STUDY GUIDE FOR SECOND MIDTERM
To study for the midterm, you should go over your homework, study
the answers, and be sure that you understand the principles involved. In
addition, I will give below some study questions for the material that
we have covered.
Topic: Overview of Star and Planet Formation
Consider a star which has 5 times the mass of the Sun (M*= 5
Msun). Assuming that the star is on the main sequence, what is its
luminosity and lifetime on the main sequence?
What is the meaning of the Jeans Mass?
What does conservation of angular momentum imply about the
collapse of a rotating object?
Topic: Radio and Infrared Astronomy
Know the different wavelength regions in the electromagnetic
spectrum. Know which wavelengths get through the atmosphere.
Explain why some radio telescopes can have mirrors full of holes.
Understand how to use the equation for the resolution of a telescope;
given two telescopes of the same size, one working at twice the
wavelength, which will have the better resolution? How much better?
Describe the difficulties of doing infrared astronomy and the
methods used to get around those difficulties.
Topic: Atoms and Molecules
Define an element, a compound, an atom, and a molecule. What
are the relationships between these concepts?
Explain ionization and recombination. What is an isotope?
Consider the H2 molecule: is the net force between the 2 H
atoms attractive or repulsive? What is activation energy? Draw the
potential energy curve for two separate H atoms coming together to make
an H2 molecule.
Describe the solar system model for the atom and explain its
major flaw. How did Niels Bohr "solve" this problem? Describe the Bohr
model in terms of orbits, then in terms of energy levels, and show how
these pictures are related.
Give the rules for energy states and show how they lead to the
equation E2 - E1 = h(nu). How does this equation
allow us to identify an
atom or molecule by observing the light that it emits?
What are the three kinds of energy levels in molecules? Draw
some diagrams to indicate the relationship between these kinds of energy
levels and their typical pattern of separations. Which electronic and
vibrational energy state are most interstellar molecules in?
Topic: Interstellar Molecules
Describe how an emission line would be detected by radio
Name three interstellar molecules and say why they are of
interest. What is the implication of the fact that so many interstellar
molecules are based on carbon?
Explain the presence of complex molecules in interstellar space.
Be sure to deal with both destruction (dissociation) and formation.
Explain the special problem in forming H2. How is this problem solved
by the presence of dust grains?
Describe the composition of the two kinds of interstellar dust
grains. What are PAH's? What kinds of ices are known to exist in
Topic: Molecules as Probes; Molecular Clouds
How do we measure the density and temperature in molecular clouds?
What results are obtained? How are sizes and masses measured and what
are the results?
Describe the various different cloud motions and how they affect
the line that is observed.
Describe the composition of molecular clouds (what are the
abundances of different molecules?). How are molecular clouds different
from atomic clouds?
What are the implications of the fact that most molecular clouds
have masses much greater than a Jeans mass? What are the implications
of the fact that the Jeans masses of most clouds are larger than the
masses of most stars?
Consider three molecular clouds: clouds A and B have the same
density, but cloud A is twice as hot; clouds B and C have the same
temperature, but cloud C is twice as dense. Which clouds have the
lowest and highest Jeans mass? Which would you choose to observe if you
were hunting for regions of star formation?
Topic: Star Formation
Show how to make a simple estimate of the star formation rate
in our whole Galaxy (in Msun yr-1) assuming that clouds more massive than
the Jeans mass are all collapsing at free fall. Compare this rate to
that which is needed. What do you conclude from this?
What has been learned about star formation from studies of
dense gas and infrared sources in the Orion cloud?
Describe two modes of star formation, explaining how they
Explain the "inside-out" collapse theory for low mass star
formation. Describe recent evidence that the object B335 is undergoing
such a collapse.
What stops the collapse of the gas which is forming a star?
What is a protostar? What distinguishes it from a main sequence star?
Why is infrared light useful in studying star formation?
Describe the theory of what should happen in the collapse of a
rotating object. How does rotation modify the theory of inside-out
Describe the observations that show that young stars often
eject matter in a bipolar outflow.
Draw a picture which shows how the accretion and outflow can
be combined. What fundamental problem of star formation might the disk
and outflow solve?
Describe the evidence that disks exist around many young
Explain what a silhouette disk is. Can stars in clusters
Describe the changes in the plot of light versus wavelength
as a collapsing region in a molecular cloud evolves through the
protostar phase to become a main sequence star. Explain these changes.
What fraction of stars are binaries? Can binaries have disks?
What effect do binaries have on disks? What does the relative rarity of
brown dwarfs tell us about planet formation?
Topic: Formation of Our Solar Sytem, Earth, and its Atmosphere
Describe the dynamical regularities in our Solar System.
Describe the spacing and composition differences of planets in
our solar system.
Describe the solar nebula model for the origin of the Solar
System. How is this model related to the disks around young stars?
Describe the role of dust grains in planet formation. Describe
the possible connections between the dust grains in molecular clouds and
those involved in planet formation.
Explain how the differences between the inner planets and the
outer planets in size and composition arose in their formation process.
Describe the formation of the Earth and the origin of its
Make up a plausible story for how an oxygen atom in an
interstellar cloud eventually winds up as part of a water molecule in
the Earth's ocean. Describe both the chemical transformations of the
oxygen atom and the changes in physical location leading from
interstellar cloud to ocean.
Topic: Planets around other Stars
Describe why it is so difficult to directly detect planets around other
stars. Explain why this would be somewhat easier (though still beyond
current capabilities) using infrared light.
Explain how the two main methods for indirect detection of
planets work. Which would be favored for detecting a large planet
circling, in a large orbit, a small star that is close to the Sun?
How can we learn the mass and orbital radius of a planet that we
cannot even see?
Summarize the results so far of searches for planets around other
stars. What are the implications of the considerable differences from
our own solar system?