Trialpha

Trialpha juggles three helium nuclei inside a star. For a moment they become carbon, and that is why you exist.

The Physics

Stars make carbon by fusing three helium-4 nuclei (alpha particles), which only works well because carbon-12 has an excited state at just the right energy. The Hoyle state sits at 7.654 MeV, spin-parity 0⁺, about 0.38 MeV above the 7.275 MeV three-alpha threshold. It almost always falls apart again; only rarely does it settle down into ordinary carbon.

3 ⁴He → ¹²C*(7.654 MeV, 0⁺) → ¹²C + γγ (rarely)

• ⁴He: helium-4 nucleus, alpha particle
• ¹²C*: excited carbon-12 nucleus
• MeV: mega-electronvolt, 1.602 × 10⁻¹³ J
• 0⁺: spin 0, positive parity
• γ: gamma-ray photon

In 1953 Fred Hoyle predicted a carbon level near 7.7 MeV; a Caltech team measured 7.68 ± 0.03 MeV, and Cook, Fowler and Lauritsen assigned it 0⁺ in 1957.

On the shirt

Front: Trialpha, the pixel-art character, with the title. Back: the three-alpha reaction, the resonance energy and threshold, the 1953 prediction and measurement and the 1957 assignment. Printed on a unisex tee.