Fusia

Meet Fusia, a proton in the Sun's core who, after a long chain of steps, ends up as part of a helium nucleus.

The Sun shines by fusing hydrogen into helium. In its core, at about 15 million kelvin, protons move fast enough to approach each other closely despite their electric repulsion, helped by quantum tunnelling. In the proton–proton chain, two protons fuse into deuterium, one turning into a neutron and releasing a positron and a neutrino. Further steps build helium-3 and then helium-4. Overall, four protons become one helium-4 nucleus, two positrons and two neutrinos.

The helium-4 nucleus is lighter than four protons. Computed from atomic masses, the missing mass corresponds to about 26.7 MeV for each helium nucleus made, released as gamma rays and particle motion. A small share is carried away by the neutrinos, which escape the Sun directly.

Detecting those solar neutrinos on Earth confirmed that fusion powers the Sun. This chain dominates in stars of the Sun's mass or less; hotter, heavier stars rely mainly on the CNO cycle.

The first step is extremely slow, because it needs the weak nuclear force. That slowness is why the Sun burns steadily for billions of years.