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 7
Astronomy Place tutorial “Stellar Evolution”
complete and do exercises
Astronomy Place tutorial “Black Holes”
lessons plus exercises
Main Sequence Star =
Fusing H -> He in Core
Luminosity - Mass Relation
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
HertzsprungRussell
Diagram
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
Luminosity
Very massive
stars are rare
Low-mass
stars are
common
Why no stars
with less
than 0.08
Msun ?
Temperature
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
a
RED
GIANT
Helium Fusion
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
Evolution
of low
mass star
Evolution
Small Mass
Star
Quantum Mechanics
Fundamental Principle of Quantum
Mechanics:
ONE CAN’T OBSERVE
WITHOUT DISTURBING
Expressed mathematically as Heisenberg’s
Uncertainty Principle
Dx Dv > h/m
Position
Velocity
A number
Mass
Pressure of “Degenerate” Gas
In words:
Uncertainty in Position x Uncertainty in Speed is
Greater 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 Nebula
Instability
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
QuickTime™ and a
Sorenson Video 3 decompressor
are 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
Not to scale!
5. Planetary Nebula leaves white
dwarf behind
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 heavier
elements 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
Not to scale!
5. Supernova leaves neutron star
behind
http://instruct1.cit.cornell.edu/courses/a
stro101/java/evolve/evolve.htm
Test:
Cluster
HR
Diagrams
Same Distance
Same Age
Question:
Why must
stars evolve?
Life History of a Star
Loss of Energy to Space
Gravitational Contraction of Core
Contraction is halted temporarily
by nuclear fusion
Energy generation in core
Death of Stars
1) White Dwarf
2) Neutron Star
3) Black Hole
4) Nothing
Small Mass Stars
End 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 protons
to 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 a
Sorenson Video 3 decompressor
are needed to see this picture.
What is the source of Energy for a
Supernova Explosion
• Gravitational Potential Energy