Dead stars could break one of physics’ most famous limits

The discovery 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 these stellar remnants are roughly the size of Earth, they can contain as much mass as the Sun or even more.

For white dwarfs without strong magnetic fields or rapid rotation, physicists have long recognized an upper mass boundary of approximately 1.4 times the Sun’s mass. Known as the Chandrasekhar limit, this threshold plays an important role in understanding how some stars end their lives in spectacular explosions.

When a white dwarf made primarily of carbon and oxygen approaches this limit, the increasing pressure and density inside its core can ignite carbon, potentially setting off a powerful thermonuclear explosion known as a Type Ia supernova.

Scientists Discover How Magnetic Fields Could Break a Famous Cosmic Limit

New simulations by researchers from the Department of Physics at the Indian Institute of Science (IISc) and their collaborators suggest that the traditional mass 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, substantially exceeding the conventional limit of 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,” explains Banibrata Mukhopadhyay, Professor at the Department of Physics and corresponding author of the study published in The Astrophysical Journal Letters.

The possibility of white dwarfs exceeding the Chandrasekhar limit is not entirely new. Theoretical physicists have explored these unusual objects, sometimes called “super-Chandrasekhar” white dwarfs, for decades.

Astronomers have also observed Type Ia supernovae that shine much more brightly than expected. These exceptionally luminous explosions have raised the possibility that some white dwarfs may reach masses as high as 2.8 times that of the Sun before exploding.

However, demonstrating 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.

Following a Star’s Evolution From Birth to Death

“The important question was not simply whether a super-Chandrasekhar white dwarf is possible, but whether a star can actually evolve into one,” says Zenia Zuraiq, first author of the study and 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 this process, the researchers modified a stellar evolution computer program called STARS, originally developed at the University of Cambridge.

The team expanded the software to account for magnetic fields and the cooling of white dwarfs. These additions allowed them to simulate how magnetized stars change over time, beginning with the main-sequence stage, when stars generate energy by fusing hydrogen into helium.

The simulations then followed the stars through their later evolutionary stages and into their final state as white dwarfs. Afterward, the researchers modeled a binary star system in which a white dwarf gradually pulls matter from a neighboring companion star.

This process, known as accretion, allows the white dwarf to gain mass over time. The researchers wanted to determine whether its growing magnetic field could change how much additional material it could support.

A Hidden Magnetic Force Helps Dead Stars Resist Gravity

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 in turn provides additional pressure that helps counteract gravity.

The stronger magnetic support allows the white dwarf to hold more mass than would otherwise be possible. It also changes the familiar relationship between a white dwarf’s mass and its size, potentially creating different maximum mass limit(s) depending on how magnetic fields behave inside the star.

One simulation illustrates just how dramatic the difference could be.

The researchers modeled a carbon-oxygen white dwarf with an initial mass of 1.02 times that of the Sun. It had formed from a main-sequence star that originally contained eight solar masses.

In the simulation, 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.

When magnetic effects were included, the white dwarf 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 that magnetic fields could fundamentally alter how massive a white dwarf can become.

The same mechanism may also help explain another astronomical puzzle. Some well-studied white dwarfs have unexpectedly large radii for their relatively low masses. Magnetic pressure could provide a possible explanation for why these stars appear larger than conventional models predict.

Could the Discovery Change How We Measure the Universe?

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. Such measurements have helped 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.

However, the new findings suggest that some exploding white dwarfs could have substantially different masses and internal magnetic fields.

Those differences could influence the properties and luminosity of the resulting supernovae, potentially helping explain why certain Type Ia explosions are unusually bright.

Understanding how magnetic fields affect these stellar explosions could therefore help astronomers refine their interpretations of supernova brightness and improve their understanding of cosmic expansion.

Although 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.

The research suggests that one of astronomy’s most famous mass limits may have important exceptions, with powerful magnetic fields allowing some dead stars to become far heavier than previously expected.