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Stars Part Two:
Stellar Evolution
Overview of the life of a star:
1. Formation of protostar from
a cloud of mostly Hydrogen
gas.
2. Main sequence star
3. Red giant
• White dwarf or…
• Supernova • Neutron star or
• Black hole
Formation of protostar:
1. Gaseous clouds contract
under their own gravity.
2. Regional areas of initial high
density accrete more and
more gas.
3. Gravitational potential turns
to heat.
4. Heat and pressure start
fusion.
Birth of a star
IP Demo: Star_Birth.ip
Birth of a star
1. As the cloud of gas and dust collapses,
a small rotation becomes big
(Conservation of angular momentum)
2. The rapidly spinning protostar often
needs to get rid of angular momentum
before it can start fusion.
3. The magnetic field channels rapidly
spinning material out of polar jets
Birth of a star
1. Eventually, the spin slows enough to
allow fusion
2. The newly born star often blows away
the nebula it came from with its
radiation.
3. The remaining material (still spinning)
stays around the newly formed star in
an accretion disk.
Birth of a solar system:
Accretion Disk
Icy and Gassy Stuff
Rocky Stuff
The New Star
The Solar System
National Geographic Magazine
The Inner Planets::
Mercury Venus
Earth
Mars
•Close together (Relatively)
•Terrestrial (made of rock like Earth)
Asteroids
The Outer Planets::
Jupiter Saturn
Uranus
•Spread out (Relatively)
•Gas giants
Neptune
Pluto
Life on the Main Sequence:
1. Energy comes primarily from
the Proton-Proton cycle:
1H + 1H = 2H + e+ + ν
1H + 2H = 3He + γ
3He + 3He = 4He + 1H + 1H
(requires heat and pressure)
Thermal Agitation balances the tendency
of gravity to crush a star:
Gravity Crushing
Pressure
Heat Thermal
Agitation
1. The rate of burn depends
roughly on the cube of the mass
2. Even though larger stars have
more fuel, they burn the fuel
they have at a much faster rate.
3.Big stars are Brief, Bright, and
Blue
4.Diminutive stars are Durable,
Dim and reD
.01 Billion Years
1 Billion Years
.1 Billion Years
100 Billion
Years
10 Billion Years
500 Billion Years
From Robert Garfinkle’s “Star Hopping”
From Jay Pasachoff’s “Contemporary Astronomy”
A Star trying to be too big
From Jay Pasachoff’s “Contemporary Astronomy”
4He
accumulates in the core of the star:
The death of a star:
1. When most of the Hydrogen in the core
has been used up, leaving a Helium
core, the star cools down. (The Helium
displaces the fusing Hydrogen)
2. Heat energy no longer balances gravity.
3. Gravity collapses the He core.
4. The heat generated by the implosion of
the core spurs more fusion of the
remaining Hydrogen.
5. The outer envelope of the star expands,
and cools. It is now a Red Giant
Collapse of the He Core:
Expands
Cools Down
Turning into a Red Giant :
1. A star the size of the sun would expand
to the orbit of Venus, or maybe the
earth.
2. As a red giant, the star blows off a great
deal of its mass into space.
3. A star 8 time as massive as the sun will
have a residual mass of 1 or 2 times the
mass of the sun after its red giant stage.
4. Stunning image from the Hubble:
Helium Fusion:
1. When the core gets hot and dense
enough, He begins to fuse:
4He + 4He = 8Be + γ
4He + 8Be = 12C + γ
2. The star contracts slightly and heats up,
moving along the horizontal branch
3. Before the He is used up these reactions
also occur:
4He + 12C = 16O + γ (mainly)
4He + 16O = 20Ne + γ
4He + 20Ne = 24Mg + γ
Helium Fusion:
Heats up and contracts
Carbon Fusion:
1. When most of the Helium in the core
has been used up, leaving a Carbon
core, the star cools down.
2. Heat energy no longer balances gravity.
3. Gravity collapses the Carbon core.
4. The heat generated by the implosion of
the core spurs more fusion of the
remaining Helium.
5. The outer layer of the star expands, and
cools briefly.
Collapse of the Carbon Core:
Cools and Expands again
Carbon Fusion:
1. If the remaining part of the star is more than
.7 times the mass of the sun, the core gets
hot and dense enough to start Carbon fusion:
12C + 12C = 24Mg + γ
16O + 16O = 28Si + 4He
2. Nuclei as heavy as 56Fe and 56Ni can be
created if the star core is hot enough.
3. Nucleosynthesis and fusion stop with 56Fe
and 56Ni as larger nuclei would require the
input of energy, because of binding energy
Most tightly bound nuclei
(If you go from less to
more bound you release
energy)
56Fe
and 56Ni
From Douglas Giancoli’s “Physics”
So far: Collapse of C core
Carbon Fusion
(if > .7 Msun)
Degenerate
Matter
Helium Fusion
Collapse of
He Core
Hydrogen Fusion stops
How do we know all this?
By observing Globular clusters…
How do we know all this?
By observing Globular clusters…
1. Globular clusters are thousands of stars that
all formed at more or less the same time.
2. Globular clusters are much smaller than
galaxies.
3. Galaxies create stars in an on-going process.
4. The stars in a globular cluster accrete
suddenly and nearly simultaneously.
Planetary Nebulas:
1. Some stars with mass 1-7 times the sun’s mass.
2. While the star is fusing carbon, it shrinks and
gets hotter.
3. The material blown off by the red giant phase
is overtaken by the material blown off by the
carbon core collapse.
4. The rapidly spinning core creates a strong
magnetic field that channels the expulsion of
the outer envelope.
5. Some planetary cores might have a companion.
If the residual mass of the star is less
than 1.4 times the current mass of the
sun, our story ends here.
A star with the mass of the sun
becomes a White dwarf about the size
of the earth.
The Pauli exclusion principle prevents
the star from collapsing any further.
It gradually runs out of Carbon fuel,
getting dimmer and dimmer, until it
becomes a black dwarf.
If the residual mass of the star is less
than 1.4 times the current mass of the
sun, our story ends here.
A star with the mass of the sun
becomes a White dwarf about the size
of the earth.
The Pauli exclusion principle prevents
the star from collapsing any further.
It gradually runs out of Carbon fuel,
getting dimmer and dimmer, until it
becomes a black dwarf.
Now for something completely
different….
Wanna hear a scary story?
Do not adjust your television set
We are on a special schedule…
Life After the Main Sequence
Starring:
Marcela Supernova
Joe Neutron Star
Bob Quasar
Mary Pulsar
Freda Black Hole
Music by “Warped Space Time”
If the mass of the star is greater than 1.4
times the mass of the sun. (This is called
the Chandrasekhar limit) it don’t care
about no Pauli exclusion principle.
When the Carbon Fusion fires burn down,
gravity crushes the star.
The collapse of the star releases an
incredible amount of energy. The star
becomes a supernova, increasing in
brightness by billions of times for a few
days, and then dies out.
The terrific energy released by the collapse of
the star creates elements heavier than Iron, and
forces electrons and protons to combine
creating neutrons.
Dogs become cats.
Republicans support campaign finance reform.
Democrats vote for tax cuts.
In February of 1987, a supernova occurred in
the Large Magellenic Cloud, 170,000 ly from
Earth. It was briefly visible to the naked eye.
(Assuming your eye was naked in Australia)
Neutron Stars:
1. The remnant of the supernova is composed
almost entirely of neutrons.
2. White Dwarfs are the size of planets.
3. Neutron stars are the size of towns.
4. Some Neutron stars spin a thousand times a
second.
5. The pressure is so high in the core atomic
nuclei cannot exist.
6. The outer envelope is about a mile thick - a
crust of nuclei and electrons.
7. The core is a super-fluid.
Picture of a Neutron Star:
Ticks are 5 seconds
1. In 1967, Antony Hewish of Cambridge University
in England was studying the scintillation of radio
sources due to the solar wind.
2. A graduate student named Jocelyn Bell Burnell
discovered a strong night time source of
“twinkling”.
3. Its location was fixed with respect to the stars.
From Jay Pasachoff’s “Contemporary Astronomy”
Pulsars:
1. Pulsars emit pulses some as short as 1/40th of a
second.
2. There are many things they could not be.
3. The only thing small enough, and rotating fast
enough was a neutron star
From Jay Pasachoff’s “Contemporary Astronomy”
Pulsars Movies
Real photos from hubble
Animation
Black Holes:
1. If the mass of the neutron star is bigger than
about 2 or 3 solar masses, it don’t care about no
neutron exclusion principle.
2. Gravity collapses the neutron star even further.
3. The star becomes a black hole - an object from
which even light cannot escape.
4. Light is really fast.
5. The curvature of space-time becomes infinite.
6. General relativity doesn’t work.
7. Um… we don’t yet have a quantum theory of
gravity.
Black Holes:
1. Black holes actually do radiate energy from the
event horizon due to the Heisenberg uncertainty
principle.
2. When stars orbit a black hole, we can see their
orbit, but not the black hole. We can infer the
mass from the mass of the star and its orbit.
3. The Andromeda galaxy has stars orbiting a dark
object that is 30 to 70 million times the mass of
the sun.
Picture of a Black Hole:
Quasars: (Quasi-stellar radio source)
1.
2.
3.
4.
5.
Massively bright.
Intense radio source.
Red shifted radiation.
Black holes eating matter.
Usually located in the centers of galaxies
Quasars:
1. In falling material forms an accretion disk.
2. Quasars are ravenous beasts.
3. The black hole’s magnetic field pumps energy into
the accretion disk.
4. The accretion disk gets hot.
5. The accretion disk has tornadoes that create jets
6. Predictions
1. Old bright Quasars are rare, young ones
common
2. Recently disturbed galaxies should have bright
quasars.