Stars Part Stars Part Two: Two: Stellar Evolution Stellar Evolution
Dec 27, 2015
Overview of the life of a star:1. Formation of protostar from
a cloud of mostly Hydrogen gas.
2. Main sequence star3. 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 star1. As the cloud of gas and dust collapses,
a small rotation becomes big (Ice skater pulls in their arms...)
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 star1. Eventually, the spin slows enough to
allow fusion2. 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.
The Inner Planets::
Mercury Venus Earth Mars Asteroids
•Close together (Relatively)•Terrestrial (made of rock like Earth)
Life on the Main Sequence:1. Energy comes primarily from
the Proton-Proton cycle:(Hydrogen fusion)1H + 1H = 2H + e+ + ν1H + 2H = 3He + γ3He + 3He = 4He + 1H + 1H(requires heat and pressure)Hydrogen becomes Helium
Gravity - CrushingPressure
Heat - ThermalAgitation& Radiation Pressure
Thermal agitation and radiation pressure balances the tendency of gravity to crush a star:
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
From Robert Garfinkle’s “Star Hopping”
.01 Billion Years
.1 Billion Years
1 Billion Years
10 Billion Years
100 Billion Years
500 Billion Years
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
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 + γ4He + 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 + γ4He + 20Ne = 24Mg + γ
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.
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 + γ16O + 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
From Douglas Giancoli’s “Physics”
Most tightly bound nuclei(If you go from less to more bound you release energy)
56Fe and 56Ni
So far:
Hydrogen Fusion stops
Helium FusionCollapse of C core
Carbon Fusion(if > .7 Msun)
Collapse of He Core
How do we know all this?
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.
By observing Globular clusters…
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.
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 SequenceStarring:
Marcela SupernovaJoe Neutron Star
Bob QuasarMary Pulsar
Freda Black HoleMusic 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, and Democrats cut taxes
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)
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.
Neutron Stars:
From Jay Pasachoff’s “Contemporary Astronomy”
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.
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
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.
Quasars: (Quasi-stellar radio source)1. Massively bright.2. Intense radio source.3. Red shifted radiation.4. Black holes eating matter.5. 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 jets6. Predictions
1. Old bright Quasars are rare, young ones common
2. Recently disturbed galaxies should have bright quasars.