Transcript Slide 1

A Solar System Overview
A dynamic solar system;
Stellar astronomy;
Subatomic particles and nuclear fusion;
White dwarfs as end to low mass stars;
High mass stellar evolution;
Cepheid variables;
Supernovae;
Neutron stars and black holes
Motivation
Not just Astro 2, 5, 10 content...
Note the origin of Type Ia supernovae
Note the origin of dense supernova remnants.
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Classifying the planets
The planets fit into two groups:
1. Outer Jovian planets;
2. Inner terrestrial planets.
Size, mass, and density
1. The Jovian planets are larger and more massive;
2. Terrestrial planets are more dense.
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Solar system belts & clouds
Asteroid belt
Most orbit in or near the plane of the ecliptic;
Most between Mars and Jupiter (2.2 to 3.3 AU from the Sun).
Kuiper belt
Comets which lie just outside the orbit of Neptune;
The largest is called Eris;
Pluto is one of these objects;
Projections suggest there is much more mass in the
Kuiper belt than is in the asteroid belt.
Öort cloud
Aphelia of billions of comets;
About 10,000–100,000 AU from the Sun;
Icy chunks ejected by from inner solar system by Jovian planets?
Cause of periodic, near-sterilizing impacts on the Earth?
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Recent results
Depends upon the semester
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Extrasolar planetary systems
Until recently, our technology could only find planets Jupiter-sized or
larger, which is exactly what we got!
Many of these super planets are closer to their star than Mercury is to the
Sun. How does one form such a “hot Jupiter”?
As of February 2012, Kepler has revealed 2321 planets
– 281 are Jupiter-sized or larger (6 REarth < R < 22 REarth);
– 1118 are Neptune-sized (2 REarth < R < 6 REarth);
– 676 are super-Earth (1.25 REarth < R < 2 REarth);
– 246 are approximately Earth-sized (R< 1.25 REarth).
– 88% are Neptune sized or smaller;
– Overall, the size peaks at 2-3 REarth.
– 54 are within the habitable zone of its parent solar system;
– 5 habitable zone planets are less than 2 REarth.
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The fiction of a clockwork solar system
Problems
Mars—being in the thick of the protostellar disk—should be 10× larger than it is.
Uranus & Neptune—at the edges of the prostellar disk—should be much less
massive than they are.
Why are inner asteroids rocky (S type), and outer asteroids carbon rich (C type)?
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The proposed, dynamic solar system
Solutions
The locations of the planetary orbits have shifted hugely over time.
Jupiter formed 3.5 a.u. from the Sun. Saturn, Uranus, and Neptune were very close.
Jupiter jostled Saturn into an unstable orbit; Saturn’s close encounters threw Uranus
and Neptune into their larger orbits.
Jupiter then crept to 1.5 a.u., plowing through the asteroids and throwing them to the
outer solar system.
In the process, Jupiter gravitationally shepherded a mini-disk near the Sun, which
ultimately created the terrestrial planets.
A 3:2 orbit-orbit resonance with Saturn saved Jupiter from spiraling into the Sun.
Instead, Jupiter was pulled away from the Sun, back to the outer solar system,
returning the (S type) asteroids back to the inner solar system.
Mars migrated outwards, also shepherding the S type asteroids.
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Further motion outwards by Jupiter and Saturn threw C type asteroids into the belt.
Stellar Astronomy
Composition
– Stars are formed from interstellar material (gas and dust).
– Gas in our part of our galaxy is 70% H, 28% He, 2% heavier
elements.
– As a result, stellar compositions reflect this.
Vital stats—NOT to be memorized in detail!
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Masses range from about 0.1M-100M
Radii range around 0.01R -1000R
Temperatures range around 3000K-30000K
Luminosities range around 0.001L - 106L
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Spectral Classes
In the 1890s, the group of female Harvard
astronomers lead by Edward Pickering developed
the stellar classification system.
Annie Jump Cannon recognized that this system
reflects, for the most part, stellar temperatures.
OBAFGKM
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The Hertzsprung-Russell diagram
The Hertzsprung-Russell diagram (ca. 1910) is a plot of
luminosity versus temperature (or spectral class) for
stars.
The strength of the HR diagram lies in the fact that it
shows structure, and is not just a “scatter diagram.”
About 90% of all stars fall into a group running
diagonally across the diagram called the main sequence.
Other categories in the HR diagram include white
dwarfs, red giants, and supergiants.
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Inside the Stellar Engines
Basic nuclear particles
– Protons—massive, positively charged;
– Neutrons—massive, no charge;
– Electrons—low mass (1/2000 proton), negatively
charge;
– Neutrinos—almost massless (~10-9 electron), no charge.
Nuclear fusion
– Reactions convert loosely atoms into more tightly bound
atoms;
– The change in binding corresponds to a mass change
from E=mc2.
– For main sequence stars, 4×(1H) atoms are converted to
a single 4He atom.
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Achieving Fusion
Overcoming nucleus-nucleus repulsion
– Protons repel each other because they are both
positively charged;
– Attracted to each other at about 1 fermi (10-15 m);
– Speed needed to overcome the “Coulomb barrier” as a
temperature is about 120 million K;
– Stars are only about 20 million K in their interior;
– How does it happen? Quantum tunneling to the rescue!
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Types of Nuclear Fusion
Proton-Proton Chain
Hydrogen to Helium, used in stars less than ~1.5 M;
CNO Cycle
Hydrogen to Helium, used in stars more than ~1.5 M;
Neutrino Production
These reactions predict the production of vast numbers of
neutrinos.
50 trillion neutrinos pass through your body/sec!
100 LY of lead shielding are needed to block neutrinos
~30%.
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Detecting Neutrinos
Homestake Experiment (1960s)
Raymond Davis, 400,000 liters of perchloroethylene.
Chlorine + neutrino → Argon;
Could not detect enough SNUs!
Super Kamiokande Experiment (now)
Cylinder, 41.4m tall, 39 m diameter;
Filled with 50 million liters of ultra pure water;
Neutrino + H2O produces high velocity electrons;
Cherenkov radiation produced!
Requires a neutrino that has a mass.
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Cepheid variables
Giant stars that are running out of hydrogen in their
interiors.
As they expand, they leave the main sequence. Stars with
a certain range of luminosities and temperatures become
variable. They are called Cepheid variables.
Cepheid variable stars have a well established periodluminosity relation. This provides a powerful means for
determining cosmic distances.
Note: there are many other types of variable stars, so be
careful when studying Cepheids.
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Energy Crisis
When stars run out of hydrogen in their interiors…
…their outer layers lift off to form a spectacular planetary
nebula.
…the furiously hot, compact core remains, and is called a
white dwarf.
…the white dwarf is destined to cool over time, to become
a black dwarf. But that will take a looooooong time.
…A typical white dwarf has 1.0 M, and a 12,000 km
diameter (90% of Earth’s). A teaspoon of white dwarf
material would weigh 2 tons.
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White Dwarf Conditions
In normal conditions, “thermal” pressure is determined by
a combination of density and temperature:
P=ρkT
In white dwarfs, pressure is instead maintained by
“electron degeneracy pressure”.
In degeneracy pressure, the electron cloud of each atom
cannot be squeezed any closer to the electron clouds of its
neighbor atoms.
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The Chandrasekhar Limit
More than half of all star systems are binaries or
multiple stars. In many cases, the stars are so
close that mass transfer can occur.
If mass from a red giant flows onto a white
dwarf, explosive brightness changes of 10000×
occur (novae), to 150,000 L.
So much matter can flow onto the white dwarf
star that the Chandrasekhar Limit (1.4 M) can
be exceeded.
Electron degeneracy is defeated!
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Type I Supernovae
The collapse raises the core temperature and new kinds
of fusion reactions occur (helium and carbon fusion).
The white dwarf BLOWS UP!
L=5×109L, which can be brighter than a whole galaxy!
This is called a Type Ia (or white dwarf) supernova.
Type Ia supernovae can be identified by their spectra.
Since they are formed by uniform, very repeatable
conditions. Therefore, all Type Ia supernovae should
reach the same maximum brightness.
This makes them exceptionally reliable “standard
candles.”
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Massive Stars
Stars more massive than about 8 M avoid the white dwarf
star stage, by having such high internal pressures and
temperatures that they can smash together helium, carbon,
and higher mass atoms.
This gives them an extension to their lives. They develop a
complicated onion-skin internal structure.
But eventually, even these fuel sources run out.
But in the meantime, they produce all the elements that we
find in the Universe, up to iron.
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The Iron Crisis
The inevitable formation of iron in the stellar core is a bad
sign.
Nuclear Fusion
Low mass atoms
High mass atoms
Nuclear Fission
High mass atoms
Low mass atoms
Iron produces no energy, either via fusion or fission.
The star’s attempt to fuse iron ends in catastrophe.
The nuclei in the stellar core are driven together and
neutronize, the core collapses, then rebounds, and riding a
wave of neutrinos, the star blows up in a Type II supernovae.
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Neutron stars
The end result of a star that was initially perhaps 6-12 M.
Neutron stars have masses between 1.4 and 3 M.
The star is supported by neutron degeneracy pressure, and is
in a superfluid state—a perfect conductor of energy.
The diameter of a typical neutron star is only about 20 km.
The outer crust of a neutron star is largely electrons and
positively charged atomic nuclei.
The neutron star is essentially a gigantic atomic nucleus,
albeit one held together by gravity and not the nuclear force.
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Neutron stars
A pulsar is a neutron star that pulses optical to radio
waves with a period 0.0015-8.5 seconds, though nearly
all fall between 0.1 and 2.5 seconds.
The lighthouse model explains pulsar behavior as being
due to a spinning neutron star whose radiation beam
sweeps by us.
If the neutron star is more than 3 M, even neutron
degeneracy pressure will not support it against collapse.
What then?
Some theories suggest that quark degeneracy pressure
might support the core, hence quark stars or strange
stars.
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Black Holes
A black hole is the end result of a supernova explosion
of a star with initial mass greater than about 12 M.
The event horizon is the spherical surface around a
black hole from which nothing can escape.
Inside a black hole, an object will be crushed out of
existence at a central singularity.
You don’t want to go into a black hole.
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