Lecture5 Stars - Stars Hawley & Holcomb, Chapter 4 and...

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2/20/09 Astronomy 309R – Spring 2009 1 Stars Hawley & Holcomb, Chapter 4 and 5
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2/20/09 Astronomy 309R – Spring 2009 2 Why do stars shine? Stars need a source of energy to shine! Possible sources of energy: – chemical energy ( energy can be produced in chemical reactions, as in combustion )? – gravitational energy ( when a star shrinks in radius under the influence of its own gravity, its gravitational energy becomes more negative; since energy is conserved, positive energy is being produced )? – nuclear energy ( when matter is very hot and dense, the nuclei of atoms bind to form heavier nuclei. In the process, a large amount of radiation, heat, etc., is produced ) ? To determine the right answer, we must place a requirement on the “mileage”: for how long can a particular source of energy keep powering the luminosity of the star? A physical system that can evolve toward a state with a more negative energy is a source of energy .
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2/20/09 Astronomy 309R – Spring 2009 3 Energy radiated per second is luminosity, which has units of power (Watts). Solar luminosity: L = 3.8 × 10 26 Watts. – 1 Watt = 1 Joule per second – compare to the light bulb Mass of the sun: M = 2 × 10 30 kg Radius of the sun: R = 7 × 10 8 m Gravitational energy = - (3/5) G M 2 / R = 2.3 × 10 41 Joules How long can the sun shine by using its gravitational energy through gradual shrinking? Lifetime = (amount of available energy) / (the power that comes out) Lifetime = (2.3 × 10 41 Joules) / (3.8 × 10 26 Watts) = 6 × 10 15 seconds One year = 3 × 10 7 seconds Lifetime = 20 million years Too short!
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2/20/09 Astronomy 309R – Spring 2009 4 Review of Energy Different from momentum!!! Like momentum, energy is conserved, but there can be many forms of energy: Kinetic energy (moving bodies) positive Thermal energy (heat) positive, it is simply the kinetic energy of microscopic particles Rest energy (Einstein’s E = m c 2 ) positive Energy in radiation (light, photons) positive Electrical potential energy negative or positive Chemical energy can be negative or positive Gravitational potential energy negative Nuclear binding energy can be negative or positive When a system can have a negative energy (gravitational potential and nuclear binding energy), positive energy can be extracted by making the energy of the system more negative . sources of energy
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2/20/09 Astronomy 309R – Spring 2009 5 Sources of Energy Electrical potential energy negative or positive , but overall, like the earth, the sun is electrically neutral, and so electrical potential energy cannot explain the sun’s luminosity. Chemical energy
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This note was uploaded on 09/14/2009 for the course AST 309 taught by Professor Johnlacy during the Spring '08 term at University of Texas at Austin.

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Lecture5 Stars - Stars Hawley & Holcomb, Chapter 4 and...

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