STELLAR EVOLUTION

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STELLAR EVOLUTION
?HR Diagram at the end of this lecture, you'll
not only understand this stellar evolution
diagram, but will be able to make one of these
yourself!
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IDEAL GAS LAW
PV nRT
(pressure) x (volume) (particle density) x
(constant) x (temp)
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How PV nRT works!
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Why Doesn't a Star Burn all its Fuel Instantly?
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What is Light?
Light is a form of energy, called radiative
energy. It is both a wave and a particle! It
can be characterized by its wavelength and
frequency
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Color and Wavelength
The color of the light depends on its wavelength.
Longer wavelengths correspond to redder light
shorter wavelengths correspond to bluer light.
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Color and Temperature
Everything with a temperature emits light. Even
as you sit there, you are emitting light in the
infrared!
The peak wavelength (or color) emitted by an
object is a function of its temperature. Hotter
objects emit more of their light at shorter
wavelengths and are said to be bluer cooler
objects emit more of their light at longer
wavelengths and are said to be redder. The
relation between wavelength and temperature (in
Kelvin) is given by Wein's Law,
?peakT 0.0029 meters
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Wein's Law
?peakT 0.0029 meters
Hotter objects emit more light at all wavelengths
than cooler objects. Hotter objects also appear
bluer than cooler objects.
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Which Horseshoe Is the Hottest?
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The Main Sequence
main sequence
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Stage 1 Protostar
Star formation begins with a dense cloud of gas.
A disturbance in the gas triggers a collapse, and
the cloud begins to condense under its own
gravity to form a protostar. A protostar is a
forming star that has not yet reached the point
where sustained fusion can occur in its
core. The energy source for a protostar is
gravitational contraction. The star is cool, so
its color is red, but it is very large so it has
a high luminosity. Sun's Age 1-3 years old

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Stage 2 Pre-Main Sequence
Once the star is close to hydrostatic
equilibrium, the contraction slows down.
However, the star must continue to contract until
the temperature in the core is high enough that
nuclear fusion can begin and support the star!
During the contraction the star's temperature
stays about the same, but its luminosity
decreases because of its shrinking size. Once
nuclear reactions begin in the core, the star
readjusts to account for this new energy source.
In the pre-main sequence star, both
gravitational contraction and nuclear fusion
provide energy. Sun's Age 10 million years old
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Stage 3 Zero- Age Main Sequence
Finally, the rate of fusion becomes high enough
to establish gravitational equilibrium. At this
point, fusion becomes self-sustaining and the
star settles into its hydrogen burning, main
sequence life. The main sequence phase is the
longest phase of a star's life, about 10 billion
years for a star with one solar mass. The main
sequence is not a line, but a band in the H-R
Diagram. The position of a star on the main
sequence is determined by its mass and
composition. Sun's Age 27 million years old
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More massive stars have shorter lifetimes!
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Chihuahuas live 14-15 years. Great Danes only
live 6-10 years.
Hummer 32 Gallon Gas tank 13 miles per
gallon so416 miles per tank
Camry 18.5 Gallon Gas tank 28.5 miles per
gallon so527 miles per tank
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A small change in a stars mass gives a big
change in luminosity
L M3.5 where L luminosity in solar
luminosities M mass in solar masses
So, for a star thats twice as massive as the
sun L 23.5 11.3 Its 11 times more
luminous!!
A stars lifetime (t, in solar lifetimes) can be
given by t
M
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Amount of fuel


M3.5
M2.5
Rate of fuel consumption
So a star thats twice as massive as the sun
lives 2-2.5 0.17 times as long as the sun. That
0.171010 1.7 billion years
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Stage 4 End of Main Sequence
A star ends its life on the main sequence when it
has used up all the hydrogen in its core. Once
the core hydrogen has been exhausted, a shell of
hydrogen surrounding the core begins to burn,
providing energy to the star. During its life on
the main sequence, the size and luminosity of the
star has changed very little. Sun's Age 10
billion years old
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Stage 5 Post Main Sequence
Now that hydrogen is exhausted in the core, there
is no energy to support the Helium core. Thus,
the core contracts and energy is released. The
hydrogen burning shell continues to provide
energy to the outer layers of the star. Sun's
Age 11 billion years
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Stage 6 Red Giant Helium Flash
As the helium core contracts, the temperature and
pressure increases. This increase in temperature
causes the rate of hydrogen fusion in the shell
surrounding the core to go up. As a result, the
star expands (by as much as 200 times!). The
star is now very cool, but luminous a Red Giant!
The contraction of the core causes the
temperature and density to increase such that, by
the time the temperature is high enough for
Helium to fuse to form Carbon, the core of the
star has reached a state of electron degeneracy.
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Stage 7 Helium Burning Main Sequence
The pressure due to electron degeneracy is
significantly different from the pressure
produced by the Ideal Gas Law it is independent
of temperature! In the core, the temperatures
reach 200 million Kelvin and Helium can now fuse
into Carbon, known as the Triple Alpha Process.
This happens quite suddenly and is known as the
Helium Flash.
This process produces only about 20 as much
energy as hydrogen burning, so the lifetime on
the Helium Burning Main Sequence is only about 2
billion years. When the Helium is exhausted in
the core of a star like the sun, no further
reactions are possible.
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Stage 8 Planetary Nebula
Helium and Hydrogen burning shells will continue
outside the core for a while. During Helium
Shell Burning, a final thermal pulse produces a
giant "hiccough" causing the star to eject as
much of 10 of its mass, the entire outer
envelope, known as a Planetary Nebula.
The Planetary Nebula phase is relatively short
lived, estimated to be about 25,000 years, and
there are about 10,000 planetaries in the Milky
Way.
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Stage 9 White Dwarf
As the nebula disperses, the shell nuclear
reactions die out leaving the stellar remnant,
known as a White Dwarf, supported by electron
degeneracy, to fade away as it cools down. The
white dwarf is small, about the size of the
Earth, with a density of order 1 million g/cm3,
about equivalent to crushing a Volkswagen down to
a cubic centimeter or a "ton per teaspoonful."
A white dwarf star will take billions of years to
radiate away its store of thermal energy because
of its small surface area. The white dwarf will
slowly move down and to the right in the H-R
Diagram as it cools until it fades from view as a
"black dwarf"
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Hertzsprung-Russell Diagram
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Branches
Red Giant Branch stars have a hydrogen burning
shell and their core is contracting. Horizontal
Branch stars have helium core-burning and
hydrogen shell-burning. Asymptotic Giant Branch
stars have a helium burning shell and their core
is contracting.