21st Century Astronomy

Laura Kay, Stacy Palen, Brad Smith

Chapter 21

The Milky Way-A Normal Spiral Galaxy - all with Video Answers

Educators

Chapter Questions

Problem 1

The size of the Milky Way is determined from studying _________ stars in globular clusters.
a. Cepheid variable
b. blue supergiant
c. RR Lyrae
d. Sun-like

Donald Albin

Donald Albin

Numerade Educator

Problem 2

Rank, in increasing order of age, globular clusters with mainsequence turnoffs at the following temperatures.
a. $10,000 \mathrm{K}$
b. $3000 \mathrm{K}$
c. $6000 \mathrm{K}$

Donald Albin

Numerade Educator

Problem 3

Detailed observations of the structure of the Milky Way are difficult because
a. the Solar System is embedded in the dust and gas of the disk.
b. the Milky Way is mostly dark matter.
c. there are too many stars in the way.
d. the galaxy is rotating too fast (about $200 \mathrm{km} / \mathrm{s}$ ).

Donald Albin

Numerade Educator

Problem 4

In general, what does the abundance of a star's heavy elements indicate about the age of the star?
a. Stars with higher abundance are older.
b. Stars with higher abundance are younger.

Donald Albin

Numerade Educator

Problem 5

Older stars are found farther from the midplane of a galactic disk because
a. the disk used to be thicker.
b. the stars have lived long enough to move there.
c. the younger stars in the thick disk were more massive and have already died.
d. none of the above

Donald Albin

Numerade Educator

Problem 6

The magnetic field of the Milky Way has been detected by
a. synchrotron radiation from cosmic rays.
b. direct observation of the field.
c. its interaction with Earth's magnetic field.
d. studying molecular clouds.

Donald Albin

Numerade Educator

Problem 7

Evidence of a supermassive black hole at the center of the Milky Way comes from
a. direct observations of stars that orbit it
b. X-rays from material that is falling in.
c. strong radio emission from the region of the accretion disk
d. all of the above

Zachary Warner

Numerade Educator

Problem 8

A most likely fate of the Large and Small Magellanic Clouds is that they will
a. become part of the Milky Way.
b. remain orbiting forever.
c. become attached to another passing galaxy.
d. escape from the gravity of the Milky Way.

Zachary Warner

Numerade Educator

Problem 9

Rank, in increasing order, the following in terms of the fraction of the Milky Way's mass that they represent.
a. stars in the bulge
b. stars in the halo
c. stars in the disk
d. dark matter

Zachary Warner

Numerade Educator

Problem 10

Globular clusters are important because
a. they provide information about how the Milky Way formed and evolved.
b. they provide information about how stars evolve.
c. they reveal the size of the Milky Way, and Earth's location in it.
d. all of the above

Zachary Warner

Numerade Educator

Problem 11

$\mathbf{T} / \mathbf{F}:$ Most of the Milky Way is visible because it is so close.

Zachary Warner

Numerade Educator

Problem 12

T/F: A globular cluster spends most of its time in the galaxy's halo.

Zachary Warner

Numerade Educator

Problem 13

T/F: The disk of the Milky Way contains most of its mass.

Zachary Warner

Numerade Educator

Problem 14

T/F: The Milky Way rotates like a solid disk.

Zachary Warner

Numerade Educator

Problem 15

$\mathbf{T} / \mathbf{F}:$ Most of the stars in the Milky Way formed at the same time.

Zachary Warner

Numerade Educator

Problem 16

Why are globular clusters so useful in determining the size of the galaxy?
a. They are large.
b. They are bright.
c. They are evenly distributed.
d. all of the above

Zachary Warner

Numerade Educator

Problem 17

In order for a variable star to be useful as a standard candle, its luminosity must be related to its
a. period of variation.
b. mass.
c. temperature.
d. radius.

Zachary Warner

Numerade Educator

Problem 18

The best evidence for dark matter in the Milky Way comes from the observation that the rotation curve
a. is quite flat at great distances from the center.
b. rises swiftly in the interior.
c. falls off and then rises again.
d. has a peak at about 2,000 light-years from the center.

Zachary Warner

Numerade Educator

Problem 19

cosmic rays are
a. a form of electromagnetic radiation.
b. high-energy particles.
c. high-energy dark matter.
d. high-energy photons.

Zachary Warner

Numerade Educator

Problem 20

In the Hubble scheme for classifying galaxies, what kind of galaxy is the Milky Way?
a. elliptical
b. spiral
c. barred spiral
d. irregular

Zachary Warner

Numerade Educator

Problem 21

Where are the youngest stars in the Milky Way Galaxy?
a. in the core
b. in the bulge
c. in the disk
d. in the halo

Zachary Warner

Numerade Educator

Problem 22

Halo stars are found in the vicinity of the Sun. What observational evidence distinguishes them from disk stars?
a. the direction of their motion
b. their speed
c. their composition
d. their temperature

Zachary Warner

Numerade Educator

Problem 23

How does the mass of the supermassive black hole at the center of the Milky Way compare with that found in most other spiral galaxies?
a. It is comparable in mass.
b. It is much less massive.
c. It is much more massive.
d. Astronomers don't yet know the masses well enough to tell.

Zachary Warner

Numerade Educator

Problem 24

Why are most of the Milky Way's satellite galaxies so difficult to detect?
a. They are very small.
b. They are very far away.
c. The halo of the Milky Way is in the way.
d. They are very faint.

Zachary Warner

Numerade Educator

Problem 25

The concept of a galactic habitable zone does not consider
a. the radiation field.
b. the ages of stars.
c. the amount of heavy elements.
d. the distance of a planet from its central star.

Zachary Warner

Numerade Educator

Problem 26

Why is it so difficult for astronomers to get an overall picture of the structure of the Milky Way Galaxy?

Zachary Warner

Numerade Educator

Problem 27

What do astronomers mean by standard candle?

Zachary Warner

Numerade Educator

Problem 28

How do astronomers know that the stars in globular clusters are among the oldest stars in the Milky Way Galaxy?

Zachary Warner

Numerade Educator

Problem 29

Compare and contrast globular and open clusters.
a. What are the main differences in the gas out of which globular and open clusters formed?
b. Why do globular clusters have such high masses while open clusters have low masses?
c. Are the characteristics of interest in parts (a) and (b) related? Explain.

Zachary Warner

Numerade Educator

Problem 30

Old stars in the inner disk of the Milky Way have higher abundances of massive elements than do young stars in the outer disk. Explain how this difference might have developed.

Zachary Warner

Numerade Educator

Problem 31

How do 21 -cm radio observations reveal the rotation of the Milky Way Galaxy?

Zachary Warner

Numerade Educator

Problem 32

What does the rotation curve of the Milky Way indicate about the presence of dark matter in the galaxy?

Zachary Warner

Numerade Educator

Problem 33

Explain the observational evidence showing that Earth is located in a spiral galaxy, not an elliptical galaxy.

Zachary Warner

Numerade Educator

Problem 34

What is one source of synchrotron radiation in the Milky Way, and where is it found?

Zachary Warner

Numerade Educator

Problem 35

Why must astronomers use X-ray, infrared, and 21-cm radio observations to probe the center of the galaxy?

Zachary Warner

Numerade Educator

Problem 36

What is $\operatorname{Sgr} A^{*}$ and how was it detected?

Zachary Warner

Numerade Educator

Problem 37

Explain the evidence for a supermassive black hole at the center of the Milky Way.

Zachary Warner

Numerade Educator

Problem 38

To observers in Earth's Southern Hemisphere, the Large and Small Magellanic Clouds look like detached pieces of the Milky Way. What are these "clouds," and why is it not surprising that they look so much like pieces of the Milky Way?

Zachary Warner

Numerade Educator

Problem 39

What has been the fate of most of the Milky Way's original satellite galaxies?

Zachary Warner

Numerade Educator

Problem 40

Use your imagination to describe how Earth's skies might appear if the Sun and Solar System were located
a. near the center of the galaxy.
b. near the center of a large globular cluster.
c. near the center of a large, dense molecular cloud.

Zachary Warner

Numerade Educator

Problem 41

From Figure 21.5 estimate the ratio between the radius of the Milky Way's halo and the radius of the disk.

Zachary Warner

Numerade Educator

Problem 42

Study Figure 21.7
a. What is the rotation velocity of a disk star located 6,000 pc from the center of the Milky Way?
b. Assuming a circular orbit, how long does it take that star to orbit once?

Zachary Warner

Numerade Educator

Problem 43

From the data in Figure 21.7 , estimate the time it would take the Large Magellanic Cloud to orbit the Milky Way, if it were on a circular orbit

Zachary Warner

Numerade Educator

Problem 44

The Sun completes one trip around the center of the galaxy in approximately 230 million years. How many times has the Solar System made the circuit since its formation 4.6 billion years ago?

Zachary Warner

Numerade Educator

Problem 45

The Sun is located about 8,300 pc from the center of the galaxy, and the galaxy's disk probably extends another 9,000 pc farther out from the center. Assume that the Sun's orbit takes 230 million years to complete.
a. With a truly flat rotation curve, how long would it take a globular cluster located near the edge of the disk to complete one trip around the center of the galaxy?
b. How many times has that globular cluster made the circuit since its formation about 13 billion years ago?

Zachary Warner

Numerade Educator

Problem 46

Parallax measurements of the variable star RR Lyrae indicate that it is located 230 pc from the Sun. A similar star observed in a globular cluster located far above the galactic plane appears 160,000 times fainter than RR Lyrae.
a. How far from the Sun is this globular cluster?
b. What does your answer to part (a) tell you about the size of the galaxy's halo compared to the size of its disk?

Donald Albin

Numerade Educator

Problem 47

Although the flat rotation curve indicates that the total mass of the Milky Way is approximately $1 \times 10^{12} M_{\mathrm{o}},$ electromagnetic radiation associated with normal matter suggests a total mass of only $9 \times 10^{10} M_{0}$. Given this information, calculate the fraction of the Milky Way mass that is made up of dark matter.

Prabhu Ramji

Numerade Educator

Problem 48

Given what you have learned about the distribution of massive elements in the Milky Way, and what you know about the terrestrial planets, where do you think such planets are most likely and least likely to form?

Zachary Warner

Numerade Educator

Problem 49

A cosmic-ray proton is traveling at nearly the speed of light $\left(3 \times 10^{5} \mathrm{m} / \mathrm{s}\right)$
a. Using Einstein's familiar relationship between mass and energy $\left(E=m c^{2}\right),$ show how much energy (in joules) the cosmic-ray proton would have if $m$ were based only on the proton's rest mass $\left(1.7 \times 10^{-27} \mathrm{kg}\right)$
b. The actual measured energy of the cosmic-ray proton is 100 J. What, then, is the relativistic mass of the cosmicray proton?
c. How much greater is the relativistic mass of this cosmicray proton than the mass of a proton at rest?

Guilherme Barros

Guilherme Barros

Numerade Educator

Problem 50

One of the fastest cosmic rays ever observed had a speed of $\left(1.0 \mathrm{minus} 10^{-20}\right) \times c .$ Assume that the cosmic ray and a photon left a source at the same instant. To a stationary observer, how far behind the photon would the cosmic ray be after traveling for 100 million years?

João Gabriel Alencar Caribé

João Gabriel Alencar Caribé

Numerade Educator

Problem 51

Consider a black hole with a mass of 5 million $M_{\circ}\left(M_{\circ}=\right.$ $\left.2 \times 10^{10} \mathrm{kg}\right) .$ A star's orbit about the black hole has a semi-major axis of 0.02 light-year $\left(1.9 \times 10^{14} \text { meters }\right)$. Calculate the star's orbital period. (Hint: For this and the questions that follow, you may want to refer back to Chapter $4 .$.

Matthew Miranda

Matthew Miranda

Numerade Educator

Problem 52

A star in a circular orbit about the black hole at the center of the Milky Way (whose mass $M_{\mathrm{BH}}=8 \times 10^{36} \mathrm{kg}$ ) has an orbital radius of 0.0131 light-year $\left(1.24 \times 10^{14}$ meters) \right. What is the average speed of this star in its orbit?

Krystal K

Numerade Educator

Problem 53

What is the Schwarzschild radius of the black hole at the center of the Milky Way? What is its density? How does this compare with the density of a stellar black hole?

Narayan Hari

Numerade Educator

Problem 54

A star is observed in a circular orbit about a black hole with an orbital radius of $1.5 \times 10^{11}$ kilometers and an average speed of $2,000 \mathrm{km} / \mathrm{s}$. What is the mass of this black hole in solar masses?

Sarah Mccrumb

Numerade Educator

Problem 55

One model of the galactic habitable zone contains stars in a doughnut between 7 and 9 kpe from the center of the galaxy, in the disk. Assuming that this doughnut is as thick as the disk itself, what fraction of the disk of the Milky Way lies in this habitable zone?

Mayukh Banik

Numerade Educator

Problem 56

Go to the "Night Sky" Web page of the National Park Service (NPS-http://nature.nps.gov/night). What is a "natural lightscape"? Click on and read "Light Pollution" and "Measuring Lightscapes." Why is it becoming more rare for people to see the Milky Way? Why does the NPS consider viewing the Milky Way an important part of the parks experience?

Matthew Miranda

Matthew Miranda

Numerade Educator

Problem 57

a. Go to the Astronomy Picture of the Day (APOD) app or website for July 2,2012 and watch the video clip "Zooming into the Center of the Milky Way" (http://apod.nasa gov/apod/ap120702.html). Why is there a shift in wavelengths of the selected pictures? On APOD, run a search for "Milky Way" to look at some of the best photographs of it. Where were the pictures taken from? Can you see the Milky Way from your location on a clear night?
b. Why does the galactic center have to be observed in infrared and X-ray wavelengths? Go to the "Milky Way Galaxy" page of the Chandra X-ray telescope website (http://chandra.harvard.edu/photo/category/milkyway .html) and to the Spitzer infrared telescope website (http://spitzer.caltech.edu). Are there any new images of the galactic center? What has been learned?

Chris Johnson

Numerade Educator

Problem 58

Go to the websites of the two main groups studying the supermassive black hole at the galactic center: the UCLA Galactic Center Group (http://astro.ucla.edu/-ghezgroup/gc) and the Galactic Center Research group at the Max Planck Institute for Extraterrestrial Physics (http://mpe,mpg.de/ ir/GC). Watch some of the time-lapse animations of the stars orbiting something unseen. Why is it assumed that the unseen object is a black hole? What new results are these groups reporting?

Zachary Warner

Numerade Educator

Problem 59

Go to the Milky Way Project website (http;//milkywayproject org. This citizen science project aims to sort Spitzer telescope observations of the dusty material in the galaxy. Log in with your Zooniverse account name, and read the information under "About" and "Tutorial." Participants in this project have already discovered some of these bubbles (http://spitzer.caltech.edu/images/4938-sig12-002-FindingBubbles-in-the-Milky-Way). Start looking.

Zachary Warner

Numerade Educator

Problem 60

Go to the HST website and watch the videos about the possible collision of the Milky Way and Andromeda (http:// hubblesite.org/newscenter/archive/releases/galaxy/2012/20/ video). Read the report under "The Full Story." Now do a Web search on this story, which received a lot of press in $2012 .$ Did other astronomers dispute this study later? Is it still receiving a lot of press attention?

Chris Johnson

Numerade Educator