Homework 11. Advanced Nuclear Burning

Consider a star which starts out with a mass of 20.0 M of pure hydrogen (H). (Note: real stars start with a mix of hydrogen and helium, but pretending this star is pure hydrogen simplifies the question!) By the end of its life, the star has the structure shown in the diagram at right, with an iron (Fe) core of 1.4 M, surrounded by 0.6 M of sulfur (S), 1.5 M of oxygen (O), 2.5 M of helium (He), and 14.0 M of unburned fuel.

1. During the star's life, how much hydrogen was used up in nuclear reactions? Give your answer in units of solar mass (M). 2. How much helium was used up by nuclear reactions? 3. How much oxygen was used up by nuclear reactions? 4. How much sulfer was used up by nuclear reactions?

At each stage of nuclear burning, a tiny fraction of the mass is converted to energy. The table below lists the fractions converted at each stage in the sequence of nuclear reactions which take place in this star.

For example, when 1 M of iron is produced by the reaction   2S  ->  Fe  , just 0.00030 M of mass is converted to energy. (These fractions are small enough to be safely ignored in describing the star's final structure; that's why the star's final mass adds up to exactly 20.0 M).

5. During the star's life, how much mass was converted to energy by the reaction    2S  ->  Fe ?

6. How much mass was converted to energy by the reaction    2O  ->  S ?

7. How much mass was converted to energy by the reaction    4He  ->  O ?

8. How much mass was converted to energy by the reaction    4H  ->  He ?

9. Finally, when the iron core collapses, gravity converts 0.2 M of mass to energy. How does this compare to the total amount of mass converted to energy by nuclear reactions?

Last modified: November 6, 2001
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