AST 104 : Midterm 4
Tuesday April 24, 2001

Answer all question.

This exam has 2 questions, each consisting of several parts. Each question is worth a total of 20 points.

The first part of each question is worth 10 points, and requires you to write a one or two paragraph prose answer describing a particular concept or process from the course. It is very important that you approach this carefully. You should write an answer that logically connects the necessary concepts, and which would provide a good explanation that you would understand if it were presented to you. Include only the information that you think is necessary and that you can logically connect to the other concepts in your answer. Any information that you write down that is not connected to the chain of your argument may lead to points being deducted.

The second part of each questions involve one short calculation, worth 5 points. Once again, it is extremely important that you explain your reasoning logically. If you just write down a numerical answer you will not receive many points, regardless of whether it is correct. If you carefully explain how you got a particular numerical answer then even if the answer is incorrect, you may receive quite a lot of partial credit.

Finally, the third part of each question requires you to write a very brief conclusion based on your calculation, and is also worth 5 points. Again, be careful, clear, precise and logical. This will save you time, and will help you to get the most points you can.

Things You Might Need

(Hubble’s Law)

(Einstein’s mass-energy relation)

(Luminosity-Brightness-distance relation)

Speed of light: c=3 x 10⁸ m/s

s=5.6705 x 10^-8 Watts/m²/degree⁴

Question 1 (20 points)

a) (10 points) Give a brief, one or two paragraph description of the basic idea of what Harlow Shapley did, and how he did it. What was the main difference between what Shapley wanted to find and what Herschel found?

b) (5 points) Here is a graph of the brightness of a Cepheid star as time passes. Also here is a period-luminosity plot for a collection of variable stars.


Brightness








0 20 40 60 80
Time (days)

Estimate the Luminosity of the Cepheid.

c) (5 points) Explain in words how you could use the luminosity to find the distance to the Cepheid.

Question 2 (20 points)

a) (10 points) Give a brief, one or two paragraph description of how we know the universe is expanding, the first piece of evidence for the big bang theory.

b) (5 points) A distant galaxy is observed to be moving away from us with a velocity 10000 m/s due to the expansion of the universe. A second galaxy is observed to recede from us at 80000 m/s in the opposite direction to the first galaxy. How far away is this second galaxy from us compared to the first one?

c) (5 points) If you lived in the second galaxy, describe how, if at all, the milky way and the first galaxy would appear to be moving with respect to you.