Type Ia supernovae are considered “standard candles” in the cosmos – reliable distance indicators for astronomy. According to conventional wisdom, these stellar explosions are always equally bright. The reason: Type Ia supernovae occur when a white dwarf sucks so much material from its companion star in a binary star system that it exceeds a critical mass limit. This so-called Chandrasekhar limit is around 1.44 solar masses. Beyond this limit, the white dwarf becomes unstable and explodes in a thermonuclear explosion.
“The Chandrasekhar mass limit for white dwarfs is considered one of the most important discoveries in astrophysics of the 20th century,” explain Zenia Zuraiq from the Indian Institute of Science and her colleagues. “The solid mass during supernova ignition makes Type Ia supernovae easily calibratable ‘standard candles’ and an important step on the cosmic distance ladder.” Among other things, they are used to measure the expansion of the cosmos and thus the influence of dark energy – a basis for the entire cosmology.
White dwarfs in binary star systems can suck material from their companion stars. If their mass reaches the Chandrasekhar limit, they explode – at least that is the conventional wisdom. — © NASA/Goddard Space Flight Center
Can the Chandrasekhar limit be exceeded?
But how reliable are these “standard candles”? There have been repeated discussions about this in recent years. Astronomers have discovered some Type Ia supernovae that contradict the usual rules – they are too bright to come from a “normal-weight” white dwarf. “Some of these particularly bright Type Ia supernovae must have been caused by white dwarfs with up to 2.8 solar masses,” report Zuraiq and her colleagues. There is also increasing evidence that white dwarfs with a strong magnetic field can break the Chandrasekhar limit.
The astronomers have now examined in more detail whether this could be possible. “The crucial question was not just whether a white dwarf could even exist above the Chandrasekhar limit: we wanted to know whether a star could develop into such a supermassive white dwarf,” explains Zuraiq. She and her team therefore used astrophysical simulations to understand the evolution of stars with different magnetic fields and masses into white dwarfs. They then modeled their fate in a binary star system.
Magnetic field protects against destabilization
The simulations showed that the magnetic field of a sun-like star plays an important role in how stable it later becomes as a white dwarf. Stronger magnetic fields stabilize the star remnant when it sucks material from a companion star and gains mass, as the astronomers determined. If the predecessor has only a weak magnetic field, it becomes unstable near the Chandrasekhar limit and its carbon core ignites and explodes.
However, if the white dwarf is more magnetized, this does not happen. “Instead, the magnetic field strengthens as the white dwarf gains mass and condenses,” explain Zuraiq and her colleagues. This typically makes the white dwarf’s magnetic field stronger by two orders of magnitude – and can thus counteract the increasing gravity. In the models, a white dwarf weighing around 1.02 solar masses was able to absorb so much material that it weighed up to 2.4 solar masses without exploding.
Strongly magnetized white dwarfs can exceed the Chandrasekhar limit: the stronger their magnetic field, the heavier they can become without exploding. — © Zuraiq et al./ The Astrophysical Journal Letters, CC by 4.0
“Our simulations show that such a path is possible under certain conditions,” report the astronomers. At the same time, the results also provide an explanation for some white dwarfs that are unusually large for their mass. “This is also due to the effect of the magnetic field,” explain the astronomers. Because the magnetic field counteracts gravity, these supersized white dwarfs are less dense than expected.
Chandrasekhar limit applies – but not for everyone
According to Zuraiq and her colleagues, their results clarify discussions about the Chandrasekhar limit and the reliability of white dwarfs as standard candles in cosmology. “By understanding the complex stellar evolution of stars up to this stage, we can dispel any doubts about the existence of highly magnetized white dwarfs beyond the Chandrasekhar limit,” they state.
This means that the Chandrasekhar limit remains valid for “normal” white dwarfs with a weak magnetic field and slow rotation, but there are exceptions. “Although the Chandrasekhar limit is sacrosanct, its value depends heavily on the physical conditions, in this case the magnetic field,” the astronomers write. This means that some type Ia supernovae can be more luminous than the concept of standard candles – with corresponding consequences for cosmology.
However, it is still unclear how often such overweight white dwarfs occur and how often they explode as superluminous Type Ia supernovae.
Source: Zenia Zuraiq (Indian Institute of Science, Bengaluru) et al., The Astrophysical Journal Letters, 2026; doi: 10.3847/2041-8213/aea97e