Stellar Evolution Stars must evolve because they shine

Gaseous Nebulae Lagoon Nebula (M8) The Trifid Nebula (M20) M24

Young Clusters Pleiades M7

Stellar Nurseries The Eagle Nebula

Orion Nebula

The Trigger Mechanism Spiral Density Waves Supernova Movie

Gas Cloud Encounters Spiral Arm Animation Cloud Spiral Arm

Angular Momentum m d v L = (mass)(velocity)(distance from axis)

Conservation of Angular Momentum Total angular momentum is unchanged FAs collapse proceeds, d is decreasing FCloud spins faster

Animation

Planet Formation Orbits should be coplanar Planets should orbit in the same direction Planets should rotate in the same direction

Temperature in the Solar Nebula M V E M Asteroids J S

The Power Struggle Gravity tries to crush the star Radiation Pressure tries to blow it up During Pre-Main Sequence Evolution, gravity is winning FStar collapses

Ideal Gas As pressure increases, there are more atomic collisions FAverage particle speed increases FTemperature goes up FP  T

More Ideal Gases Ex. – fill a balloon with air FCarefully heat it up EBalloon expands to try to cool down FPut it in the freezer EBalloon shrinks to try to heat up Ideal gases work like a safety valve

Evolution on the Blackbody Curve Visible Infrared

Evolution on the H-R Diagram Main Sequence Brightness Temperature Phase I Pre-Main Sequence Evolution

L  R 2 T 4 Stellar Luminosity Bigger objects are brighter Fpre-main sequence objects are above the main sequence

Dust Shell Region around proto-star gets too warm FDust is pushed back EProplyds EBi-polar jet

Inner Region Protostar

Proplyds

Bi-Polar Jets

Observed Protostar Brighter than indicated by temperature FVery large Redder than indicated by temperature FDust shell scatters short wavelengths T Tauri Variables

Phase II Pre-Main Sequence Evolution Protostar begins a very strong solar- type wind FT Tauri Wind FDust shell becomes vaporized FPlanet formation halted Now we see protostar for the first time FStar grows fainter (much smaller) FTemperature remains almost constant FHyashi Track

Hyashi Track

The Onset of Fusion Core temperatures reach 10 million K FFusion begins FGravity now balanced by radiation pressure FHydrostatic Equilibrium established FZero Age Main Sequence (ZAMS)

Pre-Main Sequence Evolutionary Tracks

Envelope Stellar Structure Core FFusion reactions here Envelope FSupplies gravity to keep core hot and dense

Stellar Models Start at center by guessing temperature, pressure, and composition Work outward using well-established laws of physics and chemistry Check the surface conditions against real stars.

Main Sequence Evolution Hydrogen is depleted in the current fusion zone Reaction rate falls Core can no longer support its weight Fcore shrinks Temperature in the core increases FGas is ideal Extra energy produced to - FBegin fusion reaction in next core zone FLift the envelope EStar brightens slightly

Core Main Sequence Evolution He Current hydrogen “burning” shell He New hydrogen burning shell

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