Env 384/A: Remote Sensing (RS) and Geographic Information Systems (GIS)

20. Stellar DeathLow-mass stars undergo three red-giant stagesDredge-ups bring material to the surfaceLow-mass stars die gently as planetary nebulaeLow-mass stars end up as white dwarfsHigh-mass stars synthesize heavy elementsHigh-mass stars die violently as supernovaeSupernova 1987ASupernovae produce abundant neutrinosBinary white dwarfs can become supernovaeDetection of supernova remnants1Low-Mass Stars: 3 Red Giant PhasesLow-mass definition< ~ 4 M during main-sequence lifetimeRed giant phasesInitiation ofshellhydrogenfusionRed giant branch on the H-R diagramInitiation ofcoreheliumfusionHorizontal branch of the H-R diagramInitiation ofshellheliumfusionAsymptotic giant branch of the H-R diagram2The Suns Post-Main-Sequence Fate

3Interior of Old Low-Mass AGB Stars

4Stellar Evolution In Globular Clusters

5Dredge-Ups Mix Red Giant MaterialMain-sequence lifetimeThe core remains completely separateNo exchange of matter with overlying regionsDecreasing HIncreasing Hein the coreOverlying regions retain cosmic chemical proportions~ 74 % H~ 25% He~ 1% metals[by mass]Red giant phasesThree possible stagesStage 1 dredge-upAftercoreHfusionendsStage 2 dredge-upAftercoreHefusionendsStage 3 dredge-upAftershellHefusionbeginsOnly if MStar > 2 MOne possible resultA carbon starAbundant CO ejected into spaceSame isotopes of C & O that are in human bodies6Low-Mass Stars Die GentlyHe-shell flashes produce thermal pulsesCaused by runaway core He fusion in AGB starsCyclical process at decreasing time intervals313,000 years295,000 years251,000 years231,000 yearsAll materials outside the core may be ejected~ 40% of mass lost from a1.0 M star> 40% of mass lost from a>1.0 M starHot but dead CO core exposedAt the center of an expanding shell of gasVelocities of ~ 10 km . sec-1 to ~ 30 km . sec-1Velocities of ~ 22,000 mph to ~ 66,000 mph7Carbon Star & Its CO Shell: Photo

8Carbon Star & Its CO Shell: Sketch

9Thermal Pulses of 0.7 M AGB Stars

10One Example of a Planetary Nebula

11Helix Nebula: 140 pc From Earth

12An Elongated Planetary Nebula

13Low-Mass Stars End As White DwarfsUV radiation ionizes the expanding gas shellThis glows in what we see as a planetary nebulaName given because they look somewhat like planetsNo suggestion that they have, had, or will form planetsThis gas eventually dissipates into interstellar spaceNo further nuclear fusion occursSupported by degenerate electron pressureAbout the same diameter as Earth~ 8,000 milesIt gradually becomes dimmerEventually it becomes too cool & too dim to detect14White Dwarfs & the Earth

15The Chandrasekhar LimitWhite dwarf interiorsInitially supported by thermal pressureIonized C & O atomsA sea of electronsAs the white dwarf cools, particles get closerPauli exclusion principle comes into playElectrons arrange in orderly rows, columns & layersEffectively becomes one huge crystalWhite dwarf diametersThe mass-radius relationshipThe larger the mass, the smaller the diameterThe diameter remains the same as a white dwarf coolsMaximum mass degenerate e pressure can support~ 1.4 MAfter loss of overlying gas layersWhite dwarf upper mass limit is the Chandrasekhar limit16Evolution: Giants To White Dwarfs

17White Dwarf Cooling Curves

18High-Mass Stars Make Heavy ElementsHigh-mass definition> ~ 4 M as a ZAMS starSynthesis of heavier elementsHigh-mass stars have very strong gravityIncreased internal pressure & temperatureIncreased rate of core H-fusion into HeIncreased rate of collapse once core H-fusion endsCore pressure & temperature sufficient to fuse CThe CO core exceeds the Chandrasekhar limitDegenerate electron pressure cannot support the massThe CO core contracts & heatsCore temperature > ~ 6.0 . 108 KC fusion into O, Ne, Na & Mg begins19Synthesis of Even Heavier ElementsVery-high-mass definition> ~ 8 M as a ZAMS starSynthesis of still heavier elementsEnd of core-C fusionCore temperature > ~ 1.0 . 109 KNe fusion into O & Mg beginsEnd of core-Ne fusionCore temperature > ~ 1.5 . 109 KO fusion into S beginsEnd of core-O fusionCore temperature > ~ 2.7 . 109 KSi fusion into S & Fe beginsStart of shell fusion in additional layers20The Interior of Old High-Mass Stars

21Consequence of Multiple Shell FusionCore changesCore diameterdecreaseswith each stepUltimately about same diameter as Earth~ 8,000 milesRate of core fusionincreaseswith each stepEnergy changesEach successive fusion step produces less energyAll elements heavier than iron require energy inputCore fusion cannot produce elements heavier than ironAll heavier elements are produced by other processes22Evolutionary Stages of 25-M Stars

23High-Mass Stars Die As SupernovaeBasic physical processesAll thermonuclear fusion ceasesThe core collapsesIt is too massive for degenerate electron pressure to supportThe collapse reboundsLuminosity increases by a factor of 108As bright as an entire galaxy> 99% of energy is in the form of neutrinosMatter is ejected at supersonic speedsPowerful compression wave moves outwardAppearanceExtremely bright light where a dim star was locatedSupernova remnantWide variety of shapes & sizes24The Death of Old High-Mass Stars

25Supernova: The First 20 Milliseconds

26Supernova 1987AImportant detailsLocated in the Large Magellanic CloudCompanion to the Milky Way ~ 50,000 parsecs from EarthDiscovered on 23 February 1987Near a huge H II region called the Tarantula NebulaWas visible without a telescopeFirst naked-eye supernova since 1604Basic physical processesPrimary producer of visible lightShock wave energy< 20 daysRadioactive decay of cobalt, nickel & titanium> 20 daysDimmed gradually after radioactivity was gone> 80 daysLuminosity only 10% of a normal supernova27Unusual Feature of SN 1987ARelatively low-mass red supergiantOuter gaseous layers held strongly by gravityConsiderable energy required to disperse the gasesSignificantly reduced luminosityUnusual supernova remnant shapeHourglass shapeOuterringsIonized gas from earlier gentle ejectionCentralringShock wave energizing other gases28Supernova 1987A: 3-Ring Circus

29White Dwarfs Can Become SupernovaeObserved characteristicsNo spectral lines of H or HeThese gases are goneThe progenitor star must be a white dwarfStrong spectral line of Si IIBasic physical processesWhite dwarf in a close-binary settingOver-contact situationCompanion star fills Roche lobeWhite dwarf may exceed the Chandrasekhar limitDegenerate electron pressure cannot support the massCore collapse begins, raising temperature & pressureUnrestrained core C-fusion beginsWhite dwarf blows apart30White Dwarf Becoming a Supernovae

31The Four Supernova Types

Type IaType IbType IcType IINo H or He linesStrong Si II lineNo H linesStrong He I lineNo H or He linesStrong H lines32Type Ia & II Supernova Light Curves

33Gum Nebula: A Supernova Remnant

34Pathways of Stellar Evolution

35Death of low-mass starsZAMS mass < 4 MRed giant phasesStart of shell H fusionStart of core He fusionStart of shell He fusionNo elements heavier than C & OGentle deathDead core becomes a white dwarfExpelled gases become planetary neb.Death of high-mass starsZAMS mass > 4 MRed supergiant phasesNo elements heavier than FeCatastrophic deathDead core a neutron star or black holeSupernova remnantElements heavier than Fe producedPathways of stellar evolutionLow-mass starsProduce planetary nebulaeEnd as white dwarfsHigh-mass starsProduce supernovaeEnd as neutron stars or black holesImportant Concepts36