The triple-alpha process fuses three helium-4 nuclei (alpha particles) into carbon-12. This is the primary source of carbon in the universe, occurring in red giant stars at temperatures above 100 million Kelvin.
Step 1: Two ⁴He nuclei fuse to form ⁸Be, which is extremely unstable
(half-life ~10⁻¹⁶ seconds).
Step 2: Before ⁸Be decays, a third ⁴He captures it, forming an
excited state of ¹²C (Hoyle state).
Step 3: The excited ¹²C emits gamma rays and settles to stable ¹²C.