After our Sun becomes a white dwarf and cools completely, what will be left?

In 2019, astronomers announced they’d found direct evidence of white dwarfs solidifying into oxygen-carbon crystals, as depicted in this artist’s illustration. After the white dwarf becomes completely frozen, it cools off and fades as it stops generating light. Credit: University of Warwick/Mark Garlick

After our Sun becomes a white dwarf and cools completely, what will be left?

Richard Livitski
Seal Beach, California

Our Sun will become a white dwarf in about 7 billion years. At that point, it will no longer produce energy through nuclear fusion, having exhausted its fuel (hydrogen and helium). Just like the embers of an abandoned campfire can only cool down with time, the white dwarf Sun will gradually become dimmer and cooler as it radiates its heat away into space.

The interior of this type of star is incredibly dense. To put this in perspective, a volume of white dwarf material equal to a teaspoon would weigh several tons! In such a condensed state, charged particles (oxygen and carbon nuclei) interact strongly due to the powerful electrostatic forces at play. Initially, these charged particles don’t “feel” this force too much because they’re moving quickly in all directions, thanks to the star’s very high temperature.

But as the white dwarf Sun cools, the particles in its interior slow down. Eventually, they become so slow that they can’t overcome the electrostatic interactions squeezing them from all sides — they become trapped. The interior of the white dwarf Sun will literally become a solid carbon-oxygen crystal. This freezing process, known as crystallization, is predicted to start roughly 3 billion years after our Sun becomes a white dwarf, or about 10 billion years from now.

Since diamonds are a solid form of carbon, it’s often said that white dwarfs turn into diamonds. However, this isn’t accurate — the matter inside a white dwarf is very different from a diamond. The interior of a white dwarf is not only much denser than anything found on Earth but also much hotter, even by the time the star freezes.

Once completely frozen, our white dwarf Sun will continue cooling for billions upon billions of years, slowly fading away as a dark, solid carbon-oxygen ball.

Simon Blouin
Canadian Institute of Theoretical Astrophysics National Fellow in Astrophysics, University of Victoria, Victoria, British Columbia, Canada