One of the best places to study stars is inside "open clusters", which are groups of stars that formed together from the same material and are bound together through gravity.
Open clusters act as laboratories, showing how stars of different masses and ages behave. At the same time, some stars known as "variable stars", regularly change in brightness, and their flickers and pulses help scientists learn about the physics inside stars and about the wider galaxy.
Until now, astronomers studied clusters and variable stars separately, and usually one cluster at a time. But that approach missed the bigger picture, leaving gaps in our understanding of how the lives of stars unfold across the galaxy.
Now, Richard I. Anderson, head of the Standard Candles and Distances Laboratory at EPFL and Emily Hunt at the Max Planck Institute for Astronomy, have combined these two approaches for the first time. Using data from the European Space Agency's Gaia mission , they mapped nearly 35,000 variable stars inside 1,200 open clusters across the Milky Way. This "bird's eye view" gives researchers an unprecedented map of how stars live, age, and die as part of their communities.
The findings are published as a Letter to the Editor in the journal Astronomy & Astrophysics.
New patterns in the stellar life cycle
"It is a scientific first in the way that large samples of star clusters and variable stars are analyzed together," says Anderson. "This creates synergies because the two approaches provide complementary insights."
The team built their map using the third data release from Gaia , a satellite that precisely measures the positions, brightness, and colors of more than a billion stars. They focused on clusters within 6,500 light-years away to make sure their results were reliable.
The researchers matched Gaia's catalog of variable stars to the stars in these clusters and checked the ages, distances, and brightness of each one. By tracking where each type of variable star appears in a cluster and how their numbers change with cluster age, the team pieced together new patterns in the stellar life cycle.
The results show that at least one in five stars in these clusters changes brightness over time. Young clusters host the greatest variety of variable stars, while older clusters show more stars with slow, Sun-like cycles. The study also shows that certain types of variable stars serve as markers for a cluster's age, providing a new tool for measuring how old a group of stars is without having to build complicated models.
"We are made of stardust"
The team made their catalog public, sharing the positions, types, and properties of all 35,000 variable stars in these clusters. The study also offers the cleanest diagram yet showing how different types of variable stars are distributed across the key map astronomers use to track stellar evolution (the Hertzsprung Russell Diagram).
The Gaia mission is now entering its most exciting scientific phase even though the satellite was recently turned off. Over the coming years, Gaia's vast archive of observations, which cover nearly 2 billion stars, will be processed and analyzed by scientists across Europe.
"Our work is a teaser for what is to come with Gaia [data releases 4 and 5], which will revolutionize the study of stellar populations by their light variations," says Anderson.
By showing how variable stars can be used as "clocks" and "markers" in stellar evolution, the team has opened up new ways to explore the story of the universe. "We are made of stardust," says Anderson. "Understanding the lives of stars and the physics that govern stars is crucial to understanding our origins and place in the cosmos."
Other contributors
Heidelberg University
Reference
Richard I. Anderson, Emily L. Hunt. A birds-eye view of stellar evolution through populations of variable stars in Galactic open clusters. Astronomy & Astrophysics 13 August 2025. DOI: 10.1051/0004-6361/20255511110 .1051/0004-6361/202555111
