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Nuclear Reactions in Stars

C. Why do astronomers infer that the Sun's energy comes from nuclear fusion reactions? How do we know it does not come from chemical burning?
d. Explain how the Sun produces energy by nuclear fusion.
e. When we look at stars in the sky we see a wide range of brightness. Explain the factors that would make one star appear brighter than another.

(Please explain in your own words, if possible)

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Hello and thank you for posting your questions to BrainMass.

We know the sun uses nuclear fission and not chemical burning due to the following facts:

1: The sun is about 75% hydrogen and 25% helium. This can be detected by spectral measurements. Hydrogen and Helium can not produce chemical burning by themselves (it requires oxygen and should produce water). In short - there no chemical that can produce chemical burning....

2: The vast amount of high-energy radiation (especially gamma and x-rays) the sun produces can not be the result of a chemical burning. It is more consistent with nuclear reactions (could be fission - but then again according to fact 1, there isn't any large quantity of fissile material in the sun).

3. To maintain the current ratio of mass to volume, the sun must produce huge amount of hydrostatic pressure (to prevent it from collapsing on itself due to gravity). This pressure must be produced at extremely high temperatures (millions of degrees) which is possible only during nuclear fusion ...

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