A dead star could become far more massive than predicted by a famous limit in astrophysics.
When a Sun-like star reaches the end of its life, it expels its outer layers. Its core can then become a white dwarf: an extremely dense star that retains a considerable mass within a volume comparable to that of Earth. Deprived of significant new nuclear reactions, this star gradually cools.
Artist's view of a magnetic white dwarf. The lines represent its magnetic field.
Credit: ESO/L. Calçada. CC BY 4.0.
But a white dwarf cannot accumulate matter indefinitely. Its own gravity tries to crush it, while the electrons in its matter resist this compression. For a classic, weakly magnetized, slowly rotating star, this balance fails beyond about 1.4 times the mass of the Sun. Beyond this limit comes gravitational collapse.
This value is known as the Chandrasekhar limit. Researchers wanted to find out what happens to it if the white dwarf has an extremely powerful magnetic field. Such a field can exert additional pressure inside the star and alter its resistance to gravity.
The team used a computer program capable of tracking a star's evolution from its youth to the white dwarf stage. It then simulated the arrival of matter from a companion star. This scenario makes it possible to simultaneously track the increase in mass, the star's contraction and the strengthening of its magnetic field.
In one of the models, the white dwarf starts with a mass slightly greater than that of the Sun. Without the effect of magnetism, its limit approaches 1.4 solar masses, as already known. With a sufficiently intense internal field, it could reach about 2.4 solar masses, or roughly 70% more.
The magnetic field would become stronger as the star contracts. It would then help support its layers against their own weight. The maximum mass obtained nevertheless varies depending on the shape and intensity of the field, as well as the star's cooling.
This possibility could help explain certain type Ia supernovae, explosions produced by white dwarfs. Some appear brighter than expected. Researchers are now seeking observations that could identify these potentially exceptionally massive white dwarfs before they explode.