HOMEWORK 6
Due Tuesday, April 24.
- Attempt all questions in Part A , 1 of the questions in Part B, and 1 from Part C. Please write all answers to Part A on these sheets.
- With few exceptions, Part B questions call for a paragraph of coherent English. Points will be deducted if sentences are incomplete and reduced to a list of items or a phrase or two. Do not, however, pad your answer by rewriting the question, by rewriting one sentence in several different ways, etc.
- Answers to Part C will be judged for scientific content and accuracy as well as style, grammar, etc.
Review Session: Monday April 23, 5-6pm in CPE 2.218.
Part A
| A1. | The
Sun is at a distance of 8-10 kiloparsecs from the center of the
Galaxy. True or False? |
| A2. | An important relation observed for Cepheid variables is that between |
| a.
mass and luminosity. b. luminosity and temperature. c. period and luminosity. d. period and radius. e. mass and temperature. |
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| A3. | Cepheid variable stars are important to astronomy primarily because they allow us to determine stellar |
| a.
distance. b. mass. c. temperature. d. pulsation period. e. chemical composition. |
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| A4. | Which of the following four diagrams best represents the period-luminosity law of Cepheid variables: |
 |
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| A5. | Cepheids with a period of 10 days are seen in Galaxy X. Cepheids with a period of 100 days are seen in Galaxy Y. The former are observed to be brighter than the latter. Which galaxy is closer to us? Briefly justify your answer. |
| A6. | A galaxy with open arms and a small nucleus is an Sc galaxy. True or false? |
| A7. | The Milky Way galaxy is a prime example of an E7 galaxy. True or false? |
| A8. | The Sun completes an orbit around the Galactic center in about 250 million years. How long does it take a star at 16 kpc to orbit the center? Assume the Sun is at 8 kpc from the center and the Galactic rotation curve is flat. Show all working. |
| A9. | Why do white dwarfs not make good distance indicators? |
| A10. | Which kinds of objects do NOT belong to both Population I and Population II? |
| a.
Type II supernovae b. White dwarfs c. Lower main sequence stars: M dwarfs d. Associations of OB stars |
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| A11. | The
diagram shows our galaxy and the orbit of star X. Based on this orbit, would you classify X as a Population I or II member of the Galaxy? |
| A12. |
Do you expect X to be:
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| A13. |
Match the stellar population (I and II?) with the following: ______young ______Type II supernovae |
| A14. |
Arrange the following in order of increasing size: centimeter astronomical unit |
Part B
| B1. | a. | How is the 21 cm line of hyrdrogen produced by interstellar clouds of cold hydrogen gas? |
| b. | Suppose you had
access to a radio telescope with high resolving power. Describe how you would determine the direction and speed of rotation of a nearby and well-resolved spiral galaxy. |
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| c. | Spiral galaxies contain large amounts of dark matter. Suppose you observed a spiral galaxy that had no dark matter and another with lots of dark matter in a huge galctic corona. How would you distinguish them through observations of their speed of rotation? | |
| B2. | a. | Describe one
piece of observational evidence that shows that there are small
dust grains in the plane of the Galaxy. |
| b. | If a star appears
red, how could you decide if it is a cool star, or a hot star
reddened by interstellar dust, or a hot star reddened by the Doppler shift of an incredibly rapid motion away from us? |
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| c. | How is exploration
of the Galaxy hindered by dust? Give one specific example of
a discovery or measurement that was greatly hampered by the presence of the dust in the Milky Way. |
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| B3. | a. |
Describe the difference between Population I and Population II stars in terms of the following properties (give numbers!):
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| b. | Discuss how 2 of the above differences are to be understood in terms of a model of the formation of the Galaxy. | |
| B4. | a. | Cepheid variables were discovered in the Magellanic Clouds at a time when the distance to the Clouds was unknown. Explain why Henrietta Leavitt's discovery about the brightness of Cepheid variables in the Clouds indicated that these stars obeyed a period-luminosity law. Comment, in particular, on why it was possible to deduce the law even though the distance to the Clouds was unknown. |
| b. | Explain clearly why the Hubble Space Telescope is able to use Cepheid variables to measure distance to galaxies much more distant than those measurable from Cepheids using much larger telescopes on the ground. | |
| B5. | a. | Explain how Hubble's Law, V=H0r, is established from observations of galaxies. You are expected to discuss how the radial velocities V and the distances r are measured. |
| b. | All galaxies are receding from us. Does Hubble's Law then imply that our Galaxy is in a special position from which all other galaxies are fleeing? Discuss your choice of "yes" or "no". | |
| c. | If a distant galaxy has a redshift corresponding to an expansion velocity of 7500 km/sec, what is its distance? Assume the Hubble constant is 75 km/sec/Mpc. Show all working. | |
Part C
| C1. | Surely, you began
this class with preconceived notions and even misconceptions
about astronomy. Describe in a letter to a friend one or two misconceptions that our class has highlighted for you. Clearly state what these were and follow each statement with a discussion of your present (correct, I hope) understanding. Two to three pages please! |
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| C2. |
In your view is astronomy a "useless study"? Write a reasoned essay of 2 to 3 pages on why society should or should not support study of astronomy. Draw on our syllabus as far as possible for examples of astronomy's contributions of intellectual ideas and practical benefits. |
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| C3. |
Le silence éternal de ces espaces
infinis m'effraie. Anthony Burgess, in his autobiography, Little Wilson and Big God, describes his reaction to star charts: There was[in the stockrom] an illustrated set of Dickens and there were Ball's Story of the Sun and Story of the Heavens. In both of these were folded celestial maps which opened out to show desperate blue, Pascalian emptiness. Seeing the heavens, for the first and last time in my life I fainted. I came to quickly enough but have never overcome the horror of maps unhumanised by roads and churches. I was a map-reading instructor in the army and was unfearful of tracts that could be covered in a day's march, but a world map terrifies me and visions of interstellar space bring on convulsions. This is more than agoraphobia: call it diastematophobia. I am poised till I die between fear of the cardboard darkness of the stockroom and the terror of space. I wanted the free limitations of my own skull and a world I could build with a pencil. I have not changed much since 1925. Draw on the contemporary view that stars are the sole factories for the synthesis of elements essential to life to write an essay (2 pages or so) of comfort for Pascal and Burgess. |
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