New theoretical models, published in Astronomy & Astrophysics, connect, for the first time, the magnetism at the surface of long-dead stellar remnants (white dwarfs) with recent evidence of magnetism at the cores of their dying progenitors (red giants). The team, led by astrophysicists at the Institute of Science and Technology Austria (ISTA), argues that these magnetic fields might originate early in the stars' lives, and survive their entire evolution, emerging as 'fossil fields' at the surfaces of older remnants. A better understanding of these processes can also help to better understand our own Sun's future.
For thousands of years, human civilizations have looked to the stars with a blend of curiosity and reverence. From a human perspective, these twinkling dots in the sky seem to shine eternally. However, while stars live for billions of years, their evolution is also marked by major events. While some die in a spectacular display of cosmic fireworks called supernovae, others retreat and cool down quietly, leaving behind a dead remnant called a white dwarf.
Using a theoretical model, an international team—led by PhD student Lukas Einramhof and Assistant Professor Lisa Bugnet at the Institute of Science and Technology Austria (ISTA) —links independent observations collected at different stages of stellar evolution. For the first time, they connect the evidence of magnetic fields reaching the surface of older white dwarfs to recent findings of magnetism in the cores of red giants—the dying progenitors of those remnants. Central to their model is the idea that magnetic fields formed early in a star's life can persist through all later stages, emerging at the surfaces of white dwarfs as "fossil fields" billions of years later. By incorporating recent asteroseismic data—measurements of stellar oscillations or "starquakes"—the team revisits the fossil field theory as a possible explanation for stellar magnetism.
Long-dead, and suddenly magnetic?
Magnetic fields at the surface of white dwarfs provide astrophysicists with valuable information about the remnants' past. "The magnetic field in a star is important for how the star works on the inside and how long it lives and evolves. Generally, more of the older white dwarfs tend to be more magnetic than younger white dwarfs," says Einramhof. Therefore, to explain where the magnetic fields at the surface of older white dwarfs—dead several million years earlier—come from, scientists must dig deeper into the remnants' past lives.
So far, several teams of researchers have been examining the magnetic fields of stars at different points of their stellar evolution. The ISTA team now seeks to connect these dots to clarify the processes underlying the evolution of the stars and their remnants. "As a theoretical astrophysics group, we develop theories to explain observations," Bugnet underlines.
Starquakes uncover buried magnetic fields
With asteroseismology—the study of starquakes—astronomers have only recently been able to probe the depths of red giants, the progenitors of white dwarfs. Similar to earthquakes, starquakes are natural phenomena that allow scientists to obtain measurements of the insides of stars.
The observations, carried out independently by different groups, show contrasting pictures. On the one hand, magnetic fields have been detected at the surface of older white dwarfs, suggesting that these might eventually reach the surface from within as the remnant evolves. On the other hand, observations on the 'dying' red giants using asteroseismology have provided evidence of the presence of magnetic fields at the cores of these progenitors of white dwarfs, several million years earlier in a star's evolutionary path. Using these observations to constrain their theoretical model, the ISTA team demonstrates that these two time points in a star's lifetime can be connected using a theory that had fallen out of fashion over the past decade in the white dwarf community: the fossil field scenario.
Einramhof explains, "Because a white dwarf is the exposed core of a red giant that has shed its outer layers, these different observations essentially examine the same region of a star's interior at different evolutionary stages." Therefore, after a red giant sheds its outer layers, its white dwarf remnant will display distinctive properties at its surface.
He adds, "If the magnetic field observed during the red giant phase is the same as the one that evolves to be observed at the surface of the white dwarf, then the fossil field theory can explain and connect the observations." However, the team argues that this magnetic field must originate even earlier, before the red giant phase.
Magneto-archaeology: digging into the stars' past
By revisiting the fossil field scenario with new insights, the team made several key findings about the archaeology of magnetism in stars. First, they showed that the extent of magnetism within the core of the red giant progenitor is key. "To connect the magnetic fields observed at the surface of older white dwarfs with the ones found at the core of their red giant progenitors, a larger fraction of the star must be magnetized," says Einramhof. "However, this doesn't mean the stars are more strongly magnetized, only that the magnetic fields must already reach a larger portion of their core."
Furthermore, their methodology allowed them to uncover how the evolution of a star changes the shape of a magnetic field. Rather than being centered at one point, their simulations suggest that magnetic fields can form shell‑like structures—resembling the surface of a basketball—where the field is strongest near the shell rather than at the core.
Blind at the core: what if the Sun's core is also magnetic?
Ultimately, the team's goal is to better understand how the Sun will evolve. As a 4.6-billion-year-old main-sequence star, the Sun is midway through its expected lifetime in this phase before evolving into a red giant and likely engulfing Earth. "We still don't know whether the Sun's core is magnetic. Even though it's our own star, we're practically blind to what happens at its center," says Einramhof. "Current predictions assume that the Sun's core is not magnetic. But if it turns out to be, this information would change everything we know and all the models we've based our work on."
During their longest-lived phase, called the main sequence, stars remain stable until they run out of core hydrogen 'fuel' and can no longer sustain the fusion process. When this internal mechanism fails, they puff up and evolve into red giants. "If the Sun can somehow bring hydrogen from its outer layers into its core, it would be able to live longer. One way to do this would be through strong magnetic fields," says Einramhof. However, the magnetic fields might also lead to a very different outcome. "We know that magnetic fields can significantly affect a star's evolution. But we still don't know exactly how they influence stellar evolution or how strong their effects are."
The ISTA team's findings help reestablish the fossil field theory as a plausible mechanism for the evolution of stellar magnetic fields. However, other questions remain unanswered. "Given how little we know at this stage, our work suggests that stars are most likely all magnetic. But we can't always detect this magnetism," Einramhof concludes.
