1Stellar EvolutionBefore…..During……and After….The Main SequenceIt’s all about gravity……Evolution 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 further2Main Sequence Turn-offWhat are we looking at?a) Stars of the samemass?b) Stars of the samecolor?c) Stars of the samemagnitude?d) Stars of the sameage?Main sequence3The solar corona has temperatures roughly the same levelas temperatures in the Sun's core, where nuclear fusiontakes place.Why doesn't fusion occur in the corona? a) The density in the corona is too low. b) The corona has too many free electrons. c) Atoms in the corona are mostly ionized. d) The corona has more heavy atoms than the core. e) Two of the above.Evolutionary tracks of giant stars4A (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 near…Helium Fusion• Normally, the core of a star is not hotenough to fuse helium– Electrostatic repulsion of the twocharged nuclei keeps them apart• The core of a red giant star is verydense, and can get to very hightemperatures– If the temperature is high enough,helium fuses into Beryllium, andthen fuses with another heliumnuclei to form carbon.5The Fate of Sunlike Stars• The Sun’s Lifetime:– 10 billion years on the mainsequence– Once the hydrogen is consumed,it will enter the red giant phase– Helium burning begins, startingthe yellow giant phase– Once helium is consumed, corecontracts and outer envelopeexpands, beginning the redsupergiant phase– Core begins to cool and the outerenvelope expands again, forminga planetary nebula– The core remains as a whitedwarfThe Life-path of the Sun6Formation of Planetary Nebula• As a red giant expands, itcools– Outer layers cool enough forcarbon flakes to form– Flakes are pushed outward byradiation pressure– Flakes drag stellar gas outwardwith them– This drag creates a high-speed stellar wind!– Flakes and gas form aplanetary nebulaThe Hourglass Nebula7White Dwarf Stars• At the center of theplanetary nebula lies thecore of the star, a whitedwarf– Degenerate material– Incredibly dense• Initially the surfacetemperature is around25,000 K• Cools slowly, until it fadesfrom sight.Why do stars pass quickly along theHR diagram as they reach thePlanetary Nebula Phase?• A) High velocity gas cools quickly changingthe color of the star quickly.• B) As the outer layers expand, the star theybecome more diffuse, exposing moe hot innerlayers.• C) As the atmosphere expands, the core shrinksand heats up, becoming bluer.• D) Planetary nebula are fast moving particlesthat heat the “inter stellar medium”.8White Dwarf Stars• At the center of theplanetary nebula lies thecore of the star, a whitedwarf– Degenerate material– Incredibly dense• Initially the surfacetemperature is around25,000 K• Cools slowly, until it fadesfrom sight.Eskimo Nebula9The Hourglass NebulaAnt Nebula10What’s really going on?Three thousand light-years away, a dying star throws off shells of glowing gas. This image from the HubbleSpace Telescope reveals The Cat's Eye Nebula to be one of the most complex planetary nebulae known. Infact, the features seen in the Cat's Eye are so complex that astronomers suspect the bright central object mayactually be a binary star system.11The primary source of energy for aWhite Dwarf• A) Nuclear fusion• B) Nuclear fission• C) gravitational contraction• D) stored heat, cooling passively• E) chemical heatFormation of Heavy Elements• Hydrogen and a little helium were formed shortlyafter the Big Bang• All other elements were formed inside stars!• Low-mass stars create carbon and oxygen in theircores at the end of their lifespan, thanks to the highertemperatures and pressures present in a red giant star• High-mass stars produce heavier elements likesilicon, magnesium, etc., by nuclear fusion in theircores– Temperatures are much higher– Pressures are much greater• Highest-mass elements (heavier than iron) must becreated in supernovae, the death of high-mass stars12The Lifespan of a Massive StarLayers of Fusion Reactions• As a massive star burns itshydrogen, helium is left behind, likeashes in a fireplace• Eventually the temperature climbsenough so that the helium begins toburn, fusing into Carbon. Hydrogencontinues to burn in a shell aroundthe helium core• Carbon is left behind until it toostarts to fuse into heavier elements.• A nested shell-like structure forms.• Once iron forms in the core, the endis near…13Core Collapse of Massive Stars• Iron cannot be fused into any heavier element, so itcollects at the center of the star• Gravity pulls the core of the star to a size smallerthan the Earth’s diameter!• The core compresses so much that protons andelectrons merge into neutrons, taking energy awayfrom the core• The core collapses, and the layers above fallrapidly toward the center, where they collide withthe core material and “bounce”• The “bounced material collides with the remaininginfalling gas, raising temperatures high enough toset off a massive fusion reaction. The star thenexplodes.• This is a supernova!Before and After – a Supernova14Light Curve for a Supernova• The luminosityspikes at themoment of theexplosion, andgradually fades,leaving behinda…Supernova Remnant• The supernova hasleft behind arapidly expandingshell of heavyelements that werecreated in theexplosion.• Gold, uranium andother heavies alloriginated in asupernovaexplosion!15The Crab NebulaTypes of Supernovae• Type Ia: The explosion thatresults from a white dwarfexceeding the ChandrasekharLimit (1.4 solar masses)• Type II: Supernovae resultingfrom core collapse• Less common:– Type Ib and Ic: Results fromcore collapse, but lackshydrogen, lost to stellar windsor other processes16Stellar Corpses• A type II supernova leaves behind thecollapsed core of neutrons that started theexplosion, a neutron star.• If the neutron star is massive enough, it cancollapse, forming a black