When a star like the Sun runs out of hydrogen, it swells into a red giant, and helium piles up in its core. At some point, the helium ignites — in a few seconds, it releases as much energy as the star had radiated over thousands of years. This is a helium flash.
During the flash, a stream of neutrinos is born — particles that barely interact with matter. Some of them come from the decay of fluorine-18, and they have a distinct energy—1.7 MeV. If we could catch such neutrinos, we could 'hear' the flash directly.
For now, the nearest red giants are too far away, and flashes occur only a few times per year across the entire Galaxy. So astronomers continue to study helium flashes indirectly — by the trembling of a star's surface, much like seismologists listen to earthquakes.
But if one day a detector catches that very neutrino signal, we will see for the first time how the heart of an old star ignites.
as written by the authors arXiv:2602.15119
Low-mass stars ($$M\lesssim 2\,M_\odot$$) ignite helium under degenerate conditions, eventually causing a nuclear run-away -- the helium flash. The alpha-capture process on $$^{14}$$N produces a large amount of $$^{18}$$F, whose subsequent decay spawns an intense $$ u_e$$ burst (with average energy of $$0.38$$ MeV) lasting about a day. We show that, in addition, a strong $$1.7$$ MeV neutrino line is generated by electron capture on $$^{18}$$F. Detection is hindered by large backgrounds in state-of-the-art neutrino observatories, such as JUNO. In next-generation facilities, such as the Jinping neutrino experiment, the horizon for a detection with a local significance of $$3 \sigma$$ would be extended to almost $$3$$ pc. Although helium flashes occur a few times per year in our Galaxy, there are no stellar candidates approaching the tip of the red giant branch within $$10$$ pc. Hence, to date, asteroseismology remains the most promising tool for probing the most energetic thermonuclear event in the life of a low-mass star.
A helium flash lasts about a day, but during that time it releases more energy than the Sun emits over several thousand years.