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.
A helium flash lasts about a day, but during that time it releases more energy than the Sun emits over several thousand years.