Deck 22: Our Galaxy

Question 1

Astronomers are able to study the center of the Milky Way galaxy in all of the following parts of the spectrum EXCEPT ONE. Which one is the EXCEPTION?

A)infrared
B)radio
C)visible
D)x ray

Answer 1

C

Question 2

The dimensions of our galaxy are not precisely known. For example, our distance to the galactic center is known only to 8 ± 1 kpc. One fundamental reason for this lack of further knowledge is that:

A)the Milky Way Galaxy is an irregular galaxy without much uniformity.
B)the whole system rotates rapidly, making measurements difficult.
C)our detection systems are not powerful enough to see across the entire galaxy.
D)we are inside the galaxy.

Answer 2

D

Question 3

In the absence of interstellar dust, a certain A0 star has the same magnitude through a blue filter and through a visual filter. When the light passes through a dust cloud before being observed, the:

A)B and V magnitudes remain equal.
B)B magnitude will be a smaller number than the V magnitude.
C)V magnitude will be a smaller number than the B magnitude.
D)size of the dust particles will determine whether B or V is the larger number.

Answer 3

C

Question 4

Variable stars, such as Cepheid variables and RR Lyrae stars, are used in what important measurement in astronomy?

A)rotation speeds of galaxies
B)surface temperature measurement
C)distance measurement
D)the keeping of accurate time

Question 5

Who did early studies of RR Lyrae variable stars?

A)Harlow Shapley
B)Henrietta Leavitt
C)Edwin Hubble
D)Sir Isaac Newton

Question 6

A Cepheid variable star is seen to vary regularly with a period of 25 days. How many times more luminous than the Sun would this star appear to be if it were to replace the Sun in our solar system? (See Figure 22-4 of Universe, 11th ed.)
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A)4
B)10,000
C)1000
D)100

Question 7

Harlow Shapley determined the position of the Sun in the galaxy by measuring the distances to 93 globular clusters of stars. How did Shapley obtain the distances to these very distant clusters?

A)He measured the apparent brightness and pulsation periods of RR Lyrae stars in the clusters.
B)He plotted the apparent brightness and spectral classes of a sample of the stars in each cluster on a Hertzsprung-Russell diagram.
C)He measured the proper motion and radial velocity of each globular cluster.
D)He measured the apparent brightness of the brightest red giant stars in each cluster.

Question 8

If the Milky Way Galaxy were rescaled to the size of the continental United States, the average distance between stars would be about:

A)5 cm.
B)1 m.
C)100 m.
D)10 km.

Question 9

If the Milky Way Galaxy were rescaled to the size of the continental United States, the size of an average star would be about the size of a(n):

A)automobile.
B)basketball.
C)tennis ball.
D)human cell.

Question 10

The distance to the globular cluster M55 has been determined to be 6500 pc by analysis of RR Lyrae stars in the cluster. How do the apparent brightnesses of these stars compare with the apparent brightness of the Sun? (Calculate b/b_Ÿe)

A)5.9 × 10^-17
B)2.5 × 10^-7
C)2.4 × 10^-6
D)There is no reason to believe all RR Lyrae stars in M55 have the same apparent brightness.

Question 11

What would be the wavelength of maximum emission of interstellar dust grains at a temperature of 65 K? (See Wien's law, Section 5-4, Universe, 11th ed.)

A)450 μm
B)22.5 μm
C)45 μm
D)2.25 μm

Question 12

If you want to find the distribution of dust in the Milky Way Galaxy, you should observe in which portion of the spectrum?

A)x ray
B)near infrared
C)far infrared
D)radio

Question 13

If you want to find the distribution of stars in the Milky Way Galaxy, you should observe in which portion of the spectrum?

A)x ray
B)near infrared
C)far infrared
D)radio

Question 14

The dimensions of the disk of our Milky Way Galaxy are:

A)diameter 80,000 ly; thickness 6500 ly.
B)diameter 6500 ly; thickness 2000 ly.
C)diameter 2000 ly; thickness 160,000 ly.
D)diameter 160,000 ly; thickness 2000 ly.

Question 15

The diameter of our galaxy is about:

A)50 kpc.
B)2 kpc.
C)8 kpc.
D)160 kpc.

Question 16

Where in the Milky Way Galaxy would you look for O and B stars?

A)only in the disk and spiral arms
B)everywhere in the galaxy
C)only in the halo
D)only in the central bulge

Question 17

High-velocity stars are isolated stars that swing in and out of the plane of the galaxy at relatively high velocities with respect to the solar system. Where would you expect to find them?

A)moving rapidly toward the solar system from all directions because of cosmic explosions in the open clusters where they are produced
B)in the rotating disk of the galaxy, having relatively large velocities with respect to the solar system because of their positions in the disk
C)in orbit around the galactic halo
D)circling the entire galaxy, just outside the galactic plane

Question 18

Most stars in the galactic halo are:

A)Population II stars in globular clusters.
B)Population II stars not in globular clusters.
C)Population I stars in open clusters.
D)Population I stars not in open clusters.

Question 19

The disk of the Milky Way appears bluish because of the light from the many O- and B-type stars found there. What color tint would you expect globular clusters to take on?

A)white-that is, a random mixture of all the visible colors
B)red
C)green
D)blue

Question 20

What is the net electric charge on a single unit of H I?

A)-1
B)0
C)+1
D)+2

Question 21

If you want to find the distribution of neutral hydrogen in the Milky Way Galaxy, you should observe in which portion of the spectrum?

A)x ray
B)near infrared
C)far infrared
D)radio

Question 22

A map of our galaxy deduced from radio observations of the 21-cm line emission from cool hydrogen gas reveals:

A)two spiral arms, one on each side of the galaxy.
B)two spiral arms, which wrap around the galaxy several times.
C)a smooth distribution of stars, characteristic of an elliptical galaxy.
D)at least four spiral arms and several short arm segments.

Question 23

What is the Local Bubble?

A)a region near the Sun devoid of interstellar gas and dust
B)a region of neutral hydrogen centered on the Sun
C)a region of ionized hydrogen centered on the Sun
D)a region near the Sun of very rarified but high-temperature gas

Question 24

Which physical process is thought to have created the Local Bubble near our Sun in the galaxy?

A)a nearby supernova explosion
B)Neanderthal campfires
C)the solar wind
D)hot O and B stars in the solar neighborhood

Question 25

Where is the Sun located in our galaxy? (i.e., What is our address in the universe?)

A)in the Centaurus arm, between the galactic center and the Orion arm
B)in the Sagittarius arm, which is between the Centaurus and Orion arms
C)in or close to the Orion arm, which is between the Sagittarius and Perseus arms
D)in the Perseus arm, between the Orion and Cygnus arms

Question 26

Three of the following are reasons why visible emissions from neutral hydrogen in the far regions of the Milky Way Galaxy would be difficult at best to detect on Earth. Which is NOT a correct explanation?

A)Neutral hydrogen does not emit visible light unless it receives an input of energy, which is unlikely in the cold regions of interstellar space.
B)The density of neutral hydrogen is too low to emit enough visible light to be detected.
C)Interstellar extinction would delete visible light before it crossed the galaxy.
D)At these cold temperatures, Wien's law gives an emission intensity that is essentially zero.

Question 27

If you were to view our galaxy from outside, you would find that the spiral arms:

A)stand out because they have many times the density of stars as the region between the stars.
B)stand out because the bright O and B stars are there.
C)are hard to detect because the density of stars is almost uniform across the galaxy.
D)stand out because the red giants are there.

Question 28

Which one of the following objects does not exhibit differential rotation?

A)Mercury
B)the Sun
C)the solar system
D)the disk of the Milky Way Galaxy

Question 29

The speed of the Sun in its orbit around the galaxy is deduced from observations of:

A)the galactic center, about which the Sun is orbiting.
B)Cepheid variables between spiral arms, because distances can easily be measured to them.
C)the orbital motions of stars near the Sun.
D)globular clusters in the galactic halo and distant galaxies.

Question 30

The most important reason why globular clusters are useful for finding the speed of the Sun in its orbit around the galaxy is that globular clusters:

A)on average rotate at the same speed as the Sun around the center of the galaxy.
B)are bright and easily seen at large distances.
C)are distributed uniformly around the galaxy.
D)on average do not rotate like stars in the disk of the galaxy.

Question 31

If the Sun were traveling around the galactic center along with companion stars (all in circular orbits), as depicted in Figure 22-17 (Universe, 11th ed.), from which of the stars in that diagram would you measure a Doppler shift of their light? (Hint: Think about relative velocities and orbital velocities at different orbital distances from the galactic center.)
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A)stars at the same orbital distance as the Sun
B)stars in directions at 45° angles from the Sun's direction of motion
C)stars directly between the Sun and the galactic center
D)None of them, because they are all moving along with the Sun and have no relative velocity with respect to it.

Question 32

The present estimate for the total mass of our galaxy (including a dark matter halo) is approximately:

A)10¹¹ $M_{\odot}$
B)10¹⁰ $M_{\odot}$
C)10¹² or more.
D)10⁹ or less.

Question 33

Assuming an absolutely flat rotation curve, what would be the orbital speed of a star on the outer edge of the disk of the observable galaxy?

A)160 km/s
B)220 km/s
C)510 km/s
D)680 km/s

Question 34

Dark matter cannot be seen, but we can detect its presence by its:

A)strong radio emissions.
B)strong emissions in the near infrared.
C)gravitational influence on visible stars.
D)accretion disk.

Question 35

How would the value of the orbital speed of a star on the observable outer edge of the plane of a galaxy change if there were no dark matter?

A)It would decrease to zero.
B)It would decrease, but not to zero.
C)It would remain unchanged.
D)It would increase.

Question 36

Because the rotation curve for the Milky Way Galaxy is approximately flat, a star in a circular orbit 13 kpc from the galactic center has the same orbital speed as the Sun, namely 220 km/sec. How much mass is interior to this star's orbit (in solar masses)?

A)9.0 × 10¹⁰
B)1.5 × 10¹¹
C)9.0 × 10¹¹
D)1.5 × 10¹²

Question 37

The neutrino has been proposed as a dark matter candidate. What is the current status of this idea?

A)It is still a viable candidate.
B)It has been rejected, because it is not massive enough.
C)It has been rejected, because it cannot "clump" to form the dark matter halo of our galaxy.
D)It has been rejected, because it has been found to be an unstable particle.

Question 38

According to the density-wave theory of spiral structure in galaxies:

A)collisions between galaxies create waves like the ripples around a rock dropped into a pond, and these are twisted into a spiral pattern by the galaxy's rotation.
B)collisions between stars send shock waves propagating through the disk of the galaxy.
C)star-formation regions propagate through the interstellar medium and get stretched into a spiral structure by differential rotation.
D)stars and interstellar clouds spend more time in regions of higher density, thus maintaining the higher density of the region.

Question 39

One proposed source for the energy to sustain density waves is:

A)the Kelvin-Helmholtz contraction on a galactic scale.
B)radioactivity.
C)gravitational attraction between the bar and the rest of the galaxy.
D)energy from gamma-ray bursters.

Question 40

Good tracers of spiral arms in galaxies are:

A)red giants, because they are bright, and their long wavelengths can be seen through dust.
B)O and B stars, because they have short lifetimes and are thus always close to where they were formed.
C)red dwarfs, because they are the most numerous stars.
D)neutral hydrogen, because it can be observed via the 21-cm line.

Question 41

Suppose it is discovered that the earliest galaxies were flocculent spirals. What implications might this have?

A)Flocculent spirals always evolve into grand design spirals.
B)The early universe was chaotic.
C)The stars in these early spirals formed first before the galaxies themselves (and their spiral arms).
D)The early universe was metal poor.

Question 42

The compact radio source Sagittarius A* has a mass of about 4.1 million and a size comparable to:

A)the Sun.
B)the solar system.
C)a globular cluster.
D)the galactic central bulge.

Question 43

Do we believe the black hole Sagittarius A* is growing?

A)We have witnessed the sudden disappearance of several orbiting stars in recent years, suggesting that Sagittarius A* is growing.
B)The x-ray radiation from Sagittarius is relatively weak, suggesting it is not growing.
C)The intense gravitational waves coming from Sagittarius A* suggest that it is growing.
D)While Sagittarius A* is not visible, the accretion disk around it is visible. This disk is growing, so we believe Sagittarius A* is growing.

Question 44

The center of the Milky Way Galaxy is characterized by each of the following EXCEPT ONE. Which is the EXCEPTION?

A)synchrotron (nonthermal) radiation produced by electrons spiraling in a powerful magnetic field
B)huge filaments of gas similar in structure to solar prominences
C)continuous bursts of powerful x-ray radiation
D)a halo of lobes of hot, ionized x-ray-emitting gas

Question 45

Which of the following BEST describes of the object at the center of our galaxy?

A)4 million $M_{\odot}$ in a volume the size of our solar system
B)5 billion $M_{\odot}$ in a volume smaller then Jupiter's orbit
C)20 $M_{\odot}$ in a volume the size of the Sun
D)several trillion $M_{\odot}$ in a volume 2 ly in diameter

Question 46

The most important electromagnetic bands for studying the galactic center do NOT include which one of the following?

A)infrared
B)visible
C)radio
D)x ray

Question 47

The star S2 orbits Sagittarius A* with a period of 15.2 years and a semi-major axis of 950 au. What does Kepler's third law suggest as the total amount of mass inside the orbital radius of S2?

A)1.3 × 10⁶ $M_{\odot}$
B)2.6 × 10⁶ $M_{\odot}$
C)4.1 × 10⁶ $M_{\odot}$
D)9.4 × 10⁹ $M_{\odot}$

Question 48

Gamma-ray "bubbles" have been found above and below the plane of our galaxy. (See Figure 22-30 of Universe, 11th ed.) These are:
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A)relatively small regions within the central bulge that are probably remnants of jet activity from Sagittarius A*.
B)regions from which no gamma-ray emission has been observed.
C)huge regions in which gamma rays are produced when high-speed electrons interact with photons of energies below the gamma-ray range.
D)the first gamma-ray bursters to be observed within our galaxy.