AST— Jan 31 Lecture + Chapter Notes

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

ASTThe Sun Nuclear Fusion Tuesday January 31 2012141A Closer Look At The Sun Why was the Suns energy source of a major mystery y Ancient thinkers thought the Sun was some type of fire ie lump of burning coal or wood y Before we learned the Suns distance from Earth no one realized just how much energy the Sun releases into space y Once we learned the Suns energy output scientists were at a loss to explain it until Einstein showed that mass itself contains a tremendous amount of energythy In the 19 centuryscientists suggested that the Sun generates energy by slowly contracting in size a process called gravitational contraction o Shrinking Sun would always have some gas moving inward converting gravitational potential energy into thermal energyo Sun could have been shining for 25 millions yearsflawedEarth was older than 25 million yearsWhy does the Sun shiney The Sun converts mass into energy through the process of nuclear fusiono Requires extremely high temperatures and densitiesfound deep in the Suns core y For the Sun to shine steadily it must keep the core hot and dense o Maintains these conditions through a natural balance between two competing forces gravity pulling inward and pressure pushing outward or hydrostatic equilibriumCalled gravitational equilibriumo Outward push comes from internal gas pressureBecause the weight of overlying layers is greater the pressure must increase with depth y Pressure makes the gas hot and dense enough to sustain nuclear fusion y Energy released by fusion heats the gas and maintains the pressure y The sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion y Sun was born about 4 billion years ago from a collapsing cloud of interstellar gas o Contraction of the gas released gravitational potential energy Much of this energy radiated away but the rest stayed inside o Central temperature and density grew high enough to sustain nuclear fusion energy generation in the Suns interior came into balance with the energy lost from the surface in the form of radiation y The core originally became hot through the release of energy by gravitational contraction as gravity made the Suns birth cloud contract
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