Physics in Perspective

, Volume 20, Issue 1, pp 124–158 | Cite as

The History and Impact of the CNO Cycles in Nuclear Astrophysics

  • Michael Wiescher


The carbon cycle, or Bethe-Weizsäcker cycle, plays an important role in astrophysics as one of the most important energy sources for quiescent and explosive hydrogen burning in stars. This paper presents the intellectual and historical background of the idea of the correlation between stellar energy production and the synthesis of the chemical elements in stars on the example of this cycle. In particular, it addresses the contributions of Carl Friedrich von Weizsäcker and Hans Bethe, who provided the first predictions of the carbon cycle. Further, the experimental verification of the predicted process as it developed over the following decades is discussed, as well as the extension of the initial carbon cycle to the carbon-nitrogen-oxygen (CNO) multi-cycles and the hot CNO cycles. This development emerged from the detailed experimental studies of the associated nuclear reactions over more than seven decades. Finally, the impact of the experimental and theoretical results on our present understanding of hydrogen burning in different stellar environments is presented, as well as the impact on our understanding of the chemical evolution of our universe.


Carl Friedrich von Weizsäcker Hans Bethe Carbon cycle CNO cycle 



This article is based on a presentation the author gave at the symposium in honor of the 100th birthday of Carl Friedrich von Weizsäcker, and was published in German, in the Proceedings of the Leopoldina Academy.112 This is a revised and extended version for the English-speaking audience. My special gratitude goes to Professor Karl Hufbauer for helpful discussions and his willingness to provide unpublished work and information on the history of the power generation in stars. Thanks also to Joachim Görres and Karl-Ulrich Kettner for multiple discussions of the topic and for bringing up useful information and memories on earlier days of experimental study of CNO and NeNa reactions.


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© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of PhysicsUniversity of Notre DameNotre DameUSA

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