21st Century Astronomy

Laura Kay, Stacy Palen, George Blumenthal

Chapter 19

Galaxies - all with Video Answers

Educators

Chapter Questions

Problem 1

Which of the following contributes the largest percentage to the total mass of a spiral galaxy?
a. dark matter
b. central black hole
c. stars
d. dust and gas

Sarah Mccrumb

Sarah Mccrumb

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Problem 2

In the context of spiral galaxies, Kepler's laws could be used to estimate
a. $P$, the period.
b. $A,$ the semimajor axis.
c. $M,$ the mass of the galaxy.
d. $v,$ the rotation speed of the galaxy.

Sarah Mccrumb

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Problem 3

Astronomers determine the radius of an AGN by measuring
a. how much light comes from it.
b. how hard it pulls on stars nearby.
c. how quickly its light varies.
d. how quickly it rotates.

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Problem 4

If you observed a galaxy with an $\mathrm{H} \alpha$ emission line that had a wavelength of $756.3 \mathrm{nm}$, what would be the galaxy's redshift? Note that the rest wavelength of the H $\alpha$ emission line is $656.3 \mathrm{nm}$.
a. 0.01
b. 0.05
c. 0.10
d. 0.15

Sarah Mccrumb

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Problem 5

As astronomers extend their distance ladder beyond $30 \mathrm{Mpc}$ they change their measuring standard from Cepheid variable stars to Type Ia supernovae. Why is this change necessary?
a. Type Ia supernovae are more luminous than Cepheid variables.
b. Type Ia supernovae are less luminous than Cepheid variables.
c. Type Ia supernovae vary more slowly than do Cepheid variables.
d. Type Ia supernovae vary more quickly than do Cepheid variables.

Sarah Mccrumb

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Problem 6

Which galaxy type has a spherical bulge and a well-defined disk?
a. spiral
b. barred spiral
c. elliptical
d. irregular

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Problem 7

Which galaxy type is shaped like a rugby ball?
a. $\mathrm{Sb}$
b. $\mathrm{SBb}$
c. $\mathrm{E0}$
d. $\mathrm{E5}$

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Problem 8

For a galaxy, the term morphology refers to
a. its shape.
b. its evolution over time.
c. the motion of its stars.
d. its overall density.

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Problem 9

If all the stars in an elliptical galaxy traveled in random directions in their orbits, the elliptical galaxy would be type
a. E0.
b. E2.
c. E5.
d. E7.

Sarah Mccrumb

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Problem 10

The flat rotation curves of spiral galaxies imply that the distribution of mass resembles
a. the Solar System; most mass is concentrated in the center.
b. a wheel; the density remains the same as the radius increases.
c. the light distribution of the galaxy; a large concentration occurs in the middle, but significant mass exists quite far out.
d. an invisible sphere much larger than the visible galaxy.

Sarah Mccrumb

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Problem 11

Astronomers observe two galaxies, A and B. Galaxy A has a recession velocity of $2,500 \mathrm{km} / \mathrm{s},$ while galaxy $\mathrm{B}$ has a recession velocity of $5,000 \mathrm{km} / \mathrm{s}$. According to these data,
a. galaxy $\mathrm{A}$ is 4 times as far away as galaxy $\mathrm{B}$.
b. galaxy $\mathrm{A}$ is twice as far away as galaxy $\mathrm{B}$.
c. galaxy $\mathrm{B}$ is twice as far away as galaxy $\mathrm{A}$
d. galaxy $\mathrm{B}$ is 4 times as far away as galaxy $\mathrm{A}$.

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Problem 12

The Hubble constant is found from the
a. slope of the line fit to the data in Hubble's law.
b. $y$ -intercept of the line fit to the data in Hubble's law.
c. spread in the data in Hubble's law.
d. inverse of the slope of the line fit to the data in Hubble's law.

Sarah Mccrumb

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Problem 13

Astronomers know that dark matter is present in galactic halos because the speeds of orbiting stars __________ far from the center of the galaxy.
a. decrease
b. increase
c. remain about constant
d. fluctuate dramatically

Sarah Mccrumb

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Problem 14

What accounts for the differences among various types of AGNs?
a. the type of the host galaxy
b. the size of the central black hole
c. the amount of dark matter in the galaxy's halo
d. our viewing angle

Sarah Mccrumb

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Problem 15

If a Seyfert galaxy's nucleus varies in brightness on the timescale of 10 hours, then approximately what is the size of the emitting region?
a. $20 \mathrm{AU}$
b. $70 \mathrm{AU}$
c. $90 \mathrm{AU}$
d. $140 \mathrm{AU}$

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Problem 16

What was the subject of the Great Debate, and why was it important to astronomers' understanding of the scale of the universe?

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Problem 17

How did observations of Cepheid variable stars finally settle the Great Debate?

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Problem 18

Why is it better to observe more than one type of standard candle in a distant galaxy?

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Problem 19

Explain what astronomers mean by distance ladder.

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Problem 20

Why is it important to know the type of progenitor of a Type Ia supernova in a distant galaxy?

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Problem 21

Some galaxies have regions that are relatively blue; other regions appear redder. What does this variation indicate about the differences between these regions?

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Problem 22

Describe how elliptical galaxies and spiral bulges are similar.

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Problem 23

Which is more luminous: a quasar or a galaxy with 100 billion solar-type stars? Explain your answer.

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Problem 24

The nearest observed quasar is about 750 Mpc away. Why don't astronomers observe any that are closer?

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Problem 25

What distinguishes a normal galaxy from one that contains an AGN? It is a principle of science, often attributed to Einstein, that one should make things as simple as possible, but not simpler. Explain how this principle is at work in the distinction between normal galaxies and AGNs.

Sarah Mccrumb

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Problem 26

Contrast the size of a typical AGN with the size of the Solar System. How do astronomers know the size of an AGN?

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Problem 27

Describe what astronomers think is happening at the center of a galaxy that contains an AGN.

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Problem 28

It is likely that most galaxies contain supermassive black holes, yet many galaxies display no obvious evidence for their existence. Why do some black holes reveal their presence while others do not?

Sarah Mccrumb

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Problem 29

Study Figure 19.9
a. Why is it important that the different "rungs" of the distance ladder overlap in the distances that they measure?
b. Does the figure end at the right edge because there are no more ways to measure distance or because there is no more universe to measure? How do you know?

Sarah Mccrumb

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Problem 30

Why do astronomers think that planets around stars near the centers of galaxies would not be good locations for the formation of life?

Sarah Mccrumb

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Problem 31

Study Figure $19.14 .$ The small vertical bars (known as error bars) on the data points indicate the size of the measurement error.
a. At a radius of 25,000 parsecs (pc), what is the approximate measurement error in the rotation velocity?
b. What is this value as a percentage of the measured velocity?
c. Error bars are important because they show how wrong the measurement could possibly be. One way to think about this is that the black line could be as high as the top of the error bars or as low as the bottom of the error bars. In either case, would shifting the black line change the overall conclusion about redshift and distance? Why or why not?

Sarah Mccrumb

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Problem 32

Suppose the number density of galaxies in the universe is, on average, $3 \times 10^{-68}$ galaxy $/ \mathrm{m}^{3} .$ If astronomers could observe all galaxies out to a distance of $10^{10} \mathrm{pc}$, how many galaxies would they find?

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Problem 33

The spectrum of a distant galaxy shows the H $\alpha$ line of hydro$\operatorname{gen}\left(\lambda_{\mathrm{rest}}=656.28 \mathrm{nm}\right)$ at a wavelength of $750 \mathrm{nm} .$ Assume that $H_{0}=70 \mathrm{km} / \mathrm{s} / \mathrm{Mpc}$.
a. What is the redshift $(z)$ of this galaxy?
b. What is its recession velocity $\left(v_{r}\right)$ in kilometers per second?
c. What is the distance of the galaxy in megaparsecs?

Sarah Mccrumb

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Problem 34

The nearest known quasar is $3 \mathrm{C} 273 .$ It is located in the constellation Virgo and is bright enough to be seen in a medium-sized amateur telescope. With a redshift of 0.158 , what is the distance to $3 \mathrm{C} 273$ in parsecs?

Sarah Mccrumb

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Problem 35

The quasar $3 \mathrm{C} 273$ has a luminosity of $10^{12} L_{\text {sun }}$. Assuming that the total luminosity of a large galaxy, such as the Andromeda Galaxy, is 10 billion times that of the Sun, compare the luminosity of $3 \mathrm{C} 273$ with that of the entire Andromeda Galaxy.

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Problem 36

A quasar has the same brightness as a galaxy that is seen in the foreground 2 Mpc distant. If the quasar is 1 million times more luminous than the galaxy, what is the distance of the quasar?

Sarah Mccrumb

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Problem 37

Estimate the Schwarzschild radius for a supermassive black hole with a mass of 26 billion $M_{\text {Sun. }}$.

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Problem 38

You read in the newspaper that astronomers have discovered a "new" cosmological object that appears to be flickering with a period of 83 minutes. Because you have read $21^{\text {st }}$ Century Astronomy, you are able to estimate quickly the maximum size of this object. How large can it be?

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Problem 39

A quasar has a luminosity of $10^{41} \mathrm{W}$, or $\mathrm{J} / \mathrm{s}$, and $10^{8} \mathrm{M}_{\text {sun }}$ to feed it. Assuming constant luminosity and 20 percent conversion efficiency, what is your estimate of the quasar's lifetime?

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Problem 40

A solar-type star $\left(M=2 \times 10^{30} \mathrm{kg}\right)$ approaches a supermassive black hole. As it crosses the event horizon, half of its mass falls into the black hole while the other half is completely converted to energy in the form of light. How much energy does this dying star send out to the rest of the universe?

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Problem 41

Suppose a Type Ia supernova is found in a distant galaxy. The measured supernova brightness is $10^{-17} \mathrm{W} / \mathrm{m}^{2}$. What is the distance of the galaxy?

Sarah Mccrumb

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Problem 42

Suppose that an object with the mass of Earth $\left(M_{\text {Earth }}=5.97 \times 10^{24} \mathrm{kg}\right)$ fell into a supermassive black hole with a 10 percent energy conversion.
a. How much energy (in joules) would be radiated by the black hole?
b. Compare your answer with the energy radiated by the Sun each second: $3.85 \times 10^{26} \mathrm{J}$

Sarah Mccrumb

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Problem 43

If a luminous quasar has a luminosity of $2 \times 10^{41} \mathrm{W}$, or $\mathrm{J} / \mathrm{s}$, how many solar masses $\left(M_{\text {sun }}=2 \times 10^{30} \mathrm{kg}\right)$ per year does this quasar consume to maintain its average energy output?

Sarah Mccrumb

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Problem 44

Material ejected from the supermassive black hole at the center of galaxy $\mathrm{M} 87$ extends outward from the galaxy to a distance of approximately 30,000 pc. $\mathrm{M} 87$ is approximately 17 Mpc away.
a. If this material were visible to the naked eye, how large would it appear in the nighttime sky? Give your answer in degrees $\left(1 \text { radian }=57.3^{\circ}\right)$.
b. Compare this size with the angular size of the Moon.

Sarah Mccrumb

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Problem 45

A lobe in a visible jet from galaxy $\mathrm{M} 87$ is observed at a distance of 1,530 pc $(5,000$ light-years) from the galaxy's center moving outward at a speed of 0.99 times the speed of light $(0.99 c) .$ Assuming constant speed, how long ago was the lobe expelled from the supermassive black hole at the galaxy's center?

Sarah Mccrumb

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Problem 46

Go to the Goddard Media Studios website and view the animation of a Cepheid variable star in a spiral galaxy (http://svs.gsfc.nasa.gov/goto?10145). Explain how astronomers use data like these to estimate the distance to the galaxy. What is actually observed, what is assumed, and what is calculated? (Review the discussion in Chapter 16 as needed.)

Farhanul Hasan

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Problem 47

Go to the Astronomy Picture of the Day app or website (http://apod.nasa.gov/apod) and look at some recent pictures of galaxies. In each case, consider the following questions: Was the picture taken from a large or small telescope; from the ground or from space? Are galaxies in the image face on, edge on, or at an angle? What wavelengths were used for making the image? Are any of the colors "false colors"? If the picture is a combination of images from several telescopes, what do the different colors indicate?

Ren Jie Tuieng

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Problem 48

Citizen science:
a. Go to the website for Galaxy Zoo (http://galaxyzoo.org), the original Zooniverse citizen science project. (Log in with your Zooniverse password.) The specific project in action at any given time depends on the real data that need to be examined. One of the projects is likely a classification project. Click on and read, "Story," "Science," and "Classify," and then classify some galaxies. Save a copy of your classifications for your homework if necessary.
b. Go to the website for Radio Galaxy Zoo (http://radio galaxyzoo.org/). This project asks people to help locate supermassive black holes and their associated jets. Click on "Science" to read about the project, "Begin Hunting" to see some data, and then "Classify." Work through the given example, and then classify a few more.

Joanna Quigley

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Problem 49

a. Go to the website for the Fermi Gamma-ray Space Telescope (http://fermi.gsfc.nasa.gov). Scroll down to click on “Full News Archive" and look for a story about dark matter. What has this telescope discovered about dark matter?
b. Go to the website for the Alpha Magnetic Spectrometer (http://ams02.org), a particle physics detector located on the International Space Station to search for dark matter, including WIMPs. What are the latest results?

Farhanul Hasan

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Problem 50

a. Go to the website for the NASA Swift Gamma-Ray observatory (swift.gsfc.nasa.gov), which studies gamma-ray bursts. Click on "Latest Swift News" and look for a story about supermassive black holes. What has been discovered?
b. Go to the website for NuSTAR (Nuclear Spectroscopic Telescope Array-http://www.nustar.caltech.edu), a space telescope launched by NASA in 2012 . This mission is studying active galaxies hosting supermassive black holes. What type of telescope is this (wavelengths observed, general design)? What has been discovered?

Zachary Warner

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