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