AST 324
Spring 1998

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

  1. 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?

  2. What is the meaning of the Jeans Mass?

  3. What does conservation of angular momentum imply about the collapse of a rotating object?

Topic: Radio and Infrared Astronomy

  1. Know the different wavelength regions in the electromagnetic spectrum. Know which wavelengths get through the atmosphere.

  2. 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?

  3. Describe the difficulties of doing infrared astronomy and the methods used to get around those difficulties.

Topic: Atoms and Molecules

  1. Define an element, a compound, an atom, and a molecule. What are the relationships between these concepts?

  2. Explain ionization and recombination. What is an isotope?

  3. 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.

  4. 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.

  5. 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?

  6. 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

  1. Describe how an emission line would be detected by radio telescopes.

  2. 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?

  3. 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?

  4. Describe the composition of the two kinds of interstellar dust grains. What are PAH's? What kinds of ices are known to exist in grain mantles?

Topic: Molecules as Probes; Molecular Clouds

  1. 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?

  2. Describe the various different cloud motions and how they affect the line that is observed.

  3. Describe the composition of molecular clouds (what are the abundances of different molecules?). How are molecular clouds different from atomic clouds?

  4. 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?

  5. 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

  1. 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?

  2. What has been learned about star formation from studies of dense gas and infrared sources in the Orion cloud?

  3. Describe two modes of star formation, explaining how they differ.

  4. Explain the "inside-out" collapse theory for low mass star formation. Describe recent evidence that the object B335 is undergoing such a collapse.

  5. What stops the collapse of the gas which is forming a star? What is a protostar? What distinguishes it from a main sequence star?

  6. Why is infrared light useful in studying star formation?

  7. Describe the theory of what should happen in the collapse of a rotating object. How does rotation modify the theory of inside-out collapse?

  8. Describe the observations that show that young stars often eject matter in a bipolar outflow.

  9. 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?

  10. Describe the evidence that disks exist around many young stars.

  11. Explain what a silhouette disk is. Can stars in clusters have disks?

  12. 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.

  13. 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

  1. Describe the dynamical regularities in our Solar System.

  2. Describe the spacing and composition differences of planets in our solar system.

  3. Describe the solar nebula model for the origin of the Solar System. How is this model related to the disks around young stars?

  4. 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.

  5. Explain how the differences between the inner planets and the outer planets in size and composition arose in their formation process.

  6. Describe the formation of the Earth and the origin of its atmosphere.

  7. 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

  1. 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.

  2. 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?

  3. How can we learn the mass and orbital radius of a planet that we cannot even see?

  4. 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?
Evans's 324 | Astronomy Department