lecture13_ch10 - Radius: 6.9 x 108 m (109 times Earth)...

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Unformatted text preview: Radius: 6.9 x 108 m (109 times Earth) Mass: Our Star 2 x 1030 kg (300,000 Earths) Luminosity: 3.8 x 1026 watts Lifetime Prediction Predicted life = Energy Available Why does the Sun shine? Rate of Energy Loss = Energy Available Luminosity 4 x 1038 J Chemical Energy: Estimated life expectancy ~ 30,000 years Gravitational Energy: Sun’s Energy Content 4 x 1041 J Estimated life expectancy ~ 30,000,000 years Chem. energy = 4 x 1038 J (~ 30,000 yr) Grav. energy = 4 x 1041 J (~ 30 million yr) Nuclear energy = 1 x 1044 J Nuclear Energy: (~ 10 billion yr) = 2 x 1047 J (~ 10 trillion yr) 1 x 1046 J Estimated life expectancy ~ 100 billion (1011) years Nuclear Energy: 1 x 1046 J (x10% of the total actually available, in the core:) Mass-energy Estimated life expectancy ~ 10 billion (1010) years 1 What is the Sun’s structure? Fission Fusion Big nucleus splits into smaller pieces Small nuclei stick together to make a bigger one (Nuclear power plants) (Sun, stars) Solar wind: A flow of charged particles from the surface of the Sun Corona: Very hot outer atmosphere of Sun (106 K): X-rays! Chromosphere: Hot inner atmosphere of Sun (104-105 K): UV! 5800 K Photosphere: “Surface” of Sun from which visible photons escape: Visible! 2 Convection zone: Hot gas rises and cool gas sinks Radiation zone: Million-degree gas, intense X-rays Core: Gas at 15 million Kelvin, nuclear fusion reactions Review: What is Gas Pressure? • Pressure is Force/Area • Gas pressure is proportional to particle density x temperature ( = heat / volume) • Fewer particles/volume lower pressure • Lower temperature lower pressure Stacks in gravity: Weight of upper layers compresses lower layers 3 Gravitational equilibrium: The outward push of pressure balances the inward pull of gravity How does nuclear fusion occur in the Sun? High temperature enables nuclear fusion to happen in the core Proton-proton chain is how hydrogen fuses into helium in Sun Proton-proton chain is how hydrogen fuses into helium in Sun IN 4 protons Solar Thermostat Temperature Decreases OUT nucleus 2 gamma rays 2 positrons 2 neutrinos Fusion Rate Decreases Total mass is 0.7% lower Core compresses 4He Temperature Restored Core heats up… 4 Solar Thermostat Temperature Increases Fusion Rate Increases Core expands Sun, Part 2 Temperature Restored Core cools off… How does the energy from fusion get out of the Sun? Energy gradually leaks out of radiation zone in form of randomly bouncing photons (1 million years to escape) Convection zone: Hot gas rises and cool gas sinks 5 Core Where fusion Solar LayersK happens, 15 million Radiation Zone Convection Zone Photosphere Chromosphere 5800 K Photosphere: “Surface” of Sun from which visible photons escape Corona Solar Wind Photons bounce from particle to particle Energy transport by “boiling” Sun’ s “Surface”: visible: 5500 K Ultraviolet: 10,000100,000 K X-ray, 106 - 107 K Particles, ions & dust flying away from the Sun Patterns of vibration on surface tell us about what Sun is like inside How do we know what is happening inside the Sun? Results agree very well with mathematical models of solar interior Cosmic Gall, by John Updike Neutrinos created during fusion directly escape core Neutrinos, they are very small. Tiny They have no charge and have no mass. And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a sheet of glass. They snub the most exquisite gas, Ignore the most substantial wall, Cold shoulder steel and sounding brass, Insult the stallion in his stall, And scorning barriers of class, Infiltrate you and me! Like tall and painless guillotines, they fall Down through our heads into the grass. At night, they enter at Nepal and pierce the lover and his lass From underneath the bed-you call It wonderful; I call it crass. The neutrino flux from the Sun is what we would expect from nuclear fusion and the behavior of neutrinos. 6 So how do we know what is happening inside the Sun? Data: Vibrations on the surface of the Sun; Data: Counting the neutrinos emitted by the Sun; What is solar activity? Predictions: models of H & He gas physics and nuclear reactions, constrained by laboratory physics. Sunspots Are cooler than other parts of the Sunʼs surface (4000 K) Have strong magnetic fields Loops of bright gas often connect sunspot pairs 2 things to remember about Magnetic Fields • Moving, charged particles create magnetic fields. • Charged particles are affected by magnetic fields: they spiral around magnetic fields & move along them, but resist moving across them. Sunʼ s magnetic field causes solar activity 7 Material in loops flows along magnetic field lines Corona appears bright in X-ray photos in places where magnetic fields trap hot gas Charged particles spiral along magnetic field lines How does solar activity affect humans? Charged particles streaming from Sun can disrupt electrical power grid and can disable communications satellites Energetic particles high in Earthʼs atmosphere cause auroras (Northern Lights) How does solar activity vary with time? 8 Number of sunspots rises and falls in 11-year cycle Sunspot cycle is related to winding and twisting of Sunʼs magnetic field. Magnetic fields have tension, and can snap like rubber bands when they break, and release energy by accelerating charged particles. Remember: • The sun is powered by nuclear fusion in its core. • The energy generated in the core = the luminosity (power) the sun radiates into space • How does the sun maintain equilibrium? • How we know what happens inside the sun? • How does solar activity affect the Earth? 9 ...
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This note was uploaded on 11/13/2011 for the course ISP 205 taught by Professor Donahue during the Fall '08 term at Michigan State University.

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