Stellar EvolutionStellar Evolution
Beyond the Main Sequence
On the Main SequenceOn the Main Sequence
Hydrostatic EquilibriumHydrogen to Helium in CoreAll sizes of stars do thisAfter this, evolution depends on mass
High mass > 8M
Low mass < 8M
Stellar ModelsStellar Models
3 initial conditions– Star must produce energy– Hydrostatic Equilibrium– Energy transport
Equations are developed to account for energy production
Chemical composition and mass must be included in formulae
Models InfoModels Info
Tell us how a star evolves– Evolutionary Track
How long will it live?How will it die?Our Sun is the test case
Hydrogen BurningHydrogen Burning
Star<1.5M use PP Chain
Star>1.5M use CNO cycle
Both take H and turn it into He (fusion)
CNO CycleCNO Cycle
More efficient at higher temps
More complicated reaction chain
Carbon 12 is a catalystNitrogen and Oxygen are created as intermediate products
CNO CycleCNO Cycle
12C + 1H 13N + 13N 13C + e+ + 13C + 1H 14N + 14N + 1H 15O + 15O 15N + e+ +
15N + 1H 12C + 4He4 1H go to 1 4He
So long Main SequenceSo long Main Sequence
H in CORE is exhaustedOnly He remainsCore not hot enough to fuse He
No outward pressureGravity wins battle (for a while)
Evolution of 1MEvolution of 1M Star Star
10 billion years passed on MSHelium Core, burning ceasesCore begins to contract (gravity)
Contraction = heatingNeed to reach 100 million K to fuse He
Core and EnvelopeCore and Envelope
2 parts of star– Central Core– Outer envelope
Core is not burningCore is contracting and heating
Heats envelopeH fusion begins in lower envelope (Shell Burning)
Envelope ActionEnvelope Action
Core is heating, H shell is burning
Both are heating envelopeEnvelope responds by expanding and cooling
Subgiant (T and L constant)
Envelope ActionEnvelope Action
Envelope takes time to respond fully to temperature increase
Expands dramaticallyL TRed Giant PhaseHR diagram path = Red Giant Branch
About 1 billion years to expand fully
He Core, H shellHe Core, H shellExpanding Envelope
H burning shell
Inert He Core
Helium Burning BeginsHelium Burning Begins
100 million degrees is reached
Collapse ceasesHe fusion beginsHe nucleus= alpha particleHe fusion creates CTriple-alpha process
Triple-alpha ProcessTriple-alpha Process
4He + 4He 8Be + 8Be + 4He 12C +
Called Triple-alpha since 3 He nuclei are involved
Helium FlashHelium Flash
Occurs in Low Mass StarsHe core inert, H shell burning
H shell adds He to coreCore contracts and becomes degenerate
Helium FlashHelium Flash
Degenerate electron pressureElectrons hold core upT is reached for He burningBurning ignites explosivelyChain reaction He burning in core(Helium Flash!)
Blows out H shell burning
Horizontal BranchHorizontal Branch
After Helium FlashCore burning Helium quiescently
Stays same L and T for some time
Stars line up on HR diagram by mass
Sun’s HB lifetime = 100 million years
End of He End of He
All He in core is fused to CBurning stopsContracting C core (inert)Core heats, causes He and H shell burning
Heat sources cause envelope to expand again (asymptotic giant branch)
C core, H and He shellC core, H and He shellExapanding Cooling Evenlope
He shell
H Shell
Inert C Core
Thermal PulsesThermal Pulses
Triple Alpha in shell is very T sensitive
Explosive burning pulses in shell
Causes envelope to expand rapidly (5-10years)
Luminosity varies up to 50%Some models predict 4 pulses about 100,000 years apart
Planetary NebulaPlanetary Nebula
A superwind developsPulses and wind rip envelope off star
Expanding shell of gas 20km/sHot core appears
Core?Core?
Core cannot collapse enough to burn C
Remains degenerateForms a White DwarfNo burning, no energy generation
Cools off
Lower Mass StarLower Mass Star
Never fuse He to CBurn VERY slowlyMost are still on MS
5-M5-M Star Evolution Star Evolution
Will follow similar path to sun but…
Uses CNO cycle on MSCan fuse C in coreNo He flashLeaves a larger corpse (White dwarf)
Evolution occurs much faster
Chandrasekhar LimitChandrasekhar Limit
Determines upper mass limit for white dwarf formation
WD is very dense1 teaspoon= 5 tonsLargest WD 1.4M
Above that, degeneracy pressure fails to hold up star
Massive Star EvolutionMassive Star Evolution
Similar beginning to SunSits on MS fusing H to HeOccurs much faster (7million years)
Can fuse He to C and so onStar is massive enough that fusion temperatures are reached
FusionFusion
Massive Stars can support fusion up to Iron
Carbon to NeonNeon to OxygenOxygen to SiliconSilicon to Iron
Onion Skin ModelOnion Skin Model
Star will have many layers of shells develop
Final stage has an inert Iron core with 6 shells burning
End state of star
Onion Skin ModelOnion Skin ModelH shell
He shell
C shell
Ne shell
O shell
Si shell
Fe core
Binding Energy Binding Energy
Iron has highest binding energy
All fusion reactions before Iron exothermic
After Iron, endothermicBig Problem for Star!Can’t fuse Iron!
End of LifeEnd of Life
Core contracts without stopping
Core begins to photodisintegrate
Electrons, Protons, NeutronsElectron and Protons combineFlood of neutrinos are released
Core BounceCore Bounce
Core can’t contract any moreNeutron Degeneracy causes rebound
Impacts AtmosphereViolently Ejects AtmopshereSupernova! (Type II SN)
Heavy Element Heavy Element FormationFormation
During Supernova Neutrons collect onto remaining Fe nuclei
Rapid Neutron Capture (r-process)
Form every naturally occuring element
Gold is rare!
Energy ReleaseEnergy Release
Luminosity of star increases 100 million times (108)
Amount of energy released = all stars in Milky Way
Ejects mass back into ISM25Mstar will return 23-24M
Star will leave behind a corpse but not a WD
Star ClustersStar Clusters
Examine HR diagram of stars to determine ages
Globular Cluster– Very dense– Many stars of different masses– All same age
MS turnoff tells agesProof that stars are evolving
SummarySummary
Stars live similar livesMassive stars end states are violent
Low mass stars die quietlyStars evolve around MSCan use clusters to look at evolution of Stars