A new study from Kyoto University has shown that white dwarfs, which were once thought to be relatively cool and dormant, can be significantly hotter than expected when locked in tight orbits with another star
Tidal forces cause this heating, reshaping how astronomers view the evolution of these dense white dwarf stars.
Understanding white dwarfs
White dwarf stars are the final evolutionary stage of stars like our Sun. After exhausting their nuclear fuel, these stars shed their outer layers and collapse into extremely dense, Earth-sized remnants. Despite their small size, white dwarfs can have masses close to that of the Sun, making them among the densest forms of matter in the universe.
White dwarfs usually cool over billions of years, reaching surface temperatures around 4,000 Kelvin. But recent astronomical observations have uncovered something unusual; they discovered that some white dwarfs in binary systems are far hotter, reaching temperatures between 10,000 and 30,000 Kelvin, and are also larger than expected.
Tides in space
This discovery prompted a research team led by Kyoto University to investigate whether tidal heating could explain the elevated temperatures and increased size of white dwarfs in close binary systems. In these systems, two stars orbit each other extremely closely.
Similar to how the Moon causes tides on Earth, gravitational forces in close binary systems can deform the stars involved. These tidal interactions can cause friction and internal heating, particularly in the less compact star. In this case, the tidal pull of a smaller, denser white dwarf can cause its companion to heat up and expand.
The team developed a theoretical framework to model how tidal heating affects white dwarfs over time. This model allows scientists to predict both temperature and orbital changes as the stars evolve, providing a more complete picture of how these systems develop and interact.
Hotter, larger and sooner than expected
The study found that tidal heating is a powerful force in short-period white dwarf binaries. It can inflate a white dwarf to twice its expected size, pushing its surface temperature well above what standard models predict. As a result, these stars begin interacting at more extended orbital periods than previously believed.
This changes the way astronomers understand binary white dwarf systems. If the stars are larger and hotter than thought, their interactions could begin sooner in their lifespans, and at wider separations. This also affects predictions about the kinds of phenomena these systems might produce, including gravitational waves, stellar explosions, and supernovae.
White dwarf binaries are linked to some of the universe’s most powerful events, such as type Ia supernovae, stellar explosions that serve as essential tools for measuring cosmic distances. The study’s framework could help scientists better understand which binary systems are likely to explode, and under what conditions.
Future research will focus on applying this model to systems with different white dwarf compositions, particularly those made of carbon and oxygen, which are considered strong candidates for type Ia supernova progenitors.
