It turns out that white dwarfs are not as “dead” as thought. Scientists have explained why some of these stars stop “solidifying” and where they get the energy to shine steadily for billions of years.

In 2019, astronomers created a Hertzsprung-Russell diagram based on data from the Gaia probe. This diagram shows the relationship between the brightness of stars, their spectral classes, and other quantities. The important thing is that the stars clearly show the branch classes. Scientists have noticed an unusual “cluster” in the line of white dwarfs, which they call the Q branch. It was located where massive white dwarfs that are in the process of cooling would be. But the analysis showed that five to nine percent of this “cluster” was actually very old; they just haven’t cooled for at least eight billion years. Finally, scientists explained how they managed to stop the “death” of these stars.

In the standard model of white dwarf cooling, its crystallization proceeds from the inside to the outside. A “dead” star that has lost its thermonuclear source begins to cool, the plasma inside crystallizes. At the same time, a noticeable amount of heat and energy is released. Of course, they slow down the cooling process, but they cannot stop it for billions of years.

According to astrophysicists’ calculations, the cores of most massive white dwarfs should consist of oxygen and neon. Their cooling follows a standard script. But some binary star mergers are still required to form massive white dwarfs with carbon-oxygen cores and lots of neutron-rich impurities. For example neon-22.

As noted by the authors of a new study published NatureWith this composition, the solidification process turns into a distillation process.

Floating crystals form in massive white dwarfs that contain impurities. They move “up” while the heavier, neon-22-rich liquid moves downward. According to the authors, the saturation of neon-22 increases from three percent to 30 percent of the mass. The authors used neon-22 as a “representative” of all neutron-saturated molecules.

As a result, the mass of the core increases (density increases by about eight percent), the white dwarf shrinks slightly (radius decreases by one percent), as a result of which additional energy is released. It is he who slows down the cooling. According to calculations, it lasts for 7-13 billion years, depending on the mass of the white dwarf. The greater the mass, the shorter the “stop” because the process proceeds faster and the energy is released faster.

The study’s authors modeled the evolution of massive white dwarfs, most of which are “ordinary”, with a composition suitable for the formation of floating crystals at five to nine percent. For the first time, the results accurately matched the position and width of the Q branch in the Gaia data.

“This discovery will require a revision not only of astronomy textbooks but also of methods for estimating the age of stellar populations,” said Simon Blouin, co-author of the new paper from the University of Victoria (Canada).

It turns out that for billions of years such white dwarfs shine just like normal ones. So they may be much older than they appear. As a result, scientists determine the age of clusters according to the age of the stars and reconstruct their formation history. Including our Milky Way galaxy.

The population of such white dwarfs is still not that large. Therefore, you should not expect significant revaluations. Of course, astronomers will now take note of their existence.