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problem_3

# problem_3 - ASTRONOMY 294Z The History of the Universe...

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ASTRONOMY 294Z The History of the Universe Professor Barbara Ryden Problem Set # 3 Due Tuesday, January 29 at class time NAME (please print clearly): SCORE (instructor use only):

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1) [20 points] You are given a ticket for running a red traffic light. For an observer halted at the red light, the light emits a wavelength λ 0 = 700 nm. You tell the traffic cop that because you were approaching the light, the Doppler shift made it appear green ( λ = 500 nm). How fast would you have been going if this were true? If the speed limit is 35 miles per hour, and the speeding fine is \$1 for every mile per hour over the limit, how large a fine would you have to pay? [Hint: 1 kilometer = 0.621 miles, 1 hour = 3600 seconds.]
2) [20 points] Assume that a typical galaxy contains 100 billion stars, and that there is one galaxy per cubic megaparsec, on average. How many galaxies are within a Hubble distance, c/H 0 = 4300 megaparsecs, of us? How many stars are within a Hubble distance of us?

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3) [20 points] The Sahara has an area of A = 9 , 000 , 000 square kilometers.
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