Not So Dead After All: Astronomers Reveal the Secret Behind Inflated White Dwarfs

Old binary star systems may contain stars that are even hotter than scientists previously believed.

White dwarfs are the dense, compact remains left behind when stars exhaust their nuclear fuel, a process that will one day occur to our own Sun. These stellar remnants are known as degenerate stars because their internal physics defy normal expectations: as they gain mass, they actually become smaller in size.

Many white dwarfs exist in pairs, forming what are known as binary systems, where two stars orbit each other. Most of these systems are ancient by galactic standards and have cooled over time to surface temperatures near 4,000 Kelvin.

Yet, astronomers have recently identified a remarkable group of short-period binary systems in which the stars complete an orbit in less than an hour. Surprisingly, these white dwarfs appear to be about twice as large as models predict, with much higher surface temperatures ranging from 10,000 to 30,000 Kelvin.

Investigating the Role of Tidal Heating

This inspired a team of researchers, led by Lucy Olivia McNeill of Kyoto University, to investigate the theory of tides and use it to predict the temperature increase of white dwarfs in short period binary orbits. Tidal forces often deform celestial bodies in binary orbits, determining their orbital evolution.

“Tidal heating has had some success in explaining temperatures of Hot Jupiters and their orbital properties with their host stars. So we wondered: to what extent can tidal heating explain the temperatures of white dwarfs in short-period binaries?” asks McNeill.

The researchers constructed a theoretical framework accounting for the temperature increase of white dwarfs in short-period binaries. This framework is completely generalized, allowing the prediction of past and future temperature evolution as well as the orbital evolution of white dwarf stars in binary systems.

The results revealed that tidal forces can strongly influence the evolution of such white dwarfs. Specifically, the tidal pull of a small white dwarf affects the internal heating of its larger but less massive companion, causing it to inflate and increasing its surface temperature to at least 10,000 degrees Kelvin.

Inflated Stars and Extended Orbital Interactions

Due to this inflation, the team predicts that white dwarfs should typically be twice as large as theory predicts when they begin to interact, or mass transfer. Consequently, short-period white dwarf binaries can start interacting at orbital periods that are three times longer than previously expected.

“We expected tidal heating would increase the temperatures of these white dwarfs, but we were surprised to see how much the orbital period reduces for the oldest white dwarfs when their Roche lobes come into contact,” says McNeill.

White dwarfs in binary systems with such short orbital periods will eventually interact and emit gravitational radiation, and are thought to cause astronomical phenomena like type Ia supernovae and cataclysmic variables.

In the future, the team plans to apply their framework to binary systems with carbon-oxygen white dwarfs and potentially learn about type Ia explosion progenitors, paying particular attention to whether or not realistic temperatures favor the so-called double degenerate, or merger scenario.

Reference: “Tidal Heating in Detached Double White Dwarf Binaries” by Lucy O. McNeill and Ryosuke Hirai, 10 October 2025, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ae045f

Funding: Japan Society for the Promotion of Science

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