rev.ex1.fall.10 - Cosmic Catastrophes 309N Fall 2010...

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Cosmic Catastrophes Wheeler 309N Fall 2010 September 3, 2010 (47700) Review for Test #1 BACKGROUND AND INTRODUCTION TO SUPERNOVAE Gravity – acts as if all the mass beneath were concentrated in the center. Gravity at surface depends on the mass and the radius of the object. Gravity is stronger inside massive, compact stars. Gravity gets weaker further away from any object. Stellar balancing act — dynamic equilibrium. A star spends most of its lifetime at a relatively constant size, temperature, luminosity, etc. while it fuses some fraction of its hydrogen into helium. During this time there is a balance between the forces inward and the forces outward. Forces inward — due to gravity, without the forces acting outwards the star would collapse. Forces outward — pressure Thermal pressure — For most of the lifetime of the star this is the dominant source of outward pressure. With this pressure a star can regulate its temperature. Regulated temperature – the nuclear burning is regulated on the main sequence. If too much energy is temporarily lost, the star contracts and heats, increasing nuclear input. If too much energy is temporarily gained, the star expands and cools, and nuclear input declines. Quantum pressure — Electrons cannot occupy the same region of space if they have the same energy.
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rev.ex1.fall.10 - Cosmic Catastrophes 309N Fall 2010...

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