Exam #3 Study Guide:
Exam 3 will have 40 multiple choice questions based on
material covered in lectures 18 to25. Questions from HW 8 to 11 may be on the
exam. A few of the questions will involve simple calculations, so you should
bring your calculator. You will not need your clicker. You may bring one
double-sided 8.5x11 inch sheet of paper with notes to the exam.
I Stars
- Our
Sun is a hot (5700 K) ball of gas powered by fusion reactions in its core
that convert hydrogen to helium
- Know
what is meant by Luminosity. Know that the Sun’s luminosity is 3.26E26W.
- Know
that hotter stars appear more blue; cooler stars are more red. Know that
stars evolve.
- Know
the following words: Photosphere, red giant, red supergiant, white dwarf,
main sequence, white dwarf, Hertzsprung-Russell Diagram.
- Be
able to locate various objects on a HR diagram
II The Standard Model and String Theory
- Know
that the “Standard Model” is a collection of the currently known particles
and the forces between them. It does not answer many of the “Why”
questions about nature.
- The
LHC at CERN is searching for the Higgs particle to explain where mass
comes from and for candidates for dark matter.
- Know
that String Theory tries to describe everything in terms of vibrating
strings. The size of the strings is 10-35m.
- String
theory requires at least 10 or 11 dimensions to work. We experience only 4
dimensions. The others are too small to be detected.
- String
Theory as a whole has not yet made falsifiable predictions that would
allow it to be experimentally tested. So far all the explanations it
provides could be explained in other ways.
III The Big Bang
- All
evidence is consistent with the Universe beginning in a very hot (1035K)
fireball 13.7 billion years ago. We call this event the Big Bang.
- Know
the three main pieces of evidence for the Big Bang
- The
cosmic microwave background radiation
- The
hydrogen, helium and lithium produced (this is called Big Bang
Nucleosynthesis)
- The
expansion of the Universe. Beyond our nearest neighbors all galaxies
appear to be moving away from us.
- The
structure of the Universe
- There
are approximately 200 billion other galaxies
- We
are part of a local group of about 20 galaxies, which includes the
Andromeda galaxy
- Galaxies
are clumped into groups called clusters.
- The
large scale structure of the Universe is clusters and voids (empty space)
- The
first person to discover the size of the Universe was Edwin Hubble. He did
this by studying variable stars. Variable stars work because it is
possible to know their luminosity accurately. A measurement of brightness
and a know luminosity can be used to determine a distance.
- Hubble
also discovered that on average galaxies are moving away from us and the
farther away the galaxy, the faster it is moving. This is called the
Hubble law.
- Know
the timeline for the Big Bang: Fireball with 1035K when all
forces are unified; gravity becomes distinct, around the time the strong
force separates there is a period of inflation where the size increases 1050
times; other forces become distinct, quarks form nuclei, at around 1
minute the hydrogen and helium form; 380,000 atoms form and the Universe
is transparent; 200 million years is when the first stars form; most of
the atoms of our bodies are made in stars between 200 million and today.
- Know
that at about 10100 years all the stars will have burned out,
and at about 101000 years the universe will be at maximum
entropy and nothing else will happen. This is of course based on what we
know now. It is very likely there is more to the story. (The last two
sentences will not be on the test, but had to be said anyway.)
IV Atomic Nuclei
- The
number of protons in a nucleus determines what element it is, the number
of neutrons what isotope.
- Be
able to determine the number of neutrons in a given isotope. For example
14-C (6 protons) has 8 neutrons.
- Some
isotopes are naturally radioactive and change into other forms.
- Radioisotope
dating is used to determine the age of objects. Radiocarbon dating is one
example.
- Given
a half-life, starting fraction, and final fraction, be able to determine
the age of a sample. Example: The initial sample is all 14-C (initial
fraction 1), the half-life is 6000 years, if an object is found that is
1/8th 14-C, how old is it? Three half-lives (0.5*0.5*0.5=1.8th)
implies it is 3*6000 years old.
V Black holes and Worm holes
- What
is escape velocity? What happens to escape velocity if the mass of the
planet is doubled? What happens if the radius is doubled?
- Black
holes are a large mass in a small region, so that the escape velocity
becomes greater than the speed of light.
- Once
inside the event horizon (aka the Schwarzschild radius) not even light can
escape.
- The
size of the event horizon depends on the mass of the black hole. Know this
equation. What is the radius of a supermassive black hole with a mass a
billion times our Sun?
- Black
holes
- Black
holes are formed in the collapse of massive stars at the end of their
life
- Black
holes also form at the centers of galaxies. Some galaxies have
supermassive black holes with 109 solar masses.
- Black
holes at the centers of galaxies explain active galaxies and quasars.
- Worm
holes are a possible solution of Einstein’s equations, but to be stable
something would have to keep them open. Know that no corresponding white
holes have been observed.
- The
entropy of the early Universe was far too low, hence the probability of
any process increasing entropy is vanishingly small. This gives time its
direction since to go back in time would allow entropy to be decreased.
VI Dark Matter and Dark Energy
- We are
made of atoms, atoms are made of nuclei and electrons, nuclei are made of
… Know all of these steps down to strings at 10-35m.
- This
type of luminous matter only makes up 4% of the universe.
- Know
the evidence for dark matter
- Gravitational
lensing
- Rotation
curves of galaxies
- Cosmic
microwave background radiation
- WMAP
indicates that 23% of the universe must be in the form of some type of
cold dark matter. These may be particles that interact by the weak force.
They are called WIMPS (weakly interactive massive particles).
- Know
that 73% of the Universe is made of Dark Energy. We don’t know what this
is, but it acts like an antigravity that is pushing the Universe apart.
VII Life in the Universe
- The
one ingredient for life on Earth is liquid water
- Other
places in the solar system where liquid water might be found are moons of
Jupiter and Saturn, and maybe on Mars
- Know
that the Drake equation can be used to make an estimate of the number of
possible technological civilizations we might be able to communicate with