Astronomy

Andrew Fraknoi, David Morrison, Sidney C. Wolff

Chapter 25 The Milky Way Galaxy - all with Video Answers

Educators

Chapter Questions

01:35

Problem 1

Explain why we see the Milky Way as a faint band of light stretching across the sky.

Matthew Miranda

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01:52

Problem 2

Explain where in a spiral galaxy you would expect to find globular clusters, molecular clouds, and atomic hydrogen.

Matthew Miranda

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02:00

Problem 3

Describe several characteristics that distinguish population I stars from population II stars.

Matthew Miranda

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01:35

Problem 4

Briefly describe the main parts of our Galaxy.

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01:12

Problem 5

Describe the evidence indicating that a black hole may be at the center of our Galaxy.

Matthew Miranda

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02:09

Problem 6

Explain why the abundances of heavy elements in stars correlate with their positions in the Galaxy.

Matthew Miranda

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01:14

Problem 7

What will be the long-term future of our Galaxy?

Matthew Miranda

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01:15

Problem 8

Suppose the Milky Way was a band of light extending only halfway around the sky (that is, in a semicircle). What, then, would you conclude about the Sun's location in the Galaxy? Give your reasoning.

Matthew Miranda

Numerade Educator

01:40

Suppose somebody proposed that rather than invoking dark matter to explain the increased orbital velocities of stars beyond the Sun's orbit, the problem could be solved by assuming that the Milky Way's central black hole was much more massive. Does simply increasing the assumed mass of the Milky Way's central super-massive black hole correctly resolve the issue of unexpectedly high orbital velocities in the Galaxy? Why or why not?

Matthew Miranda

Numerade Educator

01:10

Problem 10

The globular clusters revolve around the Galaxy in highly elliptical orbits. Where would you expect the clusters to spend most of their time? (Think of Kepler's laws.) At any given time, would you expect most globular clusters to be moving at high or low speeds with respect to the center of the Galaxy? Why?

Matthew Miranda

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01:22

Problem 11

Shapley used the positions of globular clusters to determine the location of the galactic center. Could he have used open clusters? Why or why not?

Matthew Miranda

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03:32

Problem 12

Consider the following five kinds of objects: open cluster, giant molecular cloud, globular cluster, group of O and B stars, and planetary nebulae.
A. Which occur only in spiral arms?
B. Which occur only in the parts of the Galaxy other than the spiral arms?
C. Which are thought to be very young?
D. Which are thought to be very old?
E. Which have the hottest stars?

Matthew Miranda

Numerade Educator

01:18

Problem 13

The dwarf galaxy in Sagittarius is the one closest to the Milky Way, yet it was discovered only in $1994 .$ Can you think of a reason it was not discovered earlier? (Hint: Think about what else is in its constellation.)

Matthew Miranda

Numerade Educator

01:55

Problem 14

Suppose three stars lie in the disk of the Galaxy at distances of 20,000 light-years, 25,000 light-years, and 30,000 light-years from the galactic center, and suppose that right now all three are lined up in such a way that it is possible to draw a straight line through them and on to the center of the Galaxy. How will the relative positions of these three stars change with time? Assume that their orbits are all circular and lie in the plane of the disk.

Matthew Miranda

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01:13

Problem 15

Why does star formation occur primarily in the disk of the Galaxy?

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01:26

Problem 16

Where in the Galaxy would you expect to find Type II supernovae, which are the explosions of massive stars that go through their lives very quickly? Where would you expect to find Type I supernovae, which involve the explosions of white dwarfs?

Matthew Miranda

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01:24

Problem 17

Suppose that stars evolved without losing mass- -that once matter was incorporated into a star, it remained there forever. How would the appearance of the Galaxy be different from what it is now? Would there be population I and population II stars? What other differences would there be?

Matthew Miranda

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03:16

Problem 18

Assume that the Sun orbits the center of the Galaxy at a speed of $220 \mathrm{km} / \mathrm{s}$ and a distance of 26,000 lightyears from the center.
A. Calculate the circumference of the Sun's orbit, assuming it to be approximately circular. (Remember that the circumference of a circle is given by $2 \pi R,$ where $R$ is the radius of the circle. Be sure to use consistent units. The conversion from light-years to $\mathrm{km} / \mathrm{s}$ can be found in an online calculator or appendix, or you can calculate it for yourself: the speed of light is $300,000 \mathrm{km} / \mathrm{s}$, and you can determine the number of seconds in a year.)
B. Calculate the Sun's period, the "galactic year." Again, be careful with the units. Does it agree with the number we gave above?

Matthew Miranda

Numerade Educator

03:54

Problem 19

The Sun orbits the center of the Galaxy in 225 million years at a distance of 26,000 light-years. Given that $a^{3}=\left(M_{1}+M_{2}\right) \times P^{2},$ where $a$ is the semimajor axis and $P$ is the orbital period, what is the mass of the Galaxy within the Sun's orbit?

Matthew Miranda

Numerade Educator

03:29

Problem 20

Suppose the Sun orbited a little farther out, but the mass of the Galaxy inside its orbit remained the same as we calculated in Exercise $25.19 .$ What would be its period at a distance of 30,000 light-years?

Matthew Miranda

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09:19

Problem 21

We have said that the Galaxy rotates differentially; that is, stars in the inner parts complete a full $360^{\circ}$ orbit around the center of the Galaxy more rapidly than stars farther out. Use Kepler's third law and the mass we derived in Exercise 25.19 to calculate the period of a star that is only 5000 light-years from the center. Now do the same calculation for a globular cluster at a distance of 50,000 light-years. Suppose the Sun, this star, and the globular cluster all fall on a straight line through the center of the Galaxy. Where will they be relative to each other after the Sun completes one full journey around the center of the Galaxy? (Assume that all the mass in the Galaxy is concentrated at its center.)

Matthew Miranda

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01:19

Problem 22

If our solar system is 4.6 billion years old, how many galactic years has planet Earth been around?

Matthew Miranda

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03:14

Problem 23

Suppose the average mass of a star in the Galaxy is one-third of a solar mass. Use the value for the mass of the Galaxy that we calculated in Exercise $25.19,$ and estimate how many stars are in the Milky Way. Give some reasons it is reasonable to assume that the mass of an average star is less than the mass of the Sun.

Matthew Miranda

Numerade Educator

02:08

Problem 24

The first clue that the Galaxy contains a lot of dark matter was the observation that the orbital velocities of stars did not decreases with increasing distance from the center of the Galaxy. Construct a rotation curve for the solar system by using the orbital velocities of the planets, which can be found in Appendix F. How does this curve differ from the rotation curve for the Galaxy? What does it tell you about where most of the mass in the solar system is concentrated?

Matthew Miranda

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05:19

Problem 25

The best evidence for a black hole at the center of the Galaxy also comes from the application of Kepler's third law. Suppose a star at a distance of 20 light-hours from the center of the Galaxy has an orbital speed of $6200 \mathrm{km} / \mathrm{s}$. How much mass must be located inside its orbit?

Matthew Miranda

Numerade Educator

04:27

Problem 26

The next step in deciding whether the object in Exercise 25.25 is a black hole is to estimate the density of this mass. Assume that all of the mass is spread uniformly throughout a sphere with a radius of 20 lighthours. What is the density in $\mathrm{kg} / \mathrm{km}^{3}$ ? (Remember that the volume of a sphere is given by $V=\frac{4}{3} \pi R^{3}$.) Explain why the density might be even higher than the value you have calculated. How does this density compare with that of the Sun or other objects we have talked about in this book?

Matthew Miranda

Numerade Educator

02:40

Problem 27

Suppose the Sagittarius dwarf galaxy merges completely with the Milky Way and adds 150,000 stars to it. Estimate the percentage change in the mass of the Milky Way. Will this be enough mass to affect the orbit of the Sun around the galactic center? Assume that all of the Sagittarius galaxy's stars end up in the nuclear bulge of the Milky Way Galaxy and explain your answer.

Matthew Miranda

Numerade Educator

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