How Stars Evolve

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Chapter 16

• How Stars Evolve

• Low-Mass Stars
• High-Mass Stars

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The Super Kamiokande Experiment
Information
Sonic Boom
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Observations of Solar Neutrino
The GALLEX detector Gran Sasso, Italy
The Sudbury Observatory Ontario, Canada
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Hydrogen Fusion in the Sun
The main nuclear reaction going on inside the Sun
is fusion of hydrogen into helium or the
proton-proton chain.
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Thermostat
Fusion rate is highly temperature dependent. The
two parameters are proportional to each other.
The core temperature rise would cause generation
of an excess energy, which would increase the
pressure and the core expansion and cooling.
The expansion and cooling would continue until
gravitational equilibrium is restored.
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How does the Light Comes Out?
Photons are created in the nuclear fusion
cycle. They collide with other charged particles
and change their direction (random walk). They
also decrease their energy while walking. It
takes 10 million year to get outside.
The random bouncing occurs in the radiation zone
(from the core to 70 of the Suns radius). At
Tlt2 million K, the convection zone carries
photons further towards the surface.
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Low-Mass Star at Main-Sequence
Low-mass stars produce helium from hydrogen
through the proton-proton chain during their
main-sequence lifetime.
The energy moves outward from the core through
random walk and convection.
The number of particles in the core reduces, the
core keeps shrinking, and the luminosity
increases over time.
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Red Giant Stage
When the core hydrogen depletes, nuclear fusion
ceases in the core. The core with no energy
source shrink faster.
The stars outer layers expand, the luminosity
rises. The stars becomes a red giant through a
subgiant.
The radius increases gt100 times. The luminosity
increases thousands times.
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Red Giant Stage
Why does the star grow bigger when the core is
shrinking?
The core is now made of helium, but the
surrounding layers contain plenty of
hydrogen. Gravity shrinks everything, so fusion
begins around the core (in a shell).
The fusion rate in the shell is higher than in
the core during the main-sequence stage. The
newly produced helium is added to the core.
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Switching Energy Sources
The core and the shell keep shrinking, while
thermal pressure keeps pushing upper layers
outward.
This cycle breaks down when the core reaches a
temperature of 100 million K. At this point
helium starts to fuse together.
Throughout the expansion phase, stellar wind
carries away a lot more matter than at the
main-sequence stage.
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Helium Burning
Helium (He) atoms 2 protons ? a higher positive
electric charge than hydrogen atoms.
Helium fusion occurs at higher temperatures. It
converts 3 He nuclei (alpha-particle) into 1
carbon (C) nucleus energy according to
Emc2. In low-mass stars it begins from a flash.
Helium fusion inflates the core, which pushes out
the hydrogen-burning shell. The shell burning
rate drops.
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Helium Burning
The total energy production rate falls from its
red giant phase peak. This reduces the stars
luminosity and decreases the stars radius,
making its surface hotter.
In the H-R diagram, the star goes down and to the
left. All low-mass stars fuse helium into carbon
at nearly the same rate ? they have almost the
same luminosity, but differ in temperature.
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Last Stages of Evolution
The core helium runs out in 100 million
years. When the helium is gone, the fusion stops,
and gravity shrinks the core again.
Now helium ignites in a shell around a carbon
core. The hydrogen shell burns around the helium
shell.
Both shells contract, driving temperatures
higher. The star grows more luminous, but not for
a long time (a few million years).
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Last Stages of Evolution
Carbon fusion is possible only at 600 million
K. But degeneracy pressure halts the collapse.
The star has a large size, no core fusion, and,
hence, low connection to the surface layers. The
stellar wind increases. Carbon is driven from the
core to the surface by convection.
Red giants with carbon-rich atmospheres are
called carbon stars.
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Last Stages of Evolution
Carbon stars have temperatures of 2000?3000
K. Dust particles may be formed in their winds.
At the end of its life, a low-mass star ejects
its outer layers into space.
The exposed core is still hot and radiates UV
photons, which cause the ejected nebula to
glow. Such nebulae are called planetary
nebulae. The dead remnant becomes a white dwarf.
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What Will Happen to Earth?
The Sun keeps increasing its luminosity. In 5
billion years from now the hydrogen burning will
stop in its core.
The Sun will then expand to a subgiant. It will
become 2?3 times brighter.
The Earths temperature will rise, the oceans
will be evaporated, the life may not survive. The
Earth may be destroyed, when the Sun becomes
planetary nebula.
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High-Mass Stars
High-mass stars are important, because they
produce the entire array of elements to create
life. Only these stars can produce elements
heavier than carbon.
The core temperatures are higher in massive stars
than in low-mass stars. Protons may collide with
CNO nuclei. Carbon, nitrogen, and oxygen act as
catalysts to increase the energy production rate.
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CNO Cycle
The CNO cycle is the chain of reactions that
leads to hydrogen fusion in high-mass stars.
The escalated fusion rate of the CNO cycle
produces many more photons than in low-mass
stars. The photons have no mass, but carry
momentum. They transfer the momentum to anything
the run into. The result is radiation pressure.
Radiation pressure is responsible for strong
stellar winds in massive stars.
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Life after Main-Sequence
When the core hydrogen is exhausted, massive
stars follow the same path as low-mass
stars. However, all the processes go more quickly.
When the carbon core forms, there is also a
helium- and a hydrogen-burning shell.
At this point the paths of intermediate- and
high-mass stars diverge. Intermediate-mass stars
blow their outer layers away and become white
dwarfs.
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Massive Stars after Main-Sequence
In massive stars the core temperature can reach
the critical 600 million K to ignite carbon. But
carbon burns away in a few hundred years.
Each successive stage of nuclear burning proceeds
more rapidly than prior stages. Many different
reactions may act at the same time.
One of the simplest fusion sequences is helium
capture, which produces elements with even
numbers of protons.
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Last Stages of Massive Stars
At the end of a massive stars life, iron begins
to form in the silicon-burning core. The star
becomes a red supergiant.
Iron cannot be ignited. Iron has the lowest mass
per nuclear particle.
Elements lighter than iron can fuse. Heavier
elements may produce energy only through fission.
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Summary of Stellar Evolution