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Unformatted text preview: UCSB Winter 2008 Astro 1 - Homework #7 Solutions Kevin Moore 3/9/08 19-4 Why do high-mass main-sequence (MS) stars have shorter lifetimes than those of lower mass. High-mass MS stars are much more luminous than their low-mass counterparts. The mass-luminosity relationship for MS stars is L M 3 . 5 , which leads high mass stars to burn through their fuel much faster than low mass stars (even though they have more fuel to burn through). Check out Box 19-2 on Pg. 501 for a derivation that shows MS lifetimes go as M- 2 . 5 and thus the larger the star, the shorter the MS lifetime. 19-9 Why does Helium fusion require much higher temperatures than Hydrogen fusion? Recall that temperature of something is just a macroscopic quantity representing the average kinetic energy of the particles that make it up. The high temperatures required for fusion are due to the fact that nuclei have positive charges and thus repel each other. The strong nuclear force which binds the nuclei together has a very short range, so they must get very close in order for the strong nuclear force to overcome the electric repulsion between the nuclei and fuse them. Helium nuclei contain two protons, and since the electric force is proportional to the product of the charges, the electric repulsion between two Helium nuclei is 4x as strong as the repulsion between two Hydrogen nuclei. Thus more energy - and hence a higher temperature - is required to bring them close enough to fuse. 19-10 How is a degenerate gas different from ordinary gasses. The pressure of a degenerate gas (unlike an ordinary gas) doesnt depend on its temperature. 19-13 What does it mean when an astronomer says a star moves from one place to another on an H-R diagram?...
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This homework help was uploaded on 04/07/2008 for the course ASTRO 1 taught by Professor Antonucci during the Winter '08 term at UCSB.
- Winter '08