Stellar Evolution Beyond the Main Sequence

On the Main Sequence Hydrostatic Equilibrium Hydrogen to Helium in Core All sizes of stars do this After this, evolution depends on mass High mass > 8M  Low mass < 8M 

Stellar 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 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 Burning Star<1.5M  use PP Chain Star>1.5M  use CNO cycle Both take H and turn it into He (fusion)

CNO Cycle More efficient at higher temps More complicated reaction chain Carbon 12 is a catalyst Nitrogen and Oxygen are created as intermediate products

CNO Cycle 12 C + 1 H  13 N +  13 N  13 C + e C + 1 H  14 N +  14 N + 1 H  15 O +  15 O  15 N + e N + 1 H  12 C + 4 He 4 1 H go to 1 4 He

So long Main Sequence H in CORE is exhausted Only He remains Core not hot enough to fuse He No outward pressure Gravity wins battle (for a while)

Evolution of 1M  Star 10 billion years passed on MS Helium Core, burning ceases Core begins to contract (gravity) Contraction = heating Need to reach 100 million K to fuse He

Core and Envelope 2 parts of star – Central Core – Outer envelope Core is not burning Core is contracting and heating Heats envelope H fusion begins in lower envelope (Shell Burning)

Envelope Action Core is heating, H shell is burning Both are heating envelope Envelope responds by expanding and cooling Subgiant (T  and L constant)

Envelope Action Envelope takes time to respond fully to temperature increase Expands dramatically L  T  Red Giant Phase HR diagram path = Red Giant Branch About 1 billion years to expand fully

He Core, H shell Expanding Envelope H burning shell Inert He Core

Helium Burning Begins 100 million degrees is reached Collapse ceases He fusion begins He nucleus= alpha particle He fusion creates C Triple-alpha process

Triple-alpha Process 4 He + 4 He  8 Be +  8 Be + 4 He  12 C +  Called Triple-alpha since 3 He nuclei are involved

Helium Flash Occurs in Low Mass Stars He core inert, H shell burning H shell adds He to core Core contracts and becomes degenerate

Helium Flash Degenerate electron pressure Electrons hold core up T is reached for He burning Burning ignites explosively Chain reaction He burning in core(Helium Flash!) Blows out H shell burning

Horizontal Branch After Helium Flash Core 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 All He in core is fused to C Burning stops Contracting 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 shell Exapanding Cooling Evenlope He shell H Shell Inert C Core

Thermal 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 Nebula A superwind develops Pulses and wind rip envelope off star Expanding shell of gas 20km/s Hot core appears

Core? Core cannot collapse enough to burn C Remains degenerate Forms a White Dwarf No burning, no energy generation Cools off

Lower Mass Star Never fuse He to C Burn VERY slowly Most are still on MS

5-M  Star Evolution Will follow similar path to sun but… Uses CNO cycle on MS Can fuse C in core No He flash Leaves a larger corpse (White dwarf) Evolution occurs much faster

Chandrasekhar Limit Determines upper mass limit for white dwarf formation WD is very dense 1 teaspoon= 5 tons Largest WD 1.4M  Above that, degeneracy pressure fails to hold up star

Massive Star Evolution Similar beginning to Sun Sits on MS fusing H to He Occurs much faster (7million years) Can fuse He to C and so on Star is massive enough that fusion temperatures are reached

Fusion Massive Stars can support fusion up to Iron Carbon to Neon Neon to Oxygen Oxygen to Silicon Silicon to Iron

Onion 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 Model H shell He shell C shell Ne shell O shell Si shell Fe core

Binding Energy Iron has highest binding energy All fusion reactions before Iron exothermic After Iron, endothermic Big Problem for Star! Can’t fuse Iron!

End of Life Core contracts without stopping Core begins to photodisintegrate Electrons, Protons, Neutrons Electron and Protons combine Flood of neutrinos are released

Core Bounce Core can’t contract any more Neutron Degeneracy causes rebound Impacts Atmosphere Violently Ejects Atmopshere Supernova! (Type II SN)

Heavy Element Formation During Supernova Neutrons collect onto remaining Fe nuclei Rapid Neutron Capture (r- process) Form every naturally occuring element Gold is rare!

Energy Release Luminosity of star increases 100 million times (10 8 ) Amount of energy released = all stars in Milky Way Ejects mass back into ISM 25M  star will return 23-24M  Star will leave behind a corpse but not a WD

Star Clusters Examine HR diagram of stars to determine ages Globular Cluster – Very dense – Many stars of different masses – All same age MS turnoff tells ages Proof that stars are evolving

Summary Stars live similar lives Massive stars end states are violent Low mass stars die quietly Stars evolve around MS Can use clusters to look at evolution of Stars