Studying red giants to understand the future of our galaxy
As stars at the end of their lives, red giants are like cosmic clocks for astronomers. Thanks to asteroseismology, a discipline that originated in part at the University of Liège, researchers are now probing their cores to better understand stellar evolution, refine astrophysical models and map our galaxy with unprecedented accuracy. This is the subject of the recently published book "The Golden Gift of Red Giants".
V
ery common in our galaxy and elsewhere in the universe, red giants are stars at the end of their lives that have exhausted their central hydrogen, causing them to expand and cool on the surface. They then become very bright and take on a reddish colour – hence their name – marking an advanced stage in their evolution. "Studying red giants is particularly important because they represent an advanced stage of stellar evolution, which allows astronomers to better understand the future of our Sun and other similar stars," explains Arlette Noels-Grotsch, astrophysicist and honorary professor at ULiège. "In addition, because they are very bright and their oscillations are easily detectable, they serve as natural beacons for exploring the structure and history of our galaxy, particularly by mapping its composition and age."
For a long time, stellar physics has mainly been successful in describing the surface properties of stars (temperature, brightness). In the case of red giants, it was almost impossible to date them accurately because they look so similar from the 'outside'. This is where asteroseismology comes in. "Asteroseismology is the science of 'listening' to the vibrations of a star in order to deduce its internal structure, mass, evolution and age," explains Andrea Miglio, astrophysicist and professor at the University of Bologna, who completed his doctoral thesis at ULiège. This discipline now allows us to probe the interior of many bodies in the Milky Way." Indeed, the tiny pulsations of the star modulate its light, variations that have been detected by missions such as CoRoT, Kepler and the recent TESS*. This analysis provides a frequency signature comparable to a barcode. By decrypting it, we can determine the internal properties of stars, namely their size, mass, age and rotation.
Asteroseismology applied to red giants provides empirical constraints that are impossible to obtain through conventional observations of the stars' surfaces. Beyond its already proven applications, the discipline promises breakthroughs in the morphology of magnetic fields, stellar activity, binary evolution and even tests of fundamental physics."By identifying precisely where our models fail, asteroseismology pushes stellar physics towards unparalleled precision and redraws our understanding of the Milky Way," says Arlette Noels-Grotsch.
These advances are shaking up our understanding of key processes in stellar evolution and refining our models with unprecedented precision. "We are entering an exciting era where stars are no longer just bright points, but precision laboratories," concludes Andréa Miglio. By putting our models to the test and accepting to discover their flaws, asteroseismology is advancing stellar physics towards a more complete understanding of the processes that govern the life and evolution of stars, and therefore that of our Sun.
* These three missions were designed to observe stars and detect exoplanets, while providing valuable data for asteroseismology. CoRoT, launched by France in 2006, pioneered the study of stellar oscillations and the discovery of exoplanets. Kepler, launched by NASA in 2009, has detected thousands of exoplanets and analysed the vibrations of many stars, particularly red giants. Finally, TESS, active since 2018, continues this exploration by observing almost the entire sky, targeting nearby bright stars to better understand their structure and evolution.
ULiège and asteroseismology
Asteroseismology has its roots in the pioneering work of Paul Ledoux, a Belgian astrophysicist and professor at the University of Liège, considered one of the founders of this discipline. In 1958, he published two major articles in the journal Handbuch der Physik, one on variable stars with Théodore Walraven, and the other, as sole author, on stellar stability, which is recognised as the founding article of asteroseismology. These publications profoundly influenced theoretical research on the evolution of stars and marked the beginning of what is now known as the Liège school of theoretical astrophysics, initiated by Paul Ledoux. At the University of Liège, the ASTA group (within the STAR Institute), currently headed by Marc-Antoine Dupret, has continued this tradition by training researchers who have played a pioneering role in the development of modern asteroseismology.
Reference to Paul Ledoux's portrait and publication (Famous Scholars)
Scientific reference
Noels-Grotsch A. and Miglio A., The Golden Gift of the Red Giants, IOP Publishing, 2025.
Online ISBN: 978-0-7503-2159-4
