crowded bulge around the galactic center. Between us and the cluster lies so much interstellar dust that its visible light is dimmed by a factor of about 5,000. Astronomers only found it in infrared images from ESO’s VISTA telescope, a small, faint group lost among millions of foreground stars.

The first problem was deciding which stars belong to it. The team compared infrared images taken with the FourStar camera on the 6.5-meter Magellan Baade telescope at Las Campanas Observatory in Chile, plus newer Hubble Space Telescope images, spanning almost 13 years. Stars in the cluster drift across the sky together, while field stars move in other directions. Sorting those motions statistically identified 155 member stars, far more than any previous study.

The second problem was distance. The team found two stars on the cluster’s horizontal branch, a stage where old stars burn helium at a predictable brightness, and used them with data from the Dark Energy Camera on the 4-meter Blanco telescope in Chile. The result is about 24,000 light-years, with an uncertainty of roughly 1,500 light-years, the most precise distance yet measured for this cluster.

Only then could the clock be read. Every star in a globular cluster was born at about the same time, and the heavier ones exhaust the hydrogen in their cores first. The point where stars are just leaving the main sequence, called the turnoff, shifts to fainter and redder stars as the cluster ages. Deep Hubble infrared images reached that turnoff for the first time. Two independent sets of stellar models gave 13.47 and 13.39 billion years, which the team combines as 13.4 billion.

How old is 13.4 billion years

The Sun is about 4.6 billion years old, so VVV CL001 is almost three times older. Its stars were already shining when the first galaxies were still assembling. The James Webb Space Telescope has begun to pick out compact, newborn star clusters inside distant, magnified galaxies, some seen as they were in the universe’s first billion years. The team argues that VVV CL001 is one of the survivors of that first generation, still orbiting today.

The Milky Way has about 170 known globular clusters, and the oldest ones sit in the bulge. NGC 6558 has been dated at 13.2 billion years, NGC 6256 at 13 billion and HP1 at 12.9 billion. Another team this year estimated 13.6 billion years for the bulge cluster Ton 2. On age alone, VVV CL001 is in the same tight group rather than clearly ahead of it.

What sets it apart is its chemistry. Relative to hydrogen, VVV CL001’s stars carry about 0.6% of the Sun’s proportion of iron, which makes it the most metal-poor globular cluster known in the bulge. Heavy elements build up with each generation of stars, so a cluster this poor in them most likely formed from nearly pristine gas. That is why the authors call it a strong candidate for the oldest cluster in the galaxy rather than simply one of the oldest.

A native of the galaxy’s core with a say on the universe’s age

The orbit tells the rest of the story. The new distance puts VVV CL001 on a stretched orbit that never strays more than about 7,300 light-years from the galactic center and dips to about 2,100 light-years from it. That path marks it as a native of the Milky Way’s core, born in the proto-galaxy itself, not a cluster stolen later from a passing dwarf galaxy. Surviving that close to the center for so long, where gravity tears most clusters apart, suggests, by the authors’ estimate, that it started out with something like a million times the mass of the Sun.

The cosmic stakes come from a long-running dispute. Measurements of how fast the universe is expanding disagree, and the faster value, if correct, implies a somewhat younger universe. Very old stars limit how young it can be. In their conclusions, the authors write that if VVV CL001 is confirmed as a native cluster of this age, the age of the galaxy, and perhaps of the universe, will need to be reconsidered.

What it doesn’t settle

The stated uncertainty of 0.4 billion years is only part of the error. The team adds a systematic uncertainty of around 1 billion years from the distance, which is still the weakest link. Using the slightly smaller distance from a previous study, the same method gives 13.7 billion years, practically the age of the universe. The authors point out that this cuts both ways: the cluster can be farther away and younger, but not much closer, or its stars would be older than the cosmos.

Even Hubble did not capture the full turnoff, the most sensitive part of the age measurement. Only two horizontal-branch stars could be confirmed, none of the pulsating RR Lyrae stars often used as distance yardsticks belongs to the cluster, and the dust in front of it is patchy. Every age also depends on stellar models, which agree here but share many assumptions.

There is also the gap with earlier work. Previous estimates near 12 billion years came from shallower images that never reached the turnoff, so the team considers its own figure more reliable, but no independent group has repeated the measurement. The cluster’s native origin rests on its orbit for now; detailed chemistry could still point to an early merger.

Common questions about the Milky Way’s oldest star clusters

What is a globular cluster?

A globular cluster is a dense ball of tens of thousands to millions of stars held together by their own gravity. Its stars formed together from one cloud of gas, so they share an age and a chemical makeup.

What is the oldest star cluster in the Milky Way?

There is no settled winner. Several clusters in the galactic bulge have been dated between about 13 and 13.6 billion years, and the error bars overlap. VVV CL001, at 13.4 billion years and with the lowest iron content of any bulge cluster, is now one of the strongest candidates.

How do astronomers know how old a star cluster is?

They find the turnoff, the point where the cluster’s stars are just exhausting the hydrogen in their cores. Heavier stars burn out first, so as a cluster ages the turnoff moves to fainter, redder stars. Comparing its position with stellar models gives an age, provided the distance and the dimming by dust are known.

Can a star be older than the universe?

No. When an age estimate edges past the 13.8 billion years of the universe, the error bars or the distance are the explanation. For VVV CL001, that limit works as a check: it rules out the cluster being much closer than the new measurement says.

Spectra will decide where VVV CL001 was born

VVV CL001 was discovered in 2011 by VISTA’s VVV survey of the inner Milky Way. The paper was submitted in September 2026, accepted on October 20 and posted on the arXiv server the same day. The team says new, deeper optical observations of the cluster will soon be available to find more of its horizontal-branch stars, and it is preparing high-resolution spectroscopy to measure several chemical elements one by one. Those abundances should show whether VVV CL001 was born in the Milky Way’s core, as its orbit suggests, or arrived with a merger in the galaxy’s first billion years.

Reference: Olivares Carvajal et al., “VVV CL001: A strong candidate for the most ancient star cluster in the Galaxy,” Astronomy & Astrophysics (accepted), 2026. arXiv: 2610.27969

Tags: astronomy, Milky Way, hubble, age of the universe, globular clusters, VISTA