Universe Today
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We don't have to rely solely on the JWST to observe the Universe's oldest stars. Some of the oldest stars in the Universe reside in globular clusters, and the Milky Way has about 150 of them. How old exactly? New research has the answer.
Globular clusters (GC) are sphere-shaped collections of stars bound together by gravity. The Milky Way's largest GC, Omega Centauri, holds about 10 million stars, while others hold only about 10,000. GCs are found in almost all galaxies. In spirals like ours, they're mostly found in the galactic halo.
Astronomers have puzzled over their place in galactic evolution, and some of the uncertainty arises from the ages of their members. They contain stars of all ages and compositions, pointing to a complex history. In new research, astronomers analyzed eight old Milky Way GCs to determine their ages.
The research is titled "The Absolute Age of Milky Way Globular Clusters," and it will be published in The Astrophysical Journal. The lead author is Jiaqi (Martin)Ying from the Department of Physics and Astronomy at Dartmouth College.
"Globular clusters (GCs) provide statistically significant coeval populations of stars spanning various evolutionary stages, allowing robust constraints on stellar evolution model parameters and ages," the authors write. The work focuses on their metallicities and ages, two intertwined factors that tell astronomers a lot about stars, galaxies, and how the Universe evolved from a low-metallicity state to an enriched one. "The age-metallicity relation of GCs is critical for understanding the formation and evolutionary history of galaxies, as well as the processes of chemical enrichment in the universe," they explain.
M55 is one of the globulars featured in the study. It's about 17,000 light years from Earth. Image Credit: By ESO/J. Emerson/VISTA -http://www.eso.org/public/images/eso1220a/, CC BY 4.0,https://commons.wikimedia.org/w/index.php?curid=19381930
There are two age-related factors with GCs: their absolute age and relative age. Scientists use relative ages to compare and group GCs. It's much simpler than determining their absolute age because it relies on fewer assumptions, which means there's less uncertainty. Their absolute ages help scientists determine concrete timelines for when the Universe and its galaxies formed. However, there are more uncertainties in determining absolute ages than relative ages.
Achieving this has been a longstanding goal," the authors write. "Recent advances in computational power and the increasing availability of observational constraints provide an opportunity to estimate the absolute ages of GCs with improved precision.
Distance is one of the big uncertainties that clouds the determination of absolute ages.Previous research by some of the same authors shows that distance uncertainties can contribute 80% of the age error for some of the Milky Way's GCs. While the Gaia mission has given astronomers much more accurate distances to billions of Milky Way objects, GCS are located in the galaxy's outer regions, which were difficult for Gaia to observe.
This work relies mainly on the Hubble Space Telescope Advanced Camera for Surveys (ACS) and its globular cluster survey treasury program. The survey provided photometric data with low signal-to-noise ratios for main-sequence stars in the Milky Way. Then they compared them with isochrone sets.Isochrones are theoretical curves on a Hertzsprung-Russell diagram—or colour-magnitude diagram—that plot stars by their temperature and brightness.
As stars age, they follow specific evolutionary tracks on these diagrams based on their masses. An isochrone connects the positions of stars with the same age but different masses. They show where stars with different masses would appear on the diagram at a specific moment. Since stars in GCs are thought to be coeval, meaning they're the same age but in different evolutionary stages, isochrones work exceptionally well in determining the age of GCs.
The researchers used this method to estimate the absolute ages of eight Milky Way globular clusters: NGC 104 (47 Tuc), 4147, 5053, 5466, 6362, 6809 (M55), 7078 (M15), and 7099 (M30).
They point out that while astronomers have long thought that stars in GCs are the same age, more recent evidence challenges that assumption.Recent studies show that 47 Tuc has a more age-diverse population than thought, possibly due to mergers. That makes it more challenging to determine the absolute ages of GCs.
"We use 47 Tuc as our test case, as it shows the strongest evidence for multiple populations around the turn-off region among the clusters we study in this paper," the authors explain. They found that 47 Tuc has two populations. Population 1 is about 11.91 billion years old, and population 2 is about 12.02 billion years old.
They also used M55 because it contains two known stars with similar metallicities to M55 overall. "We determine the absolute age of M55 to be 12.22 ± 0.57 Gyr," they write.
This figure from the research shows the absolute age of the globular cluster M55. Image Credit: Ying et al. 2025, The Astrophysical Journal.
After detailed modelling, analysis, and comparisons, the research team determined the absolute ages of eight Milky Way globular clusters. "We present the first absolute age-metallicity relation for Milky Way GCs, derived from our expanded sample with absolute ages ranging approximately from 11.5 to 13.5 Gyr and typical uncertainties between 0.5 and 0.75 Gyr," they write.
That means the oldest stars in the Milky Way's globular clusters may be almost as old as the Universe itself. These surprising veterans formed less than 300,000 years after the Big Bang.
Their results also showed what factors contribute to the uncertainty of absolute age determination.
"Our analysis highlights a consistent pattern across all clusters, identifying distance and reddening as dominating sources of uncertainty, collectively contributing over 50% to the total uncertainty in age determination," they explain.
They also determined that the mixed populations in GCs don't have much effect on their absolute ages. "We find that the existence of multiple stellar populations in GCs has limited influence on the absolute age of GCs," they write.
These results boost our understanding of how GCs form and provide valuable insights into the Milky Way's early accretion history. "Importantly, we demonstrate a clear trend toward older ages at lower metallicities," they write. That's in line with the broader understanding that the Universe has become enriched with metals over time.
Science is iterative, and results like these will strengthen existing models of stellar evolution and globular clusters. Different researchers use differing observations, models, and analyses and add to the collective effort to build better models that more accurately reflect nature.
The payoff is uncovering interesting facts of nature, like these ancient stars in our backyard.
