The History and Impact of the CNO Cycles in
The carbon cycle, or Bethe-Weizsa
cker 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
cker 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.
Key words: Carl Friedrich von Weizsa
cker; Hans Bethe; Carbon cycle; CNO
The energy source of the sun and all other stars became a topic of great interests in
the physics community in the second half of the nineteenth century. The theory of
thermodynamics had been developed and looked for new applications and the
question ‘‘what makes the stars burn’’ required a satisfying answer.
combustion energy, such as burning coal, was quickly dismissed; meteoritic
bombardment—which drove the heating of the early earth in the Hadean period—
also was unsustainable. It would translate into a growing mass of the sun in con-
tradiction of observation.
Also, the release of gravitational energy through a
continuous contraction of the sun as proposed ﬁrst by Herman von Helmholtz and
William Thomson was not sufﬁcient, since it required the sun to be rather young,
in contradiction with geological research indicating an old earth.
In the beginning
of the twentieth century, with the discovery of radioactivity, new ideas of
* Michael Wiescher is the Freimann Professor of Physics in the Department of Physics,
University of Notre Dame.
Phys. Perspect. 20 (2018) 124–158
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Physics in Perspective