A red giant in a globular cluster is a star in retirement. It has burned up the hydrogen in its core and now lives off fusion in a thin shell around a helium ash core. Throughout this phase, its brightness slowly increases until a helium flash occurs — the moment when the core suddenly "ignites" again. The brightest point just before this flash is called the tip of the red-giant branch (TRGB). It's a universal beacon: all such stars have similar luminosities. The Standard Model of particle physics predicts this luminosity very precisely, but only if no extra energy leaks out of the core.

If axions (light, nearly undetectable particles and dark matter candidates) are born inside a star, they carry away heat like a draft through an open window. The core has to burn fuel faster to compensate, making the TRGB star brighter. A team led by Rashid Sunyaev and colleagues tested this idea using a sample of 27 globular clusters. They used images from ground-based telescopes, Hubble, and Gaia mission data to construct precise color-magnitude diagrams for each cluster. Then, applying Monte Carlo corrections, they calculated the actual TRGB luminosity and compared it with models where the axion-electron coupling parameter varied smoothly.

The result was unambiguous: the stars shine exactly as standard physics dictates. No draft. The upper limit on the coupling constant $g_{ae}$ came out to 3.8×10⁻¹⁴ at 95% confidence. This is several times stricter than previous estimates. The mean bolometric absolute magnitude of the TRGB was found to be −3.53 with an uncertainty of just 0.05 magnitudes — the cosmic beacon has become even more reliable.