Magnetic Fields May Let Dead Stars Exceed a Famous Mass Limit, Simulations Suggest

In one simulation run by physicists at the Indian Institute of Science, a dead star kept growing past a boundary that has shaped stellar physics for decades. The star was a white dwarf, and when the calculation ended it held about 2.4 times the mass of the Sun. The long-accepted ceiling for such objects is about 1.4.

White dwarfs are the dense cores left when certain stars use up their nuclear fuel and shed their outer layers. They are roughly the size of Earth, yet can hold as much mass as the Sun or more. For those without strong magnetic fields or rapid rotation, physicists have long recognized a maximum of about 1.4 solar masses, the Chandrasekhar limit. It matters because of what happens near it. When a white dwarf made mostly of carbon and oxygen approaches the limit, rising pressure and density in its core can ignite carbon and set off a thermonuclear explosion called a Type Ia supernova.

The new work comes from the Department of Physics at IISc and collaborators, and appears in The Astrophysical Journal Letters. Banibrata Mukhopadhyay, a professor in the department and the study's corresponding author, traces the idea to 2011. A summer student came to him, and he casually handed over a problem: check whether a magnetic field could violate the Chandrasekhar limit. Theorists have explored so-called super-Chandrasekhar white dwarfs for decades, and astronomers have seen Type Ia supernovae far brighter than expected, which has raised the possibility that some white dwarfs reach as much as 2.8 solar masses before exploding.

Showing such objects can exist on paper is only part of the problem, though. Zenia Zuraiq, the study's first author and a PhD student in the department, says the real question was whether an ordinary star can actually evolve into one. To test that, the team modified STARS, a stellar evolution program originally developed at the University of Cambridge, adding treatment of magnetic fields and of white dwarf cooling. They followed magnetized stars from the main-sequence stage, when hydrogen fuses into helium, through their later phases and into the white dwarf state. They then modeled a binary system in which the white dwarf slowly draws matter from a companion, a process called accretion.

The simulations showed that even a relatively weak magnetic field can become more influential as matter piles on. The star grows denser and contracts under its own gravity, which strengthens its internal magnetic field. That field supplies extra pressure working against gravity, letting the star hold more mass than it otherwise could. It also alters the familiar relationship between a white dwarf's mass and its size, and could yield different maximum masses depending on how the fields behave inside.

The headline case began with a carbon-oxygen white dwarf of 1.02 solar masses, formed from a main-sequence star that started with eight solar masses. It accreted material at 10⁻⁹ solar masses per year, one billionth of the Sun's mass annually. With magnetic effects included, it reached about 2.4 solar masses. An otherwise comparable model without them stopped at roughly 1.4. The same mechanism may also explain why some well-studied white dwarfs have unexpectedly large radii for their low masses, since magnetic pressure could puff them up beyond conventional predictions.

The stakes extend to cosmology. Type Ia supernovae serve as standardizable candles: because their peak brightness can be standardized, comparing true brightness with apparent brightness gives distances, and those distances have helped researchers study the universe's size, history, and expansion. The Chandrasekhar limit underpins the expectation that these explosions behave in broadly predictable ways. If some exploding white dwarfs have substantially different masses and magnetic fields, their luminosity could differ, which might account for the unusually bright events and refine how supernova brightness is interpreted.

The caveat is plain: this is simulation, not observation. No one has directly watched a white dwarf grow to these masses. What the work offers is a possible evolutionary pathway, and the next thing to watch is whether observations of bright supernovae or oversized white dwarfs bear it out.