Lecture 12: Stellar Evolution- Death of a Star

"When I am dead, my dearest. Sing no sad songs for me."
Christina Rossetti, Song

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Date: March 7, 1995
Reading Assignment: pp. 455-463

Description : theory of post-Main Sequence stellar evolution

Objectives

be able to describe how fusion changes the composition of a stars core

be able to describe why hydrogen depletion causes structural changes in a star

be able to describe why hydrogen shell burning occurs

be able to describe why helium core burning takes place and the conditions which cause it to occur

be able to describe the properties of degenerate matter

be able to describe the helium flash occurs

be able to describe why helium shell burning occurs and the conditions which cause it to occur

be able to identify the subgiant, red giant, horizontal, and assymptotic giant branches on the HR diagram

Lecture Outline

Slide # 1: Stellar Evolution

Death of a Star

Slide # 2: The HR Diagram (GRAPHICS)

HR diagram

Slide # 3: Unanswered Questions

Why are 90% of stars on the Main Sequence?

Why is mass so important in determining luminosity?

How can a 1 solar mass star be a red giant, white dwarf, or on the main sequen

Why are red giants and white dwarfs different than MS stars?

Slide # 4: The HR Diagram (GRAPHICS)

Why?

Slide # 5: Red Giants

stars spend only a million years on the Hayashi Track

one solar mass star

stars spend 8 billion years on the main sequence

about 1 in 8000 stars should be in the red giant region if they are premain s

about 1 in 100 stars is a red giant

Slide # 6: Star Modeling- Laws of Physics

hydrostatic equilibrium

energy transport

energy generation

mass continuity

Slide # 7: Computer Simulations (GRAPHICS)

information presented graphically

Slide # 8: The Steps

gas cloud

fragmentation

protostar

Helmholtz contraction

Hayashi track

ignition

adjustment to the Main Sequence

Slide # 9: Stellar Evolution- MS to Death

stars do not end their life on the main sequence

internal changes affect the evolution of stars

Slide # 10: Energy Generation - Nuclear Fusion (GRAPHICS)

hydrogen fuses into helium

energy is released

Slide # 11: The Proton-Proton Chain

hydrogen cannot fuse directly into helium

the chances of 4 protons hitting at the same time are essentially zero

one set of reactions which lead to hydrogen fusing into helium is called the P

Slide # 12: The Proton-Proton Chain (GRAPHICS)

hydrogen into helium in 3 easy steps

Slide # 13: Solar Interior (GRAPHICS)

cross sectional view

Slide # 14: Post Main Sequence Evolution

Why do stars evolve off of the Main Sequence?

What structural changes occur inside the star?

Slide # 15: ZAMS

ZAMS stars have convective and radiative layers

radiation zones are in the center

convection zones are near the photosphere

ZAMS stars have uniform composition

zones are determined by density and temperature

zones are NOT determined by composition

uniform amounts of hydrogen and helium

Slide # 16: Composition Changes

Hydrogen is changing into Helium in the core

nuclear fusion powers the star

No circulation of material is occurring in the core

radiation zone, not convection zone

The fraction of helium in the core increases

the envelope abundance remains constant

Slide # 17: Stellar Composition (GRAPHICS)

helium fraction increase in the core

Slide # 18: Solar Interior (GRAPHICS)

cross sectional view

Slide # 19: Effects of Composition Change

very slight changes in stars luminosity and surface temperature

energy still being generated at about the same rate

Slide # 20: Stellar Composition (GRAPHICS)

helium fraction increase in the core

Slide # 21: End of the Main Sequence Lifetime

star runs out of hydrogen at the core

hydrogen remains in the envelope

nuclear fusion stops

no fuel, no fire

not hot enough for helium burning

the core begins to contract

no energy generated - less pressure

gravity overcomes pressure

Slide # 22: Core Contraction

energy generated from contraction

similar mechanism to premain sequence

core temperature rises

surrounding hydrogen becomes hotter

hydrogen shell burning begins

inert helium core

shell of hydrogen burning

Slide # 23: Hydrogen Shell Burning (GRAPHICS)

inert core- hydrogen burning shell

Slide # 24: 8. Shell Burning

luminosity increases

more energy generated in shell burning

surface temperature drops slightly

1 solar mass = 4000K (was 6000K)

star moves to red giant branch

Slide # 25: Red Giant Size (GRAPHICS)

before and after

Slide # 26: Red Giant Branch

radius 70 time larger

average density = 10-6 gm per cubic cm

core is very compact

average density = 105 gm per cubic cm

1000 times more dense than MS core

very luminous

10,000 solar luminosity

relatively slow evolution

100 million years in this stage

Slide # 27: The HR Diagram (GRAPHICS)

one solar mass star over 8 billion years

Slide # 28: Constellation Corner (GRAPHICS)

Constellation De Jour

Slide # 29: Winter Constellations (GRAPHICS)

Fairfax - 8pm - S - 4.0 - March 5

Slide # 30: Winter Constellations (GRAPHICS)

Fairfax - 8pm - S - 4.0 - March 5

Slide # 31: Perseus (GRAPHICS)

March 6 - 8pm - NW - 4.0 - Fairfax

Slide # 32: Perseus (GRAPHICS)

March 6 - 8pm - NW - 4.0 - Fairfax

Slide # 33: The HR Diagram (GRAPHICS)

one solar mass star over 8 billion years

Slide # 34: Core Contraction Continues

amount of inert helium increase

shell burning adds more helium ash

no energy generated in core

gravity contracts core

density and temperature increase

density = 105 gm per cubic cm

temperature = 100 million K

Slide # 35: 9 & 10 - Helium Core Burning

temperature > 100 million K

10 times hotter than main sequence core

helium fuses into carbon

triple alpha process

4He + 4He -> 8Be + energy

4He + 8Be -> 12C + energy

4He nuclei are called alpha particles

Slide # 36: Core Helium Burning (GRAPHICS)

two energy sources!

Slide # 37: 9. The Helium Flash

before helium burning stars, the core density is very high

density = 105 gm per cubic cm

high core density leads to degenerate gas pressure

increasing temperature does not change the pressure

weak link between temperature and pressure

the rate nuclear fusion occurs depends on temperature

Slide # 38: Hydrostatic Equilibrium (GRAPHICS)

a balance between pressure and gravity

Slide # 39: Normal Stars

core heats up

extra energy being generated in core

star expands

extra heat causes expansion

core cools down

expansion causes cooling

energy generated in core drops

Slide # 40: Red Giants

core heats up

helium burning begins

stars does not expand

degenerate gas pressure

nuclear burning increases

extra heat at core

core heats up

Slide # 41: 9. The Helium Flash

for a few hours, the helium burning is very rapid

degenerate gas causes a thermal runaway

in a few hours, the core is no longer degenerate

temperature finally is high enough to stabilize the star

helium burning slows to more normal rate

Slide # 42: The HR Diagram (GRAPHICS)

one solar mass star over 8 billion years

Slide # 43: 10. The Horizontal Branch

helium core burning, hydrogen shell burning

no degenerate core

"Helium main sequence"

50 million year stage of evolution

Slide # 44: The HR Diagram (GRAPHICS)

one solar mass star over 8 billion years

Slide # 45: 11. The Asymptotic Giant Branch

eventually, the star runs out of helium in the core

core is still not convective

the core again contracts

helium shell burning begins

Slide # 46: Helium Shell Burning (GRAPHICS)

4 layers in the star

Slide # 47: The HR Diagram (GRAPHICS)

one solar mass star over 8 billion years

Slide # 48: Red Giants

most of the stars in today's lecture are called red giants

some are subgiants

some are red supergiants

all are cool, luminous stars

upper right part of HR diagram

the time spent in these regions is about 1% of a stars' lifetime

Slide # 49: Abundance of Red Giants

90% stars are on the main sequence

8 billion years = 1 solar mass star

1% stars are red giants

80 million years (not including subgiants)

what about the remaining 9% of stars?

Slide # 50: Star Clusters (GRAPHICS)

the Pleiades - about 90 million years old

Slide # 51: The HR Diagram (GRAPHICS)

HR diagram

Slide # 52: Question 1

How long does it take a one solar mass star to evolve to the main sequence?

(starting from fragmentation of the gas cloud)

Slide # 53: Question 2

What is force causing star formation?

Slide # 54: The HR Diagram (GRAPHICS)

one solar mass star over 8 billion years