Magnetic Fields May Push White Dwarfs Past the Chandrasekhar Limit

For nearly a century, the fate of dying stars has been governed by one of the most celebrated numbers in astrophysics: the Chandrasekhar limit, an upper mass of roughly 1.4 times that of the Sun beyond which a white dwarf can no longer support itself against its own gravity. Now, new simulations from the Indian Institute of Science (IISc) and collaborators suggest that this iconic ceiling may not be as absolute as textbooks imply. According to a study published in The Astrophysical Journal Letters, strong internal magnetic fields could allow certain carbon-oxygen white dwarfs to grow to masses far above the classical limit, with one simulated star reaching about 2.4 solar masses before any collapse or explosion.

White dwarfs are the dense, Earth-sized remnants left behind when stars like the Sun exhaust their nuclear fuel and shed their outer layers. In an ordinary, non-rotating and non-magnetised white dwarf, the star is held up by electron degeneracy pressure, a quantum-mechanical effect that resists compression independently of temperature. Chandrasekhar showed in the 1930s that this support has a finite capacity: as the star’s mass approaches about 1.4 solar masses, the electrons are pushed to ever higher speeds approaching the speed of light, and the pressure they provide can no longer keep pace with gravity. For a carbon-oxygen white dwarf, crossing this threshold means the core becomes dense enough to ignite carbon fusion, potentially triggering a runaway thermonuclear explosion observed as a Type Ia supernova.

The constancy of that threshold is what makes Type Ia supernovae so valuable to cosmologists. If every such explosion arises from a star near the same limiting mass, the total energy released and the peak luminosity should be essentially fixed from event to event. Astronomers have exploited this uniformity to calibrate cosmic distances, and it was precisely this technique that revealed the accelerating expansion of the universe. Any population of white dwarfs that can exceed the classical limit therefore has consequences that ripple outward from stellar evolution all the way to measurements of the cosmos itself.

The new work began, somewhat unexpectedly, as an informal challenge. “The idea started in 2011, when a summer student came to me and I gave him a problem quite casually: to check whether the Chandrasekhar limit can be violated by a magnetic field,” explains Banibrata Mukhopadhyay, Professor at the Department of Physics at IISc and corresponding author of the study. What began as a back-of-the-envelope exercise has matured, over more than a decade, into a full evolutionary treatment of magnetised stars, carried out with first author Zenia Zuraiq, a PhD student in the same department, and collaborators.

Theorists had already speculated for decades that super-Chandrasekhar white dwarfs might exist. Observations of unusually over-luminous Type Ia supernovae have lent circumstantial support to the idea, hinting at progenitor stars whose masses, and whose effective mass limit, could be as high as 2.8 times the mass of the Sun. What remained uncertain was whether such objects could actually form through ordinary stellar evolution, rather than existing merely as theoretical curiosities. “The important question was not simply whether a super-Chandrasekhar white dwarf is possible, but whether a star can actually evolve into one,” says Zuraiq. “Our simulations allowed us to follow that evolutionary pathway from the main-sequence star to the white dwarf, and show that under certain conditions, such a pathway is possible.”

To answer that question, the team turned to STARS, a stellar evolution code originally developed at the University of Cambridge to model how stars change over their lifetimes. The researchers modified the code in two crucial ways: they incorporated the physics of magnetic fields and they added a treatment of white dwarf cooling. This allowed them to track magnetised stars continuously from the main sequence, the long-lived phase in which stars generate energy by fusing hydrogen in their cores, through their later evolution into compact white dwarfs. They then placed the resulting white dwarf in a binary system, where it could accrete additional matter from a companion star, the standard route by which white dwarfs are thought to build up mass toward explosion.

The simulations revealed a self-reinforcing mechanism that grows more powerful as the star gains weight. A magnetic field that is initially too weak to matter becomes increasingly significant as the white dwarf accumulates mass. Because accretion makes the star denser and causes it to contract, the internal magnetic field is amplified in the process. The strengthened field then exerts an additional pressure inside the star, providing extra support against the immense gravitational squeeze and allowing the star to hold up more mass than degeneracy pressure alone could sustain. This magnetic support alters the usual relationship between a white dwarf’s mass and its radius, producing new mass limits whose exact values depend on the detailed physics assumed for the magnetic field.

The quantitative contrast between magnetised and unmagnetised models is striking. In one simulation, the team started with a main-sequence star of 8 solar masses, which evolved into a carbon-oxygen white dwarf of 1.02 solar masses. Placed in a binary and fed matter at a rate of 10⁻⁹ solar masses per year, the magnetised model was able to grow to a limiting mass of about 2.4 solar masses, while the corresponding non-magnetised model stalled at the familiar value of roughly 1.4 solar masses. In other words, the magnetic field effectively raised the ceiling on the star’s mass by about a full solar mass, a difference large enough to change both the star’s ultimate fate and the character of any explosion it might eventually produce.

The magnetic mechanism may also resolve a long-standing puzzle in observations of individual white dwarfs. Some well-studied stars in this class have radii that are larger than standard models predict for their measured, relatively low masses, an anomaly that has resisted straightforward explanation. The simulations suggest that the same magnetic pressure that supports extra mass can also inflate the star’s size, offering a natural account of why certain white dwarfs appear puffier than their masses would suggest. If confirmed, this would connect the super-Chandrasekhar scenario to a broader range of observed stellar remnants and provide an independent diagnostic of strong internal magnetism in compact stars.

The implications for cosmology could be equally significant. Type Ia supernovae serve as standardisable candles, the calibrated beacons used to measure distances across the universe and to chart its expansion. If some fraction of these explosions originate from progenitors whose masses and magnetic properties differ substantially from the canonical picture, their luminosities may not follow the same calibration as the rest of the population. Understanding this diversity, the researchers argue, could be essential when interpreting the brightness of individual supernovae and, by extension, when drawing conclusions about the expansion history of the universe. The study, published in The Astrophysical Journal Letters on 8 October 2026 under the title describing super-Chandrasekhar white dwarfs from the evolution of magnetised main-sequence stars, marks a shift in the field: from asking whether magnetic fields can modify a famous limit on paper, to demonstrating with evolutionary simulations that nature may have found a way to do so in real stars.

Subject of Research: Magnetic field effects on white dwarf mass limits and super-Chandrasekhar stellar evolution

Article Title: Strong magnetic fields could allow white dwarfs to grow beyond the Chandrasekhar limit

Article References: Strong magnetic fields could allow white dwarfs to grow beyond the Chandrasekhar limit. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: white dwarfs, Chandrasekhar limit, magnetic fields, Type Ia supernovae, stellar evolution, Indian Institute of Science, The Astrophysical Journal Letters, electron degeneracy pressure, binary accretion, standard candles, cosmology, super-Chandrasekhar