Chapter 20Stellar Evolution (Pt1:20.1-20.3)20.1 Leaving the Main Sequence20.2 Evolution of a Sun-Like Star XXThe CNO Cycle20.3 The Death of a Low-Mass Star XXLearning Astronomy from HistoryTry to review at this point.20.4 Evolution of Stars More Massive than the Sun XXMass Loss from Giant Stars20.5 Observing Stellar Evolution in Star ClustersOMIT: 20.6 The Evolution of Binary-Star SystemsUnits of Chapter 20We cannot observe a single star going through its whole life cycle; evenshort-lived stars live too long for that. And when we observe largenumbers of stars, we are looking at stars borne at different times in the10Gyr history of our Galaxy, so they are in different phases of evolution.Observation of stars in star clusters help, by giving us a look at stars thatwere all born at the same time. However we use the observations, ourinterpretation must rely heavily on theoretical calculations of the evolutionof stars of various masses.20.1 Leaving the Main SequenceDuring its stay on the Main Sequence, anyfluctuations in a star’s condition are quicklyrestored; the star is in equilibrium.20.1 Leaving the Main SequenceEventually, as hydrogen in the core is consumed,the star begins to leave the Main SequenceIts evolution from then on depends very muchon the mass of the star:Low-mass stars go quietlyHigh-mass stars go out with a bang!Even while on the Main Sequence, thecomposition of a star’s core is changing20.2 Evolution of a Sun-like StarAs the fuel in the core is used up, the corecontracts; when it is used up the core beginsto collapse.Hydrogen begins to fuse outside the core:Stages of a star leaving the Main Sequence:You Should be able to describe what is happening at each of thesestages, in the core and in the envelope, as well as place it in the H-Rdiagram. However you don’t have to know these numbers.20.2 Evolution of a Sun-Like StarStage 9: The Red-Giant BranchAs the core continues to shrink,the outer layers of the star expandand cool.It is now a red giant, extendingout as far as the orbit of Mercury.Despite its cooler temperature, itsluminosity increases enormouslydue to its large size.20.2 Evolution of a Sun-Like StarThe red giant stage on the H-Rdiagram:Stage 10: Helium fusionOnce the core temperature has risen to100,000,000 K, the helium in the core starts tofuse, through a three-alpha process:4He + 4He → 8Be + energy8Be + 4He → 12C + energyThe 8Be nucleus is highly unstable and willdecay in about 10–12 s unless an alphaparticle fuses with it first. This is why hightemperatures and densities are necessary.20.2 Evolution of a Sun-Like StarThe helium flash:The pressure within the helium core isalmost totally due to “electrondegeneracy”—two electrons cannot bein the same quantum state, so thecore cannot contract beyond a certainpoint.This pressure is almost independent oftemperature—when the helium startsfusing, the pressure cannot adjust.20.2 Evolution of a Sun-Like StarHelium begins to fuse extremelyrapidly; within hours theenormous energy output is over,and the star once again reachesequilibriumStage 11: Back to the giant branchAs the helium in the core fuses to carbon,the core becomes hotter and hotter, andthe helium burns faster and faster.The star is now similar to its condition justas it left the Main Sequence, except nowthere are two shells:20.2 Evolution of a Sun-Like StarThe star has become a red giant forthe second timeMore Precisely 20-1: The CNO CycleThe proton–proton cycle is not the only pathstars take to fuse hydrogen to helium. Athigher temperatures, the CNO cycle occurs:In stars more massive than the Sun, whosecore temperatures exceed 20,000,000 K, theCNO process is dominant.This graphic shows the entire evolution of a Sun-like star. Use it asa review device. Try to describe each of the phases shown.Low mass stars like the sun never become hot enough for fusionpast carbon to take place.20.3 The Death of a Low-Mass StarThere is no more outward fusionpressure being generated in the core,which continues to contract.The outer layers become unstable andare eventually ejected.20.3 The Death of a Low-Mass StarThe ejected envelope expandsinto interstellar space,forming a “planetary nebula.”(No planet is involved!)The star now has two parts:• A small, extremely dense carbon core• An envelope about the size of our solarsystem.The envelope is called a planetary nebula,even though it has nothing to do withplanets—early astronomers viewing thefuzzy envelope thought it resembled aplanetary system.20.3 The Death of a Low-Mass StarPlanetary nebulae can have manyshapes:As the dead core of the star cools,the nebula continues to expandand dissipates into thesurroundings.Stages 13 and 14: White and black dwarfsOnce the nebula has gone, theremaining core is extremelydense and extremely hot, butquite small.When observed, this remnantcores is called a “white dwarf.”It is luminous only due to itshigh temperature. It will spendthe rest of its life radiatingaway its internal supply ofheat, gradually fading intoinvisibility.20.3 The Death of a Low-Mass StarThe small star Sirius B is awhite-dwarf companionof the much larger andbrighter Sirius A:20.3 The Death of a Low-Mass StarAs the white dwarf cools, its size does not change significantly; itsimply gets dimmer and dimmer, and finally ceases to glow.This outline of stellar formation and extinction can be comparedto observations of star clusters. Here a globular cluster:20.3 The Death of a Low-Mass StarThe “blue stragglers” in this H-R diagram are not exceptions toour model; they are stars that have formed much more recently,probably from the merger of smaller stars.Next we move on to the evolution of moremassive stars, whose lives are quite a bitdifferent, especially near the
View Full Document
Unlocking...