Why is the formation of carbon from three helium nuclei unlikely?

The formation of carbon from three helium nuclei, known as the triple alpha process, is inherently unlikely because it relies on an unstable intermediate step.

When two helium nuclei (alpha particles) fuse, they form a nucleus of beryllium-8. This isotope is highly unstable and decays back into two helium nuclei almost immediately. For carbon-12 to be produced, a third helium nucleus must collide with the beryllium-8 nucleus during its extremely short lifespan. Because this requires a rare three-body collision under specific conditions of high temperature and density, the reaction is statistically improbable and can only occur efficiently when stellar conditions allow the beryllium to capture a third helium nucleus before it disintegrates.12