AST201/A02 Ch-7

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Astronomy & Astrophysics
Dr.Parandis Khaveri

Ch-7.1 Stars are held together by support of internal heat and pressure. Structure of star is variation in temperature, density, and pressure 4 laws of stellar structure: 1. Law of conservation of mass – total mass must equal sum of shell masses 2. Conservation of energy – total luminosity = sun of energy generated in ALL of layers in EACH shell 3. Hydrostatic equilibrium – weight on each layer is balanced by pressure 4. Energy transport – energy moves form hot to cool region by conduction, radiation and convection Laws of mass/energy Mass – mass of star must = sum of masses of its shell Energy – amount of energy flowing out of top layer in star must = to amount of energy coming in at bottom Energy leaving surface of star is Luminosity means Luminosity must = sun of energy generated in all layers inside star Hydrostatic equilibrium – balance between weight and pressure Weight of each layer of star must be supported by layer below Inside of star is made up of gas, means it must be balanced by temperature and density: - If pressure too low, weight from above will compress and push down the layer - If pressure to high, weight layer will expand and lift the layers above too - Pressure in each layer much grow larger with increasing depth to support weight - Pressure depends on temperature and density – near surface-low temperature, hence needs less high pressure. Deeper – high temperature needs high pressure. Energy transport – conduction, radiation and convection Flow of energy through each shell determines its temperature Energy transport – energy must flow from hot to cool regions by either mode - Conduction – atom to atom up, like particles from candle to spoon particles o Requires close contact between atoms – only in stars with extreme high densities - Radiation – infrared photons, which are packets of energy radiated by flame. In star – photons are absorbed and re-emitted in random directions over and over from hot to cool regions o Opacity – resistance to flow of radiation. Depends strongly on temperature (high temperature gas is very transparent and energy tends to flow as radiation). If opacity is high, radiation acts like water hold behind dam. - Convection – hot gas rises upward while cool gas sinks downward – rising wisp of smoke above candle is by convection currents. - Sun – surface-convection, middle-radiation, (core-conduction) Stellar model – diameter, luminosity, mass, surface, temperature, density and its composition – numbers representing conditions within layers of stars Ch-7.3 More massive star is, more luminous it is. Star more massive than sun – - more weight pressing in its interior, hence interior must have high pressure to balance weight. To keep pressure high, interior must have high temperature too – interior hot, pressure high – massive stars are more luminous b/c they have to make more energy to support their la
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