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Chapter 7.pdf - Part III: The Stars Chapter 7 Stellar...

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Part III: The Stars
Chapter 7Stellar Evolution
By and large, the evolutions of stars with differentmasses are similarStars are formed from interstellar cloudsThey spend most of their life times in the mainsequence, fusing hydrogen to heliumAt the end they leave the main sequence and dieThe main differences areFor massive stars, each stage takes a shorter timeLess massive stars end their lives quiescently, moremassive stars die violentlyThe following table shows the different stages ofthe life of a sun-like star
Evolution of a Sun-like Star
The Interstellar MediumWhat is in-between the stars?It is mostly almost-emptyspace that is almosttransparent. The typical spacebetween stars, containing amere one atom per cm3, isemptier than the bestvacuum on Earth (around3000 atoms/cm3).Bycomparison, the Earth’satmosphere has 1019atomsper cm3.However, there are also otherdenser regions andsometimes we can even seethe gas, which is usually fuzzyin appearance.
NebulaeNebulae are ‘fuzzy’ objects, andknown since the 1750’s.Starting his observation while looking for the“Halley’s Comet” in 1758, Charles Messiercatalogued many such objects so peoplewould not mistake all of them as comets.Henamed them: M1, M2, … M110. Theseincluded what we now know as nebulae,clusters, comets, galaxies
The central portion of this cloud is very dark,and can be seen only by its obscuration of thebackground stars. Nearby are reflection and emissionnebulae; M4 is a globular cluster.
The Horsehead Nebula is a particularly distinctivedark dust cloud.
HorseheadNebula.mov
Average temperature of dark dust clouds is a few tens ofkelvins.These clouds absorb visible light (left), and emit radiowavelengths (right).
Star formation happens when part of a dust cloud begins tocontract under its own gravitational force; as it collapses, thecenter becomes hotter and hotter until nuclear fusion begins inthe core.This is the central section of the Milky Way Galaxy, showingseveral nebulae, areas of star formation.GaseousPillarsStarBirth.mov
VisitToOrionNebula.mov
Stage 1:Interstellar cloud starts to contract, probably triggered byshock or pressure wave from nearby star. As it contracts,the cloud fragments into smaller pieces.
Stage 2:Individual cloud fragments begin to collapse. Once thedensity is high enough, there is no further fragmentation.Stage 3:The interior of the fragment has begun heating, and isabout 10,000 K.
The Orion Nebula is thought to contain interstellarclouds in the process of condensing, as well asprotostars.
CloudCollapse.swf
Stage 4:The core of the cloud isnow aprotostar, andmakes its first appearanceon the HR diagram.
Planetary formation has begun, but the protostar is stillnot in equilibriumall heating comes fromthe gravitational collapse.
Thelast stagescan befollowed on the HRdiagram:Theprotostar’sluminosity decreaseseven as its temperaturerises because it isbecoming more compact.

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Term
Fall
Professor
NoProfessor
Tags
Stellar Evolution, Pauli, White dwarf

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