Stellar Evolution

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Stellar Evolution

• Stars must evolve because they shine

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Gaseous Nebulae
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Young Clusters
Pleiades
M7
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Stellar NurseriesThe Eagle Nebula
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(No Transcript)
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Orion Nebula
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The Trigger Mechanism

• Spiral Density Waves
• Supernova

Movie
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Gas Cloud Encounters Spiral Arm
Spiral Arm
Cloud
Animation
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Angular Momentum
m
L (mass)(velocity)(distance from axis)
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Conservation of Angular Momentum

• Total angular momentum is unchanged
• As collapse proceeds, d is decreasing
• Cloud spins faster

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Animation
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Planet Formation

• Orbits should be coplanar
• Planets should orbit in the same direction
• Planets should rotate in the same direction

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Temperature in the Solar Nebula
Asteroids
S
M V E M
J
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The Power Struggle

• Gravity tries to crush the star
• Radiation Pressure tries to blow it up
• During Pre-Main Sequence Evolution, gravity is
  winning
• Star collapses

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Ideal Gas

• As pressure increases, there are more atomic
  collisions
• Average particle speed increases
• Temperature goes up
• P ? T

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More Ideal Gases

• Ex. fill a balloon with air
• Carefully heat it up
• Balloon expands to try to cool down
• Put it in the freezer
• Balloon shrinks to try to heat up
• Ideal gases work like a safety valve

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Evolution on the Blackbody Curve
Infrared
Visible
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Evolution on the H-R Diagram
Phase I Pre-Main Sequence Evolution
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Stellar Luminosity
L ? R2T4

• Bigger objects are brighter
• pre-main sequence objects are above the main
  sequence

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Dust Shell

• Region around proto-star gets too warm
• Dust is pushed back
• Proplyds
• Bi-polar jet

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Inner Region
Protostar
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Proplyds
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Bi-Polar Jets
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Observed Protostar

• Brighter than indicated by temperature
• Very large
• Redder than indicated by temperature
• Dust shell scatters short wavelengths

T Tauri Variables
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Phase II Pre-Main Sequence Evolution

• Protostar begins a very strong solar-type wind
• T Tauri Wind
• Dust shell becomes vaporized
• Planet formation halted
• Now we see protostar for the first time
• Star grows fainter (much smaller)
• Temperature remains almost constant
• Hyashi Track

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Hyashi Track
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The Onset of Fusion

• Core temperatures reach 10 million K
• Fusion begins
• Gravity now balanced by radiation pressure
• Hydrostatic Equilibrium established
• Zero Age Main Sequence (ZAMS)

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Pre-Main Sequence Evolutionary Tracks
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Stellar Structure

• Core
• Fusion reactions here

• Envelope
• Supplies gravity to keep core hot and dense

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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.

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Main Sequence Evolution

• Hydrogen is depleted in the current fusion zone
• Reaction rate falls
• Core can no longer support its weight
• core shrinks
• Temperature in the core increases
• Gas is ideal
• Extra energy produced to -
• Begin fusion reaction in next core zone
• Lift the envelope
• Star brightens slightly

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Main Sequence Evolution
He
He
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End Section