Figure 1: Synthesis of carbon in a burning star. Two alpha particles react to form beryllium-$8$. Beryllium-$8$ and an alpha particle react to form carbon-$12$, proceeding via the Hoyle state. This state in carbon-$12$ is a “resonance” in the beryllium-$8$ plus alpha-particle system. Once the resonant state is formed, it tends to decay by breaking up into beryllium-$8$ and an alpha particle. However, approximately four out of ten thousand decays bring the excited carbon-$12$ nucleus to its stable ground state.