An Etymological Dictionary of Astronomy and Astrophysics

An → excited state in the → triple alpha process leading to the production of the most abundant → isotope of → carbon. The existence of this state is of extreme astrophysical importance concerning the → nucleosynthesis of ¹²C in stellar → cores:
⁴He + ⁴He ↔ ⁸Be,
⁸Be + ⁴He ↔ ¹²C,
¹²C→ ¹²C + γ.

The process proceeds as follows. First the unstable
→ ground state of ⁸Be is formed in the collision of two → alpha particles. Since ⁸Be exists roughly 7 x 10^-17 sec, it must fuse with an alpha particle before breaking up. However, the probability of three bodies merging simultaneously is extremely low. Hoyle showed that the ¹²C nucleus needs an excited state or resonance at 7.68 MeV to provide for a high reaction probability. The Hoyle state was soon found at 7.65 MeV with the predicted → spin and → parity.

See also: In honor of the British astrophysicist Fred Hoyle (1915-2001), who predicted this state in 1953 (Hoyle et al. 1953, Physical Review 92, 1095); it was discovered by W. A. Fowler in 1957; → state.