Magnetic fields may let dead stars exceed 1.4 solar mass limit

Some of the universe’s dead stars may be able to grow far more massive than a famous limit in physics was thought to allow, according to new computer simulations. The work suggests that powerful magnetic fields hidden inside these stellar remnants could help them withstand crushing gravity and reach extraordinary masses, ScienceDaily reported.

The finding challenges a long-standing understanding of white dwarfs, the dense cores left behind when certain stars exhaust their nuclear fuel and shed their outer layers. Although roughly the size of Earth, they can contain as much mass as the Sun or more. For white dwarfs without strong magnetic fields or rapid rotation, physicists have long recognized an upper mass boundary of about 1.4 times the Sun’s mass, known as the Chandrasekhar limit. When a carbon-oxygen white dwarf approaches this limit, rising pressure and density in its core can ignite carbon, potentially setting off a thermonuclear explosion known as a Type Ia supernova.

New simulations by researchers from the Department of Physics at the Indian Institute of Science (IISc) and their collaborators suggest the traditional limit may not apply to white dwarfs with sufficiently strong internal magnetic fields. In one simulation, a magnetized white dwarf reached approximately 2.4 times the mass of the Sun, well above the conventional 1.4 solar masses.

“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,” said Banibrata Mukhopadhyay, Professor at the Department of Physics and corresponding author of the study published in The Astrophysical Journal Letters.

The possibility is not entirely new. Theoretical physicists have explored these objects, sometimes called “super-Chandrasekhar” white dwarfs, for decades. Astronomers have also observed Type Ia supernovae that shine much more brightly than expected, raising the possibility that some white dwarfs may reach masses as high as 2.8 times that of the Sun before exploding. But showing that such massive white dwarfs could theoretically exist is only part of the challenge. Scientists also need to explain how an ordinary star could naturally develop into one.

“The important question was not simply whether a super-Chandrasekhar white dwarf is possible, but whether a star can actually evolve into one,” said Zenia Zuraiq, first author of the study and a PhD student at the Department of Physics. “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 investigate the process, the researchers modified a stellar evolution computer program called STARS, originally developed at the University of Cambridge. They expanded the software to account for magnetic fields and the cooling of white dwarfs, then simulated how magnetized stars change over time from the main-sequence stage, when stars fuse hydrogen into helium, through later stages to their final state as white dwarfs. They then modeled a binary star system in which a white dwarf gradually pulls matter from a companion star, a process known as accretion.

The simulations revealed a surprising effect. Even a magnetic field that starts out relatively weak can become increasingly influential as a white dwarf accumulates matter. As material builds up, the star becomes denser and contracts under its own gravity. This compression strengthens its internal magnetic field, which provides additional pressure that helps counteract gravity. The stronger magnetic support allows the white dwarf to hold more mass than would otherwise be possible, and it changes the familiar relationship between a white dwarf’s mass and its size, potentially creating different maximum mass limits depending on how magnetic fields behave inside the star.

One simulation illustrates the difference. The researchers modeled a carbon-oxygen white dwarf with an initial mass of 1.02 times that of the Sun, formed from a main-sequence star that originally contained eight solar masses. The white dwarf gradually accumulated material from its companion at a rate of 10⁻⁹ solar masses per year, equivalent to one billionth of the Sun’s mass annually. With magnetic effects included, it could reach approximately 2.4 solar masses. Without those effects, an otherwise comparable model reached the conventional limit of roughly 1.4 solar masses.

The results suggest magnetic fields could fundamentally alter how massive a white dwarf can become. The same mechanism may also help explain another puzzle: some well-studied white dwarfs have unexpectedly large radii for their relatively low masses, and magnetic pressure could explain why these stars appear larger than conventional models predict.

The implications could extend well beyond the evolution of dying stars. Type Ia supernovae are among astronomy’s most important tools for measuring distances across the cosmos. Because their peak brightness can be standardized, astronomers use them as cosmic reference points, often called standardizable candles. By comparing how bright these explosions actually are with how bright they appear from Earth, researchers can estimate their distances, helping scientists investigate the universe’s size, history and ongoing expansion.

The Chandrasekhar limit has traditionally provided an important part of the physical framework for understanding these explosions. If white dwarfs reach similar masses before exploding, their energy output and brightness can be expected to follow broadly predictable patterns. The new findings suggest some exploding white dwarfs could have substantially different masses and internal magnetic fields, which could influence the properties and luminosity of the resulting supernovae and potentially help explain why certain Type Ia explosions are unusually bright.

The findings come from computer simulations rather than direct observations of white dwarfs growing to these extreme masses. They offer a possible evolutionary pathway for stars that exceed the traditional Chandrasekhar limit, suggesting one of astronomy’s most famous mass limits may have important exceptions.