What old, dying stars teach us about axions as a candidate for dark matter
How do you search for invisible hypothetical particles? One way is to see how quickly they could kill white dwarfs — the dense, leftover cores of dead stars.
In recent years, astronomers have become increasingly interested in a theoretical particle known as the axion, which was concocted decades ago to solve a challenging problem with the strong nuclear force. After initial attempts to find it in particle collider experiments turned up empty, however, the idea sunk into the background.
Just because this little particle would be largely invisible, it doesn’t mean it would go completely unnoticed in the universe. In a pre-print paper published in November 2025 in the open access server arXiv, researchers reported a way to test axion models using old archival data from the Hubble Space Telescope. Although they didn’t find any evidence for axions, they beat other attempts and gave us a much clearer picture of what is and isn’t allowed in this universe.
The targets for this study were white dwarfs — the dense, dim cores of dead stars. A single white dwarf can pack the mass of the sun into an object smaller than Earth, making white dwarfs among the most exotic objects in the universe. Crucially, white dwarfs support themselves against collapse through something called electron degeneracy pressure, in which a huge sea of free-floating electrons resists collapse because, according to quantum mechanics, electrons can never share the same state.
Some models of how axions might behave say these particles could be created by electrons: If an electron were moving quickly enough, it would trigger the formation of an axion. And because the electrons deep inside a white dwarf are moving very, very quickly — at nearly the speed of light — as they buzz around in their tight confines, they could produce a lot of axions.
The axions would then go speeding off, leaving the white dwarf altogether. This production of escaping axions would rob the white dwarf of energy. And because white dwarfs don’t produce energy on their own, this would cause them to cool off faster than they would otherwise.
The researchers fed this model of axion cooling into a sophisticated software suite that can simulate the evolution of stars and how their temperature and brightness change as their interiors evolve.
This model allowed the researchers to predict the typical temperature of a white dwarf, given its age, both with and without axion cooling. With the results in hand, they turned to data of the globular cluster 47 Tucanae collected with Hubble. Global clusters are crucial because all of the white dwarfs in them were born at roughly the same time, giving the astronomers a large sample to study.
In short, the researchers found no evidence for axion cooling in the white dwarf population. But their results did give brand-new constraints on the ability for electrons to produce axions: They can’t do it more efficiently than once every trillion chances.
This result doesn’t rule out axions entirely, but it does say it’s unlikely that electrons and axions directly interact with each other. So, if we’re going to keep searching for axions, we’re going to have to find even more clever ways to look.