Learning Objectives Identify when and how thermal, radiation, or degeneracy pres
ID: 292514 • Letter: L
Question
Learning Objectives Identify when and how thermal, radiation, or degeneracy pressure affect stellar evolution. Understand how the evolution of high mass stars differs from the evolution of low mass stars. 1) Limits on stellar mass: a) What prevents stars smaller than 0.08 solar masses from forming? b) What prevents stars larger than 150 solar masses from forming? 2) Hydrogen fusion: a) What type of pressure supports the core of a main sequence star? b) How does the proton-proton chain generate helium? c) How does the CNO cycle generate helium? d) Why does the CNO cycle require much higher temperatures than the proton-proton chain?Explanation / Answer
1a) A main-sequence star can be no smaller than 0.08 solar mass. The reason is that the temperature in a contracting protostar of less than 0.08 solar mass does not become high enough for nuclear reactions to start.
1b) It is believed to prevent stars from being larger than a few hundred solar masses is The temperature becomes so high that the excess mass is pushed back into space by radiation from the star.
2a) Electron degeneracy Pressure supports the core of main sequence star or dwarf star.
2b) 4 protons fuse together to form helium and energy in the form of gamma rays, positrons which annihilate with electrons to form gamma rays, and the motion of nuclei during the reaction. The proton-proton chain is Nuclear fusion.
2c) The CNO cycle is a nuclear fusion cycle involving carbon, nitrogen, and oxygen. It is a more ecient way of fusing hydrogen into helium and is used by stars more massive than our Sun. You still have the basic equation as the proton-proton chain, but carbon acts like a catalyst in the reaction.
2d) There are more protons in the carbon, nitrogen, and oxygen nuclei than in hy- drogen nuclei so these nuclei must be moving with higher speeds to overcome the electrical repulsion between the nuclei and come close enough for fusion to occur. Since the speed of a gas molecule depends on the temperature of a gas, higher speeds require higher temperatures.
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