Measuring Time in the Stars: A New Way to Age the Ancient Cluster NGC 188
In this study, Yakut and collaborators present a fresh approach to measuring the age of one of the oldest known open clusters in the Milky Way, NGC 188. Open clusters are groups of stars that formed at roughly the same time, making them valuable “laboratories” for testing how stars evolve. NGC 188, located about 1,850 parsecs (∼6,000 light-years) away, is a prime target because of its old age, chemical uniformity, and the wealth of past observations. Previous estimates placed its age between 6 and 8 billion years, but results varied depending on the method and models used. The authors aim to refine this value by combining detailed data from binary star systems in the cluster with advanced stellar models.
Observations and Target Selection
The team selected six binary star systems in NGC 188, all of which show double-lined spectroscopic features (meaning the light from both stars can be measured separately in their spectra). They excluded systems with signs of past mass transfer, blue stragglers, or too faint to analyze well. The binaries were chosen from previous radial velocity surveys, mainly from the WIYN Open Cluster Radial Velocity Survey. While only two systems (PKM 4705 and PKM 5762) show eclipses, all six provided enough data for the analysis. Observations came from both ground-based telescopes and space missions, including Gaia for astrometry, TESS for light curves, and a variety of surveys for broad-wavelength photometry.
Modeling Binary Stars
For the two eclipsing systems, the authors combined light curves from TESS with radial velocity data to determine precise orbital and stellar properties. They used the PHOEBE software, which applies physical models of binary star systems, and refined the results using Markov Chain Monte Carlo simulations to quantify uncertainties. This gave highly accurate measurements of the stars’ masses, radii, temperatures, and luminosities, key ingredients for determining the age of the cluster.
Joint SED and Radial Velocity Fitting
The main novelty lies in jointly fitting the spectral energy distributions (SEDs) of all six binaries along with their measured radial velocity amplitudes. An SED shows how much light a star emits at different wavelengths, and when compared to theoretical isochrones (stellar evolution model curves for stars of the same age), it can reveal a star’s age and size. By assuming all twelve stars formed at the same time, the authors fitted 21 parameters at once, including all stellar masses, orbital inclinations, a single age, a common distance, and the amount of interstellar extinction. They tested fits for six different metallicity values ([Fe/H]) using MIST stellar models, finding that the age depends slightly on metallicity.
Astrometric Membership Analysis
To ensure they were studying genuine cluster members, the team used Gaia DR3 data to identify stars in NGC 188 with similar motions across the sky. They applied a proper motion cut-off that cleanly separated cluster stars from the background field, then removed stars whose Gaia data suggested unresolved binaries that could skew distances. This yielded 333 high-probability members, from which they measured a precise cluster distance of 1,850 ± 12 parsecs.
Results
The joint analysis produced a cluster age of 6.41 ± 0.28 (statistical) ± 0.18 (metallicity) Gyr. This precision comes largely from including evolved stars, particularly one in PKM 4705, which strongly constrains the models. Even without that binary, the age remains tightly measured at 6.0 ± 0.5 Gyr. The authors found excellent agreement between their stellar parameters and multiple sets of stellar evolution models, strengthening confidence in their results. Their independent photometric distance estimate (1,897 ± 58 pc) closely matches the Gaia-based value, and the stars’ positions on the Hertzsprung–Russell diagram align well with a 6.4 Gyr isochrone.
Conclusions
By combining spectroscopic, photometric, and astrometric data in one integrated framework, Yakut et al. have not only refined the age of NGC 188 but also demonstrated a robust method for dating other star clusters, even without requiring eclipsing binaries. This approach can help calibrate stellar evolution models and improve our understanding of how star clusters age and survive over billions of years.
Source: Yakut
