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According to common assumptions, matter in the mass range is processed through the so-called NeNa A ≥ 20 cycle during hydrogen-burning nucleosynthesis. The existence of such a reaction cycle implies that the (p, a) reaction on 23 Na is more likely to occur than the competing (p, g) reaction. However, recently evaluated thermonuclear rates for both reactions carry relatively large uncertainties and allow for both possibilities, i.e., a "closed" and an "open" NeNa cycle. We measured the 23 Na(p, g) 24 Mg reaction at the Laboratory for Experimental Nuclear Astrophysics. The present experimental results, obtained with our sensitive g-ray detection apparatus, reduce the reaction rate uncertainties significantly. We demonstrate that a closed NeNa cycle does not 23 Na ϩ p exist at stellar temperatures of GK. The new results have important implications for the nucleo-T p 0.2-0.4 synthesis in classical novae, including the amount of 26 Al ejected by the thermonuclear explosion, the elemental abundances of Mg and Al observed in nova shells, and observations of Mg and Al isotopic ratios in primitive meteorites.

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