Triple Alpha Process

If the central temperature of a star exceeds 100 million Kelvins, as may happen in the later phase of red giants and red supergiants, then helium can fuse to form beryllium and then carbon.

The Hoyle resonance
Index

Nuclear fusion in stars
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The Hoyle Resonance

Around 1950, astronomer Fred Hoyle was working on the modeling of stellar nucleosynthesis and considered carbon synthesis in the light of the observed carbon abundance in the stars. Carbon could be formed by the fusion of three alpha particles, but the probability is relatively so low that this would be too slow to explain the observed carbon abundance. The formation of 8Be from two alpha particles is energetically favorable since they have almost the same energy, and the subsequent fusion with another alpha particle would create a 12C nucleus.

42He + 42He -> 84Be -0.092 MeV

84Be + 42He -> 126C + 2γ (+7.367 MeV)

When the production of carbon by this process was modeled, it still seemed to be too slow to account for the observed carbon abundance, and this led Hoyle to propose that carbon had a nuclear resonance in the neighborhood of 7.7 MeV, even though none had been observed at that time. Hoyle, et al. did discover a resonance at 7.65 MeV which brought the modeled production rate into agreement with carbon observations.

F. Hoyle, D. N. F. Dunbar, W. A. Wensel, W. Whaling, Phys. Rev. 92, 649 (1953).

The dramatic implications of the Hoyle resonance in the modeling of the triple-alpha process are highlighted in a statement from Stephen Hawking and Leonard Mlodinow: "Such calculations show that a change of as little as 0.5% in the strength of the strong nuclear force, or 4 percent in the electric force, would destroy either nearly all carbon or all oxygen in every star, and hence the possibility of life as we know it. "

Index

Nuclear fusion in stars

Reference
Barrow & Tipler
Ch 4

Hawking & Mlodinow
Ch 7, p159
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