What old, dying stars tell us about axions as dark matter candidates

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How do you search for hypothetical invisible particles? One solution 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 an axion, concocted decades ago to solve a complex problem with the strong nuclear force. After initial attempts to find it in particle collider experiments proved futile, the idea fell to the wayside.

But further research revealed that the axion could be a contender to explain the mystery of dark matter. Theorists realized that there may be ways for axions to flood the universe, but that until now they have eluded direct detection.

It is not because this small particle would be largely invisible that it would go completely unnoticed in the universe. In a pre-print article published November 2025 in the open access server arXivresearchers reported a way to test axion models using old archival data from the Hubble Space Telescope. Although they found no evidence for the existence of axions, they beat other attempts and gave us a much clearer picture of what is and isn’t allowed in this universe.

The targets of this study were white dwarfs – the dense, dark cores of dead stars. A single white dwarf can pack mass of the sun in an object smaller than Earthmaking white dwarfs one of the most exotic objects in the universe. Basically, white dwarfs protect themselves from collapse by something called electron degeneracy pressure, in which a huge sea of ​​freely floating objects electrons resists collapse because, according to quantum mechanics, electrons can never share the same state.

Some models of axion behavior indicate that these particles could be created by electrons: if an electron moved fast enough, it would trigger the formation of an axion. And because the electrons deep inside a white dwarf are moving very, very quickly – almost at the same speed. speed of light – as they buzz around in their tight spaces, they could produce lots of axions.

The axions would then leave at full speed, leaving the white dwarf completely. This production of fleeing axions would deprive the white dwarf of energy. And because white dwarfs don’t produce energy on their own, they would cool more quickly than they otherwise would.

The researchers integrated this model of axion cooling into a sophisticated software suite capable of simulating the evolution of stars and how their temperature and brightness change as their interior evolves.

This model allowed researchers to predict the typical temperature of a white dwarf, given its age, with or without axion cooling. With the results in hand, they turned to data from the globular cluster 47 Tucanae collected with Hubble. Global clusters are crucial because all the white dwarfs they contain were born around the same time, giving astronomers a large sample to study.

In short, the researchers found no evidence of axion cooling in the white dwarf population. But their results gave entirely new constraints on the ability of electrons to produce axions: they can do it no more efficiently than once in a trillion chances.

This result does not entirely rule out the possibility of axions, but it indicates that electrons and axions are unlikely to interact directly with each other. So if we want to continue looking for axions, we’re going to have to find even smarter ways to search.