The magnetism of stars seems to leave traces long after their death. Scientists have discovered what they call a "fossil magnetization" on stellar corpses: white dwarfs. This observation could reveal how stars transition from their red giant phase to the white dwarf stage, a fate that awaits our Sun.
To understand this link, we must follow the life of a star like the Sun. After exhausting its hydrogen, its core collapses while its outer layers swell enormously, forming a red giant. Then, these layers disperse, leaving behind a compact and hot core: the white dwarf.
The hot core of a red giant star will become the future white dwarf.
Credit: Paul Beck (KU Leuven, Belgium)
The research team used stellar oscillations, or "starquakes," to probe their interiors. This technique, asteroseismology, works like terrestrial seismology with earthquakes. It revealed that red giants possess a magnetic field in their cores, while white dwarfs show one at their surface. The researchers then developed a model linking these two observations, based on the idea of a "fossil field."
According to this model, the magnetic field is not confined to the core of red giants but extends over a larger part of the star. As it ages, this field reorganizes into shell-like shapes, stronger near the surface than in the core.
This discovery has direct implications for our Sun. Currently, astronomers do not know whether the Sun's core is magnetic. If it were, that would change all predictions about its lifespan.
A magnetic field could indeed mix hydrogen from the outer layers with the core, thus prolonging the star's life. But it could also have opposite effects. The researchers hope their work will help better understand what lies deep within our star.
How the evolution of a star changes the shape of its magnetic field. Simulations suggest shell-like structures (pink lines).
Credit: Lukas Einramhof | ISTA
The study was published in the journal
Astronomy & Astrophysics. It shows that stellar magnetism could be much more widespread than previously thought, even if it is sometimes difficult to detect. As Lukas Einramhof recalls, "we cannot always detect this magnetism, but our work indicates that most stars are probably magnetic."