4 pages, 3 figures.-- PMID: 15650733 [PubMed].In the centres of stars where the temperature is high enough, three α-particles (helium nuclei) are able to combine to form C-12 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 supernova, it has hitherto been insufficiently determined. Here we report a measurement of the inverse process, where a C-12 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 10^7 K to 10^10 K and find significant deviations from the standard rates. Our rate below ~ 5 x 10^7 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 10^9 K, our rate is much less, which modifies predicted nucleosynthesis in supernovae.This research was supported by the Academy of Finland under the Finnish Centre of Excellence Programme, by the Spanish Agency CICYT, and by the EuropeanUnion Fifth Framework Programme ‘Improving Human Potential—Access to Research Infrastructure’.Peer reviewed