White dwarfs—the dense, fading cores left behind after stars like our sun die—are usually calm, cool, and ancient.
Over billions of years, they stop producing energy and slowly cool down to around 4,000 degrees Kelvin.
But astronomers have recently discovered something puzzling: some white dwarfs are unexpectedly hot, glowing at temperatures of 10,000 to 30,000 degrees Kelvin.
A new study led by Lucy Olivia McNeill from Kyoto University, published in The Astrophysical Journal, offers an explanation.
The secret, it seems, lies in the invisible power of tides—not ocean tides, but gravitational ones.
When two white dwarfs orbit very close together, powerful tidal forces can stretch and squeeze them, creating heat deep within their interiors.
This process, known as tidal heating, may be keeping these dying stars far warmer than expected.
White dwarfs are incredibly compact—roughly the size of Earth but with the mass of the Sun. Strangely, the more massive a white dwarf is, the smaller it becomes due to the physics of electron degeneracy pressure.
Many of these stellar remnants exist in binary systems, where two stars orbit each other. In some of these extreme pairs, the stars whirl around faster than once per hour.
McNeill and her team developed a theoretical model to understand how tidal heating affects white dwarfs in such short-period orbits.
They borrowed ideas from studies of “hot Jupiters,” gas giant planets that orbit very close to their host stars and become intensely heated by tidal effects. Applying similar physics, the researchers created a model that could predict both the temperature and orbital changes of white dwarfs over time.
Their results show that the gravitational tug of one star can significantly heat its companion. In these tight systems, the smaller white dwarf’s gravity distorts the larger one, generating internal friction and heat.
This causes the larger star to puff up—becoming up to twice its normal size—and reach much higher temperatures than standard cooling models predict.
This inflation also changes how the two stars interact. Normally, white dwarfs are expected to begin transferring mass to each other at very short orbital periods. But with tidal heating in play, they may start interacting when their orbits are up to three times longer than expected.
Eventually, these close pairs spiral together, emitting gravitational waves—ripples in spacetime—and may even trigger explosive events like type Ia supernovae. McNeill’s team now hopes to use their framework to study systems made of carbon-oxygen white dwarfs, the kind most likely to explode in such supernovae.
By uncovering how tides can reheat and reshape dead stars, this research adds an exciting new chapter to the story of stellar evolution—and offers fresh clues about some of the universe’s most powerful cosmic fireworks.
