Announcements• Angel Grades are updated

(but still some assignments not graded)More than half the class has a 3.0 or better

• Reading for next class: Chapter 18• Star Assignment 8,

due Wednesday April 7Astronomy Place tutorial “Stellar Evolution”

complete and do exercisesAstronomy Place tutorial “Black Holes”

lessons plus exercises

Main Sequence Star = Fusing H -> He in Core

Luminosity - Mass Relation

L~M3.5

Mass - Luminosity Relation

Larger Mass stars have larger Gravity pulling in

Need larger Pressure pushing out

Larger Pressure requires higher Temperature

Higher Temperature produces much greater Energy Generation Rate

Energy Loss balances Energy Generation

L ~ M 3. 5

Hertzsprung-Russell

Diagram

Lum

inos

ity

->

Main Sequence:• More massive stars have larger gravity

pulling in• Need larger pressure pushing out• Requires larger temperature• Produces faster nuclear fusion reactions• Faster energy generation• Balanced by larger Luminosity• Star must be larger to let photons escape

easier

Temperature

Lu

min

osi

tyVery massive stars are rare

Low-mass stars are common

Why no stars with less than 0.08 Msun ?

What happens as Hydrogen is fused into Helium in core of a star?

Number of particles decreases Less Pressure Core contracts & Heats up More Energy Generation Star Expands to let more energy out

(greater Luminosity) Star becomes a Giant

What happens when all the H in the core is finally converted to He?

He has larger charge, stronger repulsion, requires higher temperature to fuse into C

He core is inert, loses energy, can’t maintain pressure, star contracts

Converts gravitational PE -> thermal KE heats up star, including H surrounding core

Ignites H shell fusion, continued contraction raises temperature of shell, rate of energy generation increases

Luminosity increases, star expands to let energy out

Star becomes

a RED

GIANT

Star becomes

aRED

GIANT

Helium fusion requires higher temperatures than hydrogen fusion because the larger charge leads to greater repulsion. Eventually, core gets hot enough to fuse helium.

Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon

Helium Fusion

Evolution of low

mass star

Evolution Small Mass

Star

Quantum MechanicsFundamental Principle of Quantum

Mechanics:

CAN’T OBSERVE WITHOUT DISTURBING

Expressed mathematically as Heisenberg’s Uncertainty Principle

x v > h/m

Position Velocity A number ass

Pressure of “Degenerate” Gas

In words:Uncertainty in Position x Uncertainty in Speed isGreater than h (a small number) divided by the mass

Consequence:

Speed ~ h/(mass x distance between particles)

Squeeze particles closer together -> speed increases -> collide harder -> more Pressure

End of Fusion for Small Mass Stars

• When core density becomes very high,nuclei & electrons are squeezed so close together that the Uncertainty Principle makes their speed increase

• They become “Degenerate”

• Pressure increases with increasing Density(not Temperature), stops further contraction & heating

Small mass stars can not get hot enough to fuse Carbon

Planetary N

ebula

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

Instability

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

Life Stages of Low-Mass Star

1. Main Sequence: H fuses to He in core

2. Red Giant: H fuses to He in shell around He core

3. Helium Core Burning: He fuses to C in core while H fuses to He in shell

4. Double Shell Burning: H and He both fuse in shells

5. Planetary Nebula leaves white dwarf behind

Not to scale!

Reasons for Life Stages

1. Core shrinks and heats until it’s hot enough for fusion

2. Nuclei with larger charge require higher temperature for fusion

3. Core thermostat is broken while core is not hot enough for fusion (shell burning)

4. Core fusion can’t happen if degeneracy pressure keeps core from shrinking

Not to scale!

Large mass stars

get hot enough to fuse heavy nuclei

Fusion reactions in late stages of evolution of massive stars make heavierelements up to Iron

Evolution of high

mass star

Betelgeuse:

Red Supergiant

Life Stages of High-Mass Star

1. Main Sequence: H fuses to He in core

2. Red Supergiant: H fuses to He in shell around He core

3. Helium Core Burning: He fuses to C in core while H fuses to He in shell

4. Multiple Shell Burning: Many elements fuse in shells

5. Supernova leaves neutron star behind

Not to scale!

http://instruct1.cit.cornell.edu/courses/astro101/java/evolve/evolve.

htm

Test:Cluster

HR Diagrams

Same DistanceSame Age

Question:

Why must stars evolve?

Life History of a Star

Loss of Energy to SpaceGravitational Contraction of CoreContraction is halted temporarily

by nuclear fusionEnergy generation in core

Death of Stars

1) White Dwarf

2) Neutron Star

3) Black Hole

4) Nothing

Small Mass StarsEnd Life as WHITE DWARFS

• Core becomse so dense can not contract and heat any more

• Star supported by pressure of degenerate electrons

• Size about size of Earth

• Star slowly cools

More Massive White Dwarfs are Smaller

• More Mass -> More gravity

• Need larger Pressure

• Must squeeze electrons more to increase their speed and pressure

• Smaller White Dwarf

Maximum Mass for White Dwarfs

• Pressure of “degenerate” electrons can only support so much mass before electron speed would need to be the speed of light.

• Maximum mass of White Dwarfs 1.4 Msun

Maximum Mass of Star becomes WD

• Stars larger than 1.4 Msun can become WD because they throw off mass as Planetary Nebula

What happens to Stars with too much mass to become White Dwarfs?

• Core contracts

• Gets hotter

• Can fuse elements up to Iron and release energy

• Iron is most tightly bound nucleus (has smallest mass per nucleon of all)

• To make heavier nuclei -> must provide energy

Energy from Fusion

Iron builds up in core until electrons get squeezed onto protonsto make neutrons.

Degeneracy pressure goes away and no longer resists gravity

Core then suddenly collapses, creating supernova explosion

Neutrons collapse to the center, forming a neutron star

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

What is the source of Energy for a Supernova Explosion

• Gravitational Potential Energy