Question 1

Describe a scale model of the disk of the Milky Way using a scale in which 1 cm = 1000 light-years.Note that the diameter of the disk is approximately 100,000 light-years,the thickness of young stars is 500 light-years,and the thickness of the older stars is 4,000 light-years.

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Question 2

The energy of a cosmic ray is given by γmc²,where m is the mass of the particle,c is the speed of light,and γ is the Lorentz factor and .If a cosmic ray proton is traveling at a speed of ν = 0.999999c,what is its energy? How does that compare to the kinetic energy of a grape thrown up in the air and caught in your mouth? Assume the grape has a mass of about 5 grams and a speed of 0.5 m/s when you catch it.The mass of a proton is 1.7 × 10⁻²⁷ kg.

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Question 3

If you wanted to study the properties of nearby stars that were some of the first stars to form in the Milky Way,but were NOT members of a globular cluster,how could you go about finding them?

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Question 4

Suppose it was discovered that the galaxy has constantly been absorbing large amounts of gas that never had any stars form in it.Would this complicate our interpretation of ages and chemical enrichment?

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Question 5

How do we know that globular cluster stars were not the first stars that formed in our galaxy? Would this still be true if some or all globular clusters came into the galaxy by accretion of small satellite galaxies?

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Question 6

The Sun is rotating about the center of the galaxy at roughly 220 km/s at a distance of about 8 kpc.How long will it take to complete one revolution? How many times has the Sun orbited the galaxy in its lifetime?

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Question 7

The Sun has 2 percent of its mass in heavy elements (those elements that are not hydrogen and helium).What does this tell us about chemical enrichment in the galaxy?

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Question 8

You find a star orbiting about the center of the galaxy at a speed of 220 km/s at a distance of 15 kpc.How much mass is within the orbit of that star? You can assume the star is on a circular orbit around the center of the Milky Way.

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Question 9

What objects are used to determine that the galaxy is mostly dark matter?

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Question 10

Calculate the size of the event horizon of the supermassive black hole at the center of the Milky Way,the mass of which is 4 × 10⁶ M_⨀.Give your answer in units of AU.

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Question 11

Why is the supermassive black hole in the center of our galaxy not an AGN right now? Could it be active in the future? Why or why not?

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Question 12

Why can the observed chemical abundance of a star tell you something about its age?

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Question 13

Where would we find the stars with the richest abundance of massive elements? Where would we find those with the smallest amounts of massive elements? Why?

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Quiz 21: The Milky Waya Normal Spiral Galaxy