Star-Crossed Clusters: When Ages Are at Odds

There’s more than one way to measure the age of a star in a cluster. But do different age-assigning methods agree?

Assessing Ages

The ages of stars are critical components for many fields of astronomy, from the study of stars themselves to the planets that orbit them and the galaxies they make up. But measuring stellar ages is tricky. It’s considered relatively “easy” to measure the ages of stars in clusters; assuming all stars in a cluster are born from the same material at the same time, the age of the cluster’s stars can be determined using isochrones — theoretical curves describing the temperatures and luminosities of a group of stars of the same age.

However, most stars do not reside in clusters, and to measure the ages of stars that roam the galaxy solo or in small groups, astronomers must use other methods. One method is asteroseismology, in which researchers measure the changes in a star’s brightness or the motion of its surface due to sound waves bouncing around in its interior. Asteroseismology can reveal critical information about a star’s interior structure, composition, and age, but it’s not yet clear how asteroseismic age estimates compare to well-established isochrone-fitting methods for star clusters.

Measurement Matchup

To investigate how these methods compare, Jamie Tayar (University of Florida) and Meridith Joyce (University of Wyoming) compared the age estimates from asteroseismology and isochrone-fitting methods for seven well-studied open and globular clusters. The team scoured the literature for asteroseismic measurements of red giant stars, which are commonly used to calibrate studies of the Milky Way’s history and evolution, and selected a representative age for each cluster from isochrone-fitting methods from the literature.

While the median ages derived from asteroseismic methods fell close to the cluster age for many of the clusters, the team found an enormous amount of spread in the asteroseismic ages of stars within a single cluster. The spread is far greater than expected from the uncertainty in a given asteroseismic measurement, or from the uncertainty arising from the use of different models.

In addition, two of the seven samples had median ages far from the isochrone-fitting cluster age. For these two clusters, especially the well-constrained open cluster NGC 6819, it’s possible that the asteroseismic ages and cluster age are systematically offset from one another — a possibility that would require a dedicated asteroseismic search with a single observatory and a single analysis pipeline to investigate further.

Ages at Odds

Asteroseismic stellar ages are increasingly used to probe the history of the Milky Way and calibrate other age determination methods — so what does it mean for the asteroseismic results to be so varied and potentially offset from the cluster fitting method?

It may mean that asteroseismic measurements require further investigation and calibration before they can achieve the accuracy necessary for galactic archaeology. Part of this endeavor may involve an investigation of binary processes such as accretion or mergers that could alter the masses of individual stars, affecting their inferred ages. These efforts could potentially unlock hundreds of thousands of stellar age measurements from past, present, and future datasets, aiding studies that explore everything from individual stars and their planets to the histories of entire galaxies.

Citation

“Star-Crossed Clusters: Asteroseismic Ages for Individual Stars Are in Tension with the Ages of Their Host Clusters,” Jamie Tayar and Meridith Joyce 2025 ApJL 984 L56. doi:10.3847/2041-8213/adcd6f