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• Question
3 (1 point) Which of the following is not a characteristic of the stars of the disk component of our
galaxy?
a. circular orbits b. random highly inclined orbits c.higher metal abundance d. young stars e. star
formation regions
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Question 4
(1 point) If the sun is 5 billion years old, how many times has it orbited the galaxy? Assume a circular orbit for
the sun around the center of the
galaxy, and look up in the text its orbital speed and distance from the galactic center.
a. 5 times b. 5 billion times c. 200 times d. 20 times
e. 5000 times
•Approach this problem like a time, rate, distance
problem to figure out an orbital
period, then see how many go into 5 billion years. If the orbital speed is 220 km/s, and the oribital radius is 8.5 kpc,
we can get the period
(time): time = speed/distance and we must convert.
8.5 kpc = 8500 pc x 3.1
x 10 13 meters/pc = 2.6x1017 km. So time = 2πx2.6x1017 km/220
km/s = 7.5x1015 s. What is that in years? There are about 31 million seconds per year, so this period is 7.5x1015/3.1x107 = 2.4x108 years, or about 240 million years. 5 billion divided by 240 million is 5x109/2.4x108 = 20.8.
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