Beryllium-8 has four protons and four neutrons and is unstable against breakup into two alpha particles. Its ground-state resonance lies just above the two-alpha threshold.
This short-lived intermediate is important in the Triple-alpha process: in a sufficiently dense stellar helium environment, a further alpha particle can be captured before breakup. The nuclear resonance therefore connects the structure of a very light unstable nucleus with the production of carbon in stars.
