Binary Stars Illuminate the Secrets of NGC 2506: A Precise Age and Distance for a Middle-Aged Star Cluster

The paper by Kadri Yakut et al. (2025) takes a closer look at the open cluster NGC 2506, a large group of stars about 3,000 light-years from Earth. Using data from Gaia, TESS, and ground-based telescopes, Yakut and collaborators focused on five binary star systems, pairs of stars orbiting each other, to pin down the cluster’s age, distance, and chemical composition with greater precision than ever before. This approach, which combines light curve, radial velocity, and spectral energy distribution (SED) analyses, shows how binary stars can serve as powerful tools for understanding the life cycles of stars in clusters.

The Cluster and Its Importance

NGC 2506 lies toward the outer part of the Milky Way’s disk, in what astronomers call the “Galactic anti-center” direction. It’s an intermediate-age, metal-poor cluster, meaning its stars formed about 2 billion years ago from gas containing fewer heavy elements than the Sun. Its clearly defined main sequence and giant branch make it a key testing ground for stellar evolution theories. Over the years, astronomers have disagreed about its exact age and metallicity, with values ranging from 1.5 to 3.4 billion years and [Fe/H] ≈ −0.3 (where lower numbers indicate fewer heavy elements). These discrepancies motivated Yakut and colleagues to use binary stars as a more reliable benchmark.

Observations and Data Collection

The authors selected five double-lined spectroscopic binaries (SB2s) in NGC 2506, systems where both stars’ spectral lines are visible and can be measured accurately. Two of these systems, WOCS 5002 and WOCS 17003, also show eclipses in their TESS light curves, making them especially valuable. The team combined radial velocity data (showing how the stars move toward and away from Earth), TESS photometry, and Gaia astrometry to model each system’s orbit and physical properties. They also used multi-band photometry from surveys like 2MASS, Pan-STARRS, and WISE to build SEDs for each star, covering wavelengths from visible to infrared light.

Modeling Binary Systems

For the two eclipsing systems, Yakut et al. modeled the light curves using the PHOEBE code, a program that simulates how the brightness of a binary changes over time. These analyses yielded precise measurements of masses, radii, and temperatures, as well as orbital inclinations and eccentricities. For example, WOCS 5002 showed a notably elliptical orbit (e ≈ 0.6), while WOCS 17003 had a nearly circular orbit. By combining light and velocity data for all five systems, the researchers were able to model the cluster in a self-consistent way, ensuring that each binary’s parameters aligned with a single shared age and distance.

Fitting the Cluster’s Age and Distance

The researchers performed a joint SED fit for all ten stars (five binaries), solving for 18 parameters: ten stellar masses, five orbital inclinations, and common values of age, distance, and interstellar extinction (AV). Using MIST stellar evolution models, they found a cluster age of 1.94 ± 0.03 billion years and a distance of 3189 ± 53 parsecs. These results were cross-checked against Gaia-based distances for 919 cluster members, which yielded a nearly identical value of 3105 ± 75 parsecs, confirming the robustness of their approach. The cluster’s light absorption by interstellar dust was modest, with AV = 0.21 ± 0.04, consistent with previous measurements.

Astrometric Verification and Isochrone Comparison

To confirm that their binary-derived parameters were consistent with the cluster, the authors used Gaia’s parallaxes and proper motions to identify probable cluster members and construct a Hertzsprung–Russell diagram (HRD). The HRD showed a tight main sequence and giant branch, indicating a clean member sample. The five binary systems aligned perfectly with the theoretical isochrones for 1.94 Gyr, reinforcing the accuracy of the derived parameters. The team also compared multiple stellar evolution models (MIST, PARSEC, YaPSI/Y2) and found that the model-dependent uncertainty in age was only about ±0.07 Gyr, demonstrating remarkable precision for such a complex system.

Conclusions and Broader Implications

Yakut and collaborators have produced one of the most precise and self-consistent characterizations of an open cluster to date. Their method, jointly modeling multiple binary systems, overcomes limitations of traditional isochrone fitting by directly measuring stellar masses and radii. The resulting parameters not only refine NGC 2506’s properties but also set a new standard for how astronomers can calibrate stellar evolution models across the Milky Way. With an age of about 1.94 billion years, NGC 2506 now serves as a benchmark for studying midlife stars and the chemical evolution of the Galactic disk.

Source: Yakut