Telling Time with Stars: Using Kepler Data to Build Galactic Clocks

In their paper Casali et al. present a method for measuring the ages of stars using their chemical composition. This approach, known as “chemical clocks,” takes advantage of the fact that some elements in a star’s atmosphere change in predictable ways over time. By carefully analyzing the ratios of certain elements, the authors provide a way to estimate stellar ages with high precision—opening new windows into the history of the Milky Way.

Using Starquakes to Measure Ages

The introduction highlights the importance of stellar ages for understanding the Galaxy’s past. However, determining the age of individual stars—especially those not part of clusters—is notoriously difficult. Asteroseismology, the study of stellar oscillations (like "starquakes"), provides a powerful tool by revealing a star’s internal structure, which in turn helps calculate its mass, radius, and most importantly, age. Observations from space missions like Kepler, which collected long-term data on star brightness, make it possible to identify and measure these pulsations with great precision.

Building the Sample and Gathering Spectra

The team selected 68 red giant stars from the Kepler field that already had high-quality seismic data. They observed these stars using two powerful telescopes in Spain and measured their light at very high resolution. By analyzing this light, they could determine the abundances of many elements in the stars’ atmospheres, especially those important for chemical clock studies—namely s-process elements (like Yttrium, Zirconium, and Cerium) and alpha elements (like Magnesium, Calcium, and Titanium). These elements are produced in different types of stars and on different timescales, so their ratios carry information about when a star formed.

Calibrating the Clocks

A crucial step in their work involved calibrating the chemical clocks using the known ages of the Kepler stars, which had been determined with great accuracy using individual oscillation frequencies. Among the many ratios studied, they found that [Ce/Mg] and [Zr/Ti] showed the strongest and most precise correlations with stellar age. These ratios varied smoothly with age and had very small scatter, making them reliable age indicators. This calibration is what transforms these abundance ratios into "clocks" that can tell how old a star is.

Applying the Clocks to the Milky Way

To test their method, Casali et al. applied their chemical clocks to stars from large surveys like APOGEE and Gaia-ESO, which contain thousands of stars across the Milky Way. This allowed them to reveal the age distribution of different stellar populations. For example, they were able to distinguish between stars with high and low alpha-element content—two groups that likely formed in different parts of the Galaxy or at different times. They also recovered the expected age-metallicity relationship, observed signs of the Milky Way's disc structure (like flaring), and even identified old stars that are unexpectedly rich in heavy elements.

Conclusion: A New Way to Tell Time in the Galaxy

In conclusion, this study represents a major step forward in “Galactic archaeology.” By combining the precision of Kepler’s asteroseismology with detailed chemical analysis, Casali and colleagues have built a set of reliable tools for estimating stellar ages. These chemical clocks offer a way to study the Galaxy’s history with thousands of stars—many more than could be aged with traditional methods. Their results not only improve our ability to tell time in the cosmos but also provide new clues about how our Galaxy formed and evolved over billions of years.

Source: Casali