Download Stars: from Adolescence to Old Age

Stars:
From Adolescence to Old Age
12 April 2005
AST 2010: Chapter 21
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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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Main Sequence: Lifetime vs. Mass
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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 main
red giant stage
short for massive stars
•as low as 10 million
(107) years
long for low-mass stars
•up to 10 billion (1010)
years
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.
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 different stars in a cluster provide a test for theories of
stellar evolution
• 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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Testing the Theory: Relatively Young Stars
• Comparison of the model 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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Testing the Theory: An Older Cluster
• Comparison of the model prediction for a 4.24billion-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
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
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 Main-Sequence Stars
Stage 7: Red Giant or Supergiant
• When 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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Red Supergiant
• core radius earth-sized
• heavy element fusion in shells
• envelope 5 AU
Betelgeuse
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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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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 are 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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