Whispers of the Stars: The Tale of ²³Na and Elusive Axions

On a clear night, the stars look calm and eternal. But deep inside them, nature may be running experiments far more extreme than anything we can build on Earth. In the crushing heat and density of stellar interiors, ordinary matter behaves in extraordinary ways, and, if some bold ideas in theoretical physics are correct, stars may also be producing entirely new kinds of particles: axions and axion-like particles, or ALPs.

Axions were first proposed in the late 1970s to fix a flaw deep inside the equations of particle physics. The equations that describe the strong nuclear force seem to allow a violation of a fundamental symmetry of nature, but experiments stubbornly show no such violation. This mismatch, commonly known as the strong CP problem, has puzzled physicists for decades. The axion offers an elegant fix. Its existence would naturally restore the missing symmetry, bringing theory and experiment back into harmony.

ALPs are their close relatives. They behave much like axions but are not tied to this specific theoretical problem. Many modern theories that extend beyond the Standard Model, especially those inspired by extra dimensions or string theory, predict entire families of such light, weakly interacting particles. In these theories, axions are not an exception but part of a much larger hidden world, spread across a wide range of masses and interaction strengths.

Stars are natural laboratories for studying such elusive particles. Deep inside them, nature runs particle-physics experiments no human lab can match: temperatures reach millions of degrees and particles collide relentlessly. In these extreme conditions, axions and axion-like particles can be produced naturally and in enormous numbers. The crucial difference between them and ordinary particles like photons is escape. Photons bounce around inside a star for thousands—or even millions—of years before finally emerging as starlight. Axions and ALPs, by contrast, would slip out, carrying energy straight from the stellar core into space.

In this work, we explore a production mechanism for such elusive particles in the stellar cores that arises naturally from nuclear physics. When a nucleus is thermally excited, it typically relaxes by emitting a monochromatic photon whose energy equals the spacing between the two levels. If axions couple to nucleons, however, another possibility opens up. In certain nuclear transitions—most notably magnetic dipole (M1) transitions, which involve changes in the magnetic structure of the nucleus—an excited nucleus can emit an axion instead of a photon. The axion carries away essentially the same energy as the corresponding gamma ray, but unlike light, it can escape the dense stellar interior essentially unimpeded. Although such axion-emitting transitions are individually rare, stars compensate through sheer numbers: enormous populations of nuclei undergo thermal excitations and de-excitations every second, rendering even extremely feeble axion–nucleon couplings potentially observable.

In our study, we spotlight an unlikely hero hiding inside massive stars: sodium-23 (²³Na). Most stellar nuclei come in neatly paired forms, making odd isotopes rare and fleeting. ²³Na, which has an unpaired proton, breaks this rule. During the carbon-burning phase of massive stars, it is produced in substantial quantities and survives for long periods in the hot stellar interior. Crucially, ²³Na also possesses low-lying nuclear transitions at 440.2 keV, that can be thermally excited under these conditions, making it an efficient and novel source of stellar axions.

To quantify this emission, we followed the lives of massive stars using detailed computer simulations, tracing their evolution from birth through the carbon-burning phase and carefully tracking when and where sodium-23 is produced and excited. We then scaled this picture up to the full population of massive stars in our Milky Way galaxy, combining stellar evolution with galactic demographics. The result is remarkable: a steady, invisible drizzle of axions streaming out from carbon‑burning stars across the galaxy. And here’s the exciting twist —as these monochromatic axions traverse the galactic magnetic field, a fraction can convert into gamma-ray photons of the same energy, creating a sharp spectral line, an unmistakable signal that all-sky telescopes like COSI are designed to spot (see Fig 1).