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1If we know that a group of stars are at the same distance wecan plot the following two parameters in place ofLuminosity and Temperature on the H-R diagrama) Period and luminosityb) Surface gravity and colorc) Brightness and colord) Diameter and brightnesse) None of the aboveStars come in all sizes…• L vs T! b =L4"d2b=brightness,d=distance away!nmK109.26"#=T2Magnitude vs colorIf two stars are on the mainsequence, and one is moreluminous than the other, we canbe sure that the a) more luminous star will havethe longer lifetime b) fainter star is the moremassive c) more luminous star is themore massive d) more luminous star will havethe redder color3Massive stars burn brighterStellar Evolution –Models and Observation• Stars change very little over a human lifespan, so it isimpossible to follow a single star from birth to death.• We observe stars at various stages of evolution, and canpiece together a description of the evolution of stars ingeneral• Computer models provide a “fast-forward” look at theevolution of stars.• Stars begin as clouds of gas and dust, which collapse toform a stellar disk. This disk eventually becomes a star.• The star eventually runs out of nuclear fuel and dies. Themanner of its death depends on its mass.4Preview: Evolution of low-mass starsPreview: Evolution of high-mass stars5Tracking changes with theHR Diagram• As a star evolves, itstemperature and luminositychange.• We can follow a starsevolution on the HR diagram.• Lower mass stars move on tothe main sequence, stay for awhile, and eventually movethrough giant stages beforebecoming white dwarfs• Higher mass stars moverapidly off the main sequenceand into the giant stages,eventually exploding in asupernovaStellar EvolutionBefore…..During……and After….The Main SequenceIt’s all about gravity……6Protostars• Once a dense coreforms in the disk,the system hasentered theprotostar stage• Protostars aredifficult to find –they are shroudedby gas and dust• Infrared telescopescan detect them.The Eagle NebulaProtostars7Tracking the birth of starsProtostar•Gravitational contraction•heats to thousands of K•Huge! Hundreds of timesthe sun•Thousands of times thesurface area•Much more luminous thanthe sunFrom Protostar to Star• Low-mass protostars become stars very slowly– Weaker gravity causes them to contract slowly, sothey heat up gradually– Weaker gravity requires low-mass stars to compresstheir cores more to get hot enough for fusion– Low-mass stars have higher density!• High-mass protostars become stars relativelyquickly– They contract quickly due to stronger gravity– Core becomes hot enough for fusion at a lowerdensity– High-mass stars are less dense!8The birth tracks of low-and high-mass starsStar•Gravitational contractionreaches hot enough forfusion•0.08 M sun•88 M Jupiter• smaller > Brown dwarf•Upper limit ~ 100MsunTwo Young Star ClustersHow do we know these clusters are young?9Stellar Evolution on the Main SequenceStellar Evolution on the Main Sequence10The Main-Sequence Lifetime of a Star• The length of time a star spends fusing hydrogen into helium is called itsmain sequence lifetime– Stars spend most of their lives on the main sequence– Lifetime depends on the star’s mass and luminosity– More luminous stars burn their energy more rapidly than less luminous stars.– High-mass stars are more luminous than low-mass stars– High mass stars are therefore shorter-lived!• Cooler, smaller red stars have been around for a long time• Hot, blue stars are relatively young.High-mass stars 10 mpg Low-mass stars 60 mpgWhat determines when a star becomes a main-sequence star?a) When nuclear fusion generates the energy required to balancegravityb) When convection begins in the core.c) When optical radiation leaves the stard) When the temperature in the core reaches a highertemperature than the corona.11Stellar Evolution on the Main SequenceHigh-mass starsLow-mass starsEvolution to red giant phase• The star is expanding and cooling, so its luminosityincreases while its temperature decreases• Position on the HR diagram shifts up and to the right…Fuel runs outCore pressure dropsGravity compressescoreCore temperature risesShell burningPressure puffs outer layers Core heats up moreShell burning grows strongerAtmosphere expands and cools further12Evolutionary tracks of giant starsA (temporary) new lease on life• The triple-alphaprocess provides anew energy sourcefor giant stars• Their temperaturesincrease temporarily,until the helium runsout• The stars cool, andexpand once again• The end is


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UW-Madison AST 103 - AST 103 Lecture Notes

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