A short course in Stellar Evolution Based on a lecture by Dr' Maura McLaughlin, WVU

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A short course in Stellar EvolutionBased on a
lecture by Dr. Maura McLaughlin, WVU
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What can we learn about the Sun?
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• Average star

• Spectral type G2

• Only appears so bright because it is so close.

• Absolute visual magnitude 4.83 (magnitude if
  it were at a distance of 32.6 light years)

• 109 times Earths diameter

• 333,000 times Earths mass

• Consists entirely of gas (av. density 1.4
  g/cm3)

• Central temperature 15 million K

• Surface temperature 5800 K

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Organizing the Family of Stars The
Hertzsprung-Russell Diagram
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We know Stars have different temperatures,
different luminosities, and different sizes.
To bring some order into that zoo of different
types of stars organize them in a diagram of
Luminosity
versus
Temperature (or spectral type)
Absolute mag.
Hertzsprung-Russell Diagram
or
Luminosity
Temperature
Spectral type O B A F G K M
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The Hertzsprung Russell Diagram
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Most stars are found along the main sequence
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The Hertzsprung Russell Diagram
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Same temperature, but much brighter than MS stars
? Must be much larger
Stars spend most of their active life time on the
Main Sequence.
? Giant Stars
Same temp., but fainter ? Dwarfs
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Radii of Stars in the HR Diagram
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1,000 times the Suns radius
100 times the Suns radius
As large as the Sun
100 times smaller than the Sun
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Masses of Stars in the Hertzsprung-Russell Diagram
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The higher a stars mass, the more luminous
(brighter) it is
L M3.5
High-mass stars have much shorter lives than
low-mass stars
tlife M-2.5
Sun 10 billion yr.
10 Msun 30 million yr.
0.1 Msun 3 trillion yr.
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A Census of the Stars
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Faint, red dwarfs (low mass) are the most common
stars.
Bright, hot, blue main-sequence stars (high-mass)
are very rare.
Giants and supergiants are extremely rare.
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Life Cycle of a Star
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?
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The Contraction of a Protostar
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Evidence of Star Formation
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Nebula around S Monocerotis (Foxfur)
Contains many massive, very young stars,
including T Tauri stars strongly variable
bright in the infrared.
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Energy generation in the SunNuclear Fusion
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Need large proton speed to overcome Coulomb
barrier (electromagnetic repulsion between
protons).
Basic reaction 4 1H ? 4He energy
4 protons have more mass than 4He.
T 107 K 10 million K

• Energy gain Dmc2

What reaction rate does Sun need to resist its
own gravity?
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Hydrostatic Equilibrium
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Outward pressure force must exactly balance the
weight of all layers above everywhere in the
star.
This condition uniquely determines the interior
structure of the star.
This is why we find stable stars on such a narrow
strip (main sequence) in the HR diagram.
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Masses of Main-Sequence Stars
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Stars with masses greater than 100 solar masses
are unstable.
Stars with masses less than 0.08 solar masses
cannot fuse hydrogen in their cores - brown
dwarfs.
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The Life of Main-Sequence Stars
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Stars gradually exhaust their hydrogen fuel.
In this process of aging, they are gradually
becoming brighter, evolving off the zero-age main
sequence.
Death line
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The Lifetimes of Stars on the Main Sequence
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Expansion onto the Giant Branch
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Once Hydrogen in the core is completely
converted into He, H burning continues in shell.
Lots of energy produced -gt star expands to form
Red Giant.
Giants and supergiants are 10-1000 times
larger than the Sun and 10 - 106 times less
dense. Expansion cools the star and makes it
more luminous.
Sun will expand beyond Earths orbit!
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Red Giant Evolution
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H-burning shell keeps dumping He onto the core.
He core gets denser and hotter until the next
stage of nuclear burning can begin in the core
He fusion through the triple-alpha process
4He 4He ? 8Be g 8Be 4He ? 12C g
The onset of this process is termed the helium
flash
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Endpoints of stellar evolution
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Depends almost completely on its mass. Lets
start with the least massive stars and go to the
most!
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Red Dwarfs (0.08 - 0.4 solar masses)
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Stars with less than 0.4 solar masses are
completely convective.
Hydrogen and helium remain well mixed throughout
the entire star.
No phase of shell burning with expansion to
giant.
Star not hot enough to ignite He burning. Could
live for 100 billion yrs or more! Universe only
14 billion years old so cant test this though
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Sunlike Stars
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Sunlike stars ( 0.4 8 solar masses) develop a
helium core.
? Expansion to red giant during H burning shell
phase
? Ignition of He burning in the He core
? Formation of a degenerate C,O core
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The Fate of our Sunand the End of Earth
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• Sun will expand to a red giant in 5 billion
  years
• Expands to Earths orbit
• Earth will then be incinerated!
• Sun may form a planetary nebula (but uncertain)
• Suns C,O core will become a white dwarf

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Low luminosity high temperature gt White dwarfs
are found in the lower center/left of the H-R
diagram.
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White Dwarfs
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Degenerate stellar remnant (C,O and degenerate
electrons)
Matter in the He core has no energy source left.
? Not enough thermal pressure to resist and
balance gravity
Pressure in degenerate core is due to the fact
that electrons can not be packed arbitrarily
close together and have small energies.
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White Dwarfs
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Extremely dense 1 teaspoon of white dwarf
material mass 16 tons!!!
Chunk of white dwarf material the size of a beach
ball would outweigh an ocean liner!
white dwarfs Mass Msun Temp. 25,000
K Luminosity 0.01 Lsun
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The Chandrasekhar Limit
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The more massive a white dwarf, the smaller it is.
? Pressure becomes larger, until electron
degeneracy pressure can no longer hold up against
gravity.
WDs with more than 1.4 solar masses can not
exist!
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The Deaths of Massive Stars
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Final stages of fusion in high-mass stars (gt 8
Msun), leading to the formation of an iron core,
happen very rapidly Si burning lasts only for
1 day.
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The Deaths of Massive Stars
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Iron core ultimately collapses, triggering an
explosion that destroys the star Supernova
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The Famous Supernova of 1987 Supernova 1987A
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Before
At maximum
Unusual supernova in the Large Magellanic Cloud
in Feb. 1987
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Where do we get elements heavier than Iron???
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Only in Supernovae!!! The huge heat generated in
the explosion makes elements like Gold, Silver
and Platinum.
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Cas A Supernova Remnant
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Only 300 years old -gt youngest known remnant in
MW!
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Neutron Stars
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The central core will collapse into a compact
object supported by neutron degeneracy pressure.
A supernova explosion of an M gt 8 Msun star blows
away its outer layers.
Pressure becomes so high that electrons and
protons combine to form stable neutrons
throughout the object.
Typical size R 10 km
Mass M 1.4 3 Msun
Density r 1014 g/cm3
? Piece of neutron star matter of the size of a
sugar cube has a mass of 100 million tons!!!
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Black Holes
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Just like white dwarfs (Chandrasekhar limit 1.4
Msun), there is a mass limit for neutron stars
Neutron stars can not exist with masses gt 3 Msun
We know of no mechanism to halt the collapse of a
compact object with gt 3 Msun.
It will collapse a singularity
gt A black hole!
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Escape Velocity
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Velocity needed to escape Earths gravity from
the surface vesx 11.6 km/s.
vesc
Now, gravitational force decreases with distance
( 1/d2) gt Starting out high above the surface
gt lower escape velocity.
vesc
If you could compress Earth to a smaller radius
gt higher escape velocity from the surface.
vesc
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The Schwarzschild Radius
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gt There is a limiting radius where the escape
velocity reaches the speed of light, c
Vesc c
2GM
___
Rs
c2
G gravitational constant
M mass
Rs is called the Schwarzschild radius.
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Schwarzschild Radius and Event Horizon
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No object can travel faster than the speed of
light.
gt Nothing (not even light) can escape from
inside the Schwarzschild radius.

• We have no way of finding out whats happening
  inside the Schwarzschild radius.

• Event horizon

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What is the Schwarzchild radius of a typical
human being?
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Observing Black Holes
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No light can escape a black hole
gt Black holes can not be observed directly.
If an invisible compact object is part of a
binary, we can estimate its mass from the orbital
period and radial velocity.
Mass gt 3 Msun gt Black hole!
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Compact object with gt 3 Msun must be a black hole!
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Review - Compact Objects
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Sunlike stars (masses 0.4 - 8 solar masses) -
become white dwarfs (R 10,000 km) - maximum
mass limit of 1.4 solar masses - supported by
electron degeneracy pressure - teaspoon of WD
material would weigh as much as 100
elephants More massive stars (roughly 8-10 solar
masses) - become neutron stars (R 10 km) -
maximum mass limit of 2-3 solar masses -
supported by neutron degeneracy pressure -
teaspoon of NS star material would weigh as much
as a supertanker Most massive stars (roughly
gt 10 solar masses) - become black holes (Rs 10
km) - no maximum mass limit - too massive to be
supported by degeneracy pressure