Revised rates for the stellar triple-α process from measurement of 12C nuclear resonances

In the centres of stars where the temperature is high enough, three α-particles (helium nuclei) are able to combine to form 12C because of a resonant reaction leading to a nuclear excited state. (Stars with masses greater than ∼0.5 times that of the Sun will at some point in their lives have a central temperature high enough for this reaction to proceed.) Although the reaction rate is of critical significance for determining elemental abundances in the Universe, and for determining the size of the iron core of a star just before it goes supernova2, it has hitherto been insufficiently determined. Here we report a measurement of the inverse process, where a 12C nucleus decays to three α-particles. We find a dominant resonance at an energy of ∼11 MeV, but do not confirm the presence of a resonance at 9.1 MeV (ref. 3). We show that interference between two resonances has important effects on our measured spectrum. Using these data, we calculate the triple-α rate for temperatures from 107 K to 10 10 K and find significant deviations from the standard rates. Our rate below ∼5 × 107 K is higher than the previous standard, implying that the critical amounts of carbon that catalysed hydrogen burning in the first stars are produced twice as fast as previously believed. At temperatures above 109 K, our rate is much less, which modifies predicted nucleosynthesis in supernovae.