Transcript Stars: from Adolescence to Old Age

Chapter 21: Stars: From
Adolescence to Old Age
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Recall: the H-R Diagram
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Mass Determines Life Stages
Mass determines stages stars go through and how
long they last in each stage
with just little bit of dependence on composition
Massive stars evolve faster than small stars
Relationship between the luminosity and mass
determined by how compressed gases behave.
Small increase in mass produces a large increase
in the luminosity of a star.
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Lifetime vs. Mass
main sequence: more mass  hotter and shorter life
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star mass
(solar masses)
time (years)
Spectral type
60
3 million
O3
30
11 million
O7
10
32 million
B4
3
370 million
A5
1.5
3 billion
F5
1
10 billion
G2 (Sun)
0.1
1000's billions
M7
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Old Age: Main Sequence to Red Giant
Stage 5: Red Giant
collapse: fusion stops when the hydrogen in the core runs out
shell burning: hydrogen shell surrounding the core ignites
star expands and becomes a subgiant, then a red giant
Stage 6: Helium Fusion
helium fusion begins in the core
star passes through a yellow giant phase
equilibrates as a red giant or supergiant
Stage 7: Stellar Nucleosynthesis – fusion of heavier elements
(up to iron)
core fuel in stage 6 runs out and collapse resumes
fusion of heavier elements may ignite if star is sufficiently
massive
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Stage 5, part 1:
Collapse
main sequence: inward gravity balanced by the
outward pressure
pressure due to fusion in core
hydrogen in the core eventually converted to
helium
 nuclear reactions stop!
gravity takes over and the core shrinks
outside layers also collapse
layers closer to the center collapse faster than
those near the surface.
As the layers collapses, the gas compresses and
heats up
Stage 5, part 2:
Shell Burning
shell layer outside the core becomes hot and
dense enough for fusion to start
fusion in the layer just outside the core is
called shell burning
shell fusion is very rapid because the shell
layer is still compressing and increasing in
temperature
luminosity of the star increases from its main
sequence value
Gas surrounding the core puffs outward under
the action of the extra outward pressure
The star expands and becomes a subgiant
and then a red giant.
surface has a red color because star is
puffed out and cooler
red giant is very luminous because of its
huge surface area
time to reach red
giant stage
short for big stars
•as low as 10 million
(107) years
long for little stars
•up to 10 billion (1010)
years for low mass
stage 5: shell burning  red giant
End of Life on Earth …
When the Sun becomes a red giant, it will swallow
Mercury,Venus and perhaps the Earth too.
Or conditions on Earth’s surface will become impossible for life to exist.
Water oceans and atmosphere will evaporate away.
21.2 Star Clusters
We saw that stars tend to form in clusters
The stars in the cluster have different masses but
about the same age.
The evolution of the stars in a cluster should be
consistent with the theory.
Three types of clusters:
Globular clusters -- only contain very old stars
Open clusters -- contain relatively young stars
Stellar associations -- small groups of young stars
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21.3 Checking Out the Theory
Comparison of the prediction for the stars of a 3
million year old cluster (left) with measurements of the
stars in cluster NGC 2264 (right).
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An Older Cluster
Comparison of the model for a 4.24 billion year old
cluster (left) with measurements of stars in 47
Tucanae (right). Note the different scales.
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Stage 6: Helium Fusion
red giant: dead helium core plus
hydrogen burning shell
gravity plus inward pressure from
burning shell heats core
helium fusion starts at 100 million K
triple alpha process: three 4He  12C
helium flash: onset of helium fusion produces a burst of
energy
reaction rate settles down
Fusion in the core releases more energy/second than core
fusion in main sequence
star is smaller and hotter, but stable!
hydrostatic equilibrium holds until the core fuel runs out.
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stage 6: helium flash  yellow giant
Stage 6: Helium Fusion
hydrostatic equilibrium holds until the core fuel runs
out
star is a yellow/orange giant
dead carbon core shrinks under its weight
gravity  pressure and heat
heats helium shell surrounding core
fusion of hydrogen surrounding helium shell
star again puffs out to red giant
Sun-like or smaller stars: terminal stage
heavier stars:
helium shell flashes
pulsation (as in Cephied variable stars)
heavier elements fuse
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stage 6: yellow giant  red giant or supergiant
Pulsating Stars
In ordinary stars hydrostatic equilibrium works to
dampen (diminish) the pulsations.
But stars entering and leaving stage 6 can briefly (in
terms of star lifetimes!) create conditions where the
pressure and gravity are out of sync and the
pulsations continue for a time.
Larger, more luminous stars will pulsate with longer
periods than the smaller, fainter stars
because gravity takes longer to pull the more extended outer
layers of the larger stars back.
The period-luminosity relation can be used to
determine the distances of these luminous stars from
the inverse square law of light brightness.
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Upper mainsequence star
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Stage 7: Red Giant or Supergiant
When the core fuel runs out
again, the core resumes its
collapse.
If the star is massive enough,
it will repeat stage 5.
The number of times a star
can cycle through stages 5 to
7 depends on the mass of the
star.
Each time through the cycle,
the star creates new heavier
elements from the ash of
fusion reactions in the
previous cycle.
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Stellar Nucleosynthesis
Fusion creates heavier elements from lighter elements
very massive stars produce elements up to iron in the
core
nuclear fusion releases energy for elements lighter than iron
past iron, fusion absorbs energy
stars like our Sun produce elements up to carbon and
oxygen
heavier elements produced in supernova explosions
of very massive stars
density gets so great that protons and electrons are
combined to form neutrons (+ neutrinos)
outer layers are ejected in a huge supernova explosion
elements heavier than iron are formed and ejected
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red supergiant
core radius earth-sized
heavy element fusion in shells
envelope 5 AU
Betelgeuse
Stage 5 begins: collapse of the star generates heat
post-main
sequence evolution
of a 5 solar mass
star
hydrogen fuel in core runs out
Stage 5 continues: star collapses further as shell burning begins
Star adjusts as collapse continues
•further collapse of core
•exterior of star cools
Stage 5-6: Young Red Giant forms, Shell Burning starts
Shell burning starts
•outer layers of star expand
•core continues to contract
Stage 6: Core Burning drives further expansion
• strong new heat source
from helium fusion
• hotter  more yellow
Stage 6-7: Red Giant matures
mature red giant
region (AGB)
young red giant
region (RGB)
Planetary Nebula
Planetary nebula got their name because some
looked like round, green planets in early
telescopes.
Now known to be formed when old, low-mass
stars are unable to fuse heavier elements, and
their cores collapse.
The outer layer of the star is ejected by wind.
About one or more light years across
much larger than our solar system!
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The Helix Nebula
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