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STELLAR EVOLUTION
Stellar evolution is driven
entirely by the never
ending battle between
pressure and gravity. As
imbalances are reached,
the star is driven to find a
new energy source. Each
stage in stellar evolution
is marked by a different
energy generation
mechanism.
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!
IDEAL GAS LAW
PV = nRT
(pressure) x (volume) = (particle density) x (constant) x (temp)
How PV = nRT works!
Increasing the temperature increases the volume
and decreases the pressure.
Decreasing the volume increases the pressure (and
the increases the temperature)
Why Doesn't a Star Burn all its
Fuel Instantly?
If the fusion
rate
increases,
the
temperature
and pressure
go up...
Stars regulate their internal
pressure and temperature via the
ideal gas law PV = nRT. The rate
at which atoms fuse together is a
function of both pressure and
temperature. So, if a star gets
really hot and the pressure gets
really high, the star will expand
and cool down, thus lowering the
fusion rate.
...the star will
expand, and in
the process lower its
temperature and
pressure, and thus
its fusion rate.
Eventually,
Hydrostatic
Equilibrium will
be
reached. The
pressure
caused by the
energy
generation rate
will
balance the inward
force of gravity
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:
c=
n
speed of light in vacuum
wavelength
frequency
Color and Wavelength
The color of the light depends on its wavelength.
Longer wavelengths correspond to “redder”
light; shorter wavelengths correspond to “bluer”
light.
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
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.
Which Horseshoe Is the
Hottest?
The Main Sequence
<---------- luminosity ---------->
A star on the main sequence
is one that is generating light
and heat by the conversion of
hydrogen to helium by
nuclear fusion in its core.
brighter
dimmer
hotter
cooler
<---------------- temperature ---------------->
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
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.
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
More massive stars have shorter lifetimes!
Chihuahuas live 14-15 years.
Great Danes only live 6-10 years.
Hummer: 32 Gallon Gas
tank
13 miles per gallon
so…416 miles per
tank
Camry: 18.5 Gallon Gas
tank
28.5 miles per
gallon
so…527 miles per
tank
A small change in a star’s 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 that’s twice as massive as the sun:
L = 23.5 = 11.3
It’s ~11 times more luminous!!
A star’s lifetime (t, in solar lifetimes) can be
given by:
M
M
1
Amount of
=
=
=
t = fuel
Rate of fuel consumption
L
M3.5
M2.5
So a star that’s twice as massive as the sun lives 2-2.5 =
0.17 times as long as the sun. That 0.17*1010 = 1.7
billion years
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
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
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.
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.
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.
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"
Hertzsprung-Russell Diagram
Branches
Red Giant Branch stars have a hydrogen burning
shell and their core is contracting.
Horizontal Branch stars have helium coreburning and hydrogen shell-burning.
Asymptotic Giant Branch stars have a helium
burning shell and their core is contracting.