Lecture 15 - Stellar Energy (3/4/99)

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ASTR012 Reading:
Chapter 16-1, 16-2 (ZG4)

pages 57 - 61

Artists conception of the Sudbury Neutrino Observatory. Observation of solar neutrinos can probe conditions at the very core of the Sun. (Courtesy SNO)
? Key Question: What energy source powers the Sun?
! Key Principle: Nuclear Binding Energy
# Key Problem: Calculate rate of Fusion in Sun.


  1. Stellar Energy
  2. Nuclear Binding Energy
  3. The Proton-Proton Chain
  4. Other Fusion Chains

Solar Neutrinos:

Neutrino Astrophysics has a long tradition at Penn. There is a good discussion of the implications of the results of solar neutrino experiments set up by the group.

Some links to neutrino experiment sites:

Stellar Energy in Outline:

  1. Nuclear Energy in Stars
  2. Other Nuclear Reactions
    • The CNO cycle (for Carbon - Nitrogen - Oxygen) also turns 4 H -> He, but using carbon as a catalyst. This provides about 10% of the energy of the Sun, and requires temperatures of 16 million K.
    • The Triple Alpha reaction turns 3 He -> C ("alpha particles" are another term for the helium nucleus). This reaction needs temperatures above 100 million K, and do not happen in the Sun at this time. Later, however ...
    • Carbon Fusion can occur above about 600 million K, and produces oxygen, neon, magnesium and silicon. At still higher temperatures around 3.5 billion K, silicon can be fused to form heavier elements, including iron.
    • The iron barrier: if you remember our consideration of the binding energy per nucleon of the nucleus as a function of the atomic weight of the nucleus, we showed that you could extract energy from fusion only for elements lighter than iron. Thus, iron is the "end of the line" for fusion as an energy generator.

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smyers@nrao.edu Steven T. Myers