Deck 13: The Galaxies

Question 1

In the 1920s,Edwin Hubble began answering the question about the nature of spiral "nebulae" when he discovered Cepheid variables in the:

A) Milky Way Galaxy.
B) Whirlpool Galaxy (M51).
C) Andromeda Galaxy (M31).
D) Small Magellanic Cloud.

Answer 1

Andromeda Galaxy (M31).

Question 2

Distances to a nearby galaxy can be determined MOST accurately by:

A) using pulsating stars as beacons.
B) measuring the chemical compositions of the brightest stars.
C) measuring the shifts of spectral lines from the Milky Way Galaxy.
D) measuring the total amount of energy received from the Milky Way Galaxy.

Answer 2

using pulsating stars as beacons.

Question 3

The idea that some of the "nebulae" that are observed in the sky might be "island universes," immense collections of stars far beyond the Milky Way,was first proposed by:

A) Immanuel Kant in 1755.
B) Albert Einstein in 1909.
C) Heber Curtis in 1920.
D) Lord Rosse in 1845.

Answer 3

Immanuel Kant in 1755.

Question 4

The famous Curtis-Shapley debate in 1920 concerned which fundamental astronomical question in astronomy?

A) whether all stars were like the Sun or fundamentally different
B) whether the spiral "nebulae" were part of the Milky Way Galaxy or more distant, separate entities
C) whether the universe was expanding outward in all directions
D) whether the Sun was at the center of the Milky Way Galaxy

Question 5

Variable stars such as Cepheid variables are used in what important measurement in astronomy?

A) the measurement of the distances to stars
B) the measurement of the rotation speeds of galaxies
C) the measurement of the surface temperatures of stars
D) the keeping of accurate time

Question 6

The period-luminosity relationship for Cepheid variable stars,relating variability to absolute overall brightness,thereby providing identifiable beacons throughout our local space,was discovered by:

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

Question 7

The Milky Way Galaxy:

A) is one of many billions of galaxies in the universe.
B) is unique in the universe in showing definite spiral structure.
C) contains the whole universe; everything observable is within its volume.
D) is one of only a few spiral galaxies; most other galaxies in the universe are amorphous collections of stars shaped like ellipsoids.

Question 8

The twentieth century opened with a major unresolved question among astronomers.This was the subject of the famous Shapley-Curtis debate.What was this question?

A) How old is the Milky Way Galaxy?
B) What is the nature of dark matter?
C) Is general relativity true?
D) Is the Milky Way Galaxy the whole universe, or only one of many galaxies?

Question 9

What important role do Cepheid variables stars have in astronomy?

A) distance measurements to distant galaxies
B) the determination of stellar luminosities
C) the determination of speeds of stars in galactic arms from the Doppler shift of their spectra
D) the keeping of accurate time

Question 10

You are looking through a very old astronomy book,and you find the Messier object M31 referred to as the Andromeda Nebula.You might suspect that this book was written before:

A) 1780.
B) 1845.
C) 1924.
D) 1954.

Question 11

Who won the Shapley-Curtis debate when it concluded in 1920?

A) Shapley won.
B) Curtis won.
C) Both sides won in the sense that each successfully explained part of the data that were being debated.
D) Neither side won because there was no hard, reliable evidence to support either side.

Question 12

The intrinsic brightness (represented by luminosity)of a Cepheid variable star compared with that of the Sun is:

A) generally between 10 and 100 times larger.
B) about the same.
C) significantly less.
D) several hundred to several thousand times larger.

Question 13

The event that settled the Shapley-Curtis debate about "spiral nebulae" was:

A) Edwin Hubble measuring the distance to the Andromeda Galaxy.
B) Arno Penzias and Robert Wilson detecting the cosmic microwave background radiation.
C) Albert Einstein showing that gravity can bend the path of light.
D) Edwin Hubble showing that the universe was expanding.

Question 14

The first suggestion that there were collections of stars beyond the Milky Way in the universe was made by:

A) Edwin Hubble in 1923.
B) Immanuel Kant in 1755.
C) William Parsons, Earl of Rosse, in 1845.
D) Sir Isaac Newton in 1690.

Question 15

Who was the first person to observe the spiral structure of distant galaxies with a telescope?

A) Sir Isaac Newton
B) Sir William Herschel
C) William Rosse
D) Galileo Galilei

Question 16

The famous nineteenth-century observational astronomer Lord Rosse built the largest telescope of his time (and discovered the spiral nature of many so-called "nebulae")in which country?

A) England
B) the United States
C) Ireland
D) Germany

Question 17

Which concept concerning the diffuse objects observed by Herschel was proposed by Kant in the 1700s and was strongly supported by observations made in 1845 by the Earl of Rosse with his world-class telescope in Ireland?

A) Many of the diffuse objects in Earth's sky are moving rapidly away from the Milky Way Galaxy.
B) Many of the nebulae in the sky are separate entities beyond the Milky Way Galaxy.
C) The diffuse objects observed by Herschel and known as planetary nebulae were in fact planets orbiting other stars in the Milky Way Galaxy.
D) All the diffuse nebulae seen in the sky are gas clouds in the Milky Way Galaxy.

Question 18

The major features of our Milky Way Galaxy have been found to include all of the following EXCEPT ONE.Which is the exception?

A) more than one hundred billion stars
B) a bar of stars running through the center
C) a spherical symmetry
D) spiral arms

Question 19

In the mid-nineteenth century,Count Rosse built the world's largest telescope,the "Leviathan of Parsonstown." What major disadvantage did Rosse have in using his telescope at this time?

A) The mirror, although large, was not made using modern techniques. It had so much distortion that deep sky objects like galaxies could not be seen clearly.
B) No catalogs of deep sky objects were available, so Rosse did not know where to locate anything of interest.
C) The telescope used an objective lens rather than a mirror and thus suffered from distortion due to sagging and chromatic aberration.
D) Photography was not available to record the images Rosse observed.

Question 20

Cepheid variable stars are useful to astronomers as indicators of:

A) stars with very high-speed motion.
B) white dwarf behavior.
C) the existence of black holes.
D) distance, particularly to nearby galaxies.

Question 21

In the 1780s,Sir William Herschel tried to measure the Sun's position in the Milky Way Galaxy by:

A) counting the density of stars in different directions along the Milky Way.
B) measuring distances to star clusters and H II regions in the disk of the Galaxy.
C) measuring the locations of globular clusters around the Galaxy.
D) comparing the Milky Way with the Andromeda Galaxy.

Question 22

The factor that misled Herschel into concluding that the stars of the Milky Way were distributed with the Sun at the center of the Milky Way Galaxy was:

A) hot hydrogen gas in the Galaxy, its emission hiding the more distant stars.
B) interstellar dust that obscured the more distant stars and thereby localized his observations.
C) gravitational bending of light by the mass of the Galaxy, distorting the relative positions of the stars.
D) that most of the "stars" he measured were in fact distant galaxies that are distributed uniformly around the Sun.

Question 23

Why are we on Earth able to see only a relatively small part of the Milky Way Galaxy?

A) There are so many stars in the Milky Way that the more distant ones are hidden behind the nearer ones.
B) Distant stars are obscured by gas in interstellar space.
C) Expansion of the universe has carried the more distant stars out of our view.
D) Distant stars are obscured by dust in interstellar space.

Question 24

The Andromeda Galaxy (M31)is BEST described as a(n):

A) extension of the Milky Way.
B) vortex surrounding a black hole.
C) gaseous nebula extending for 6° across our sky.
D) spiral collection of stars, dust, and gas 2 million ly away.

Question 25

How far away from us is the Andromeda Galaxy compared with the diameter of the Milky Way Galaxy?

A) The Andromeda Galaxy is about three Milky Way diameters away.
B) The Andromeda Galaxy is about 20 Milky Way diameters away.
C) The Andromeda Galaxy is between 100 and 120 Milky Way diameters away.
D) The Andromeda Galaxy is several million Milky Way diameters away.

Question 26

Which component of the Milky Way Galaxy accounts for interstellar extinction,the dimming of light from distant objects?

A) cool hydrogen gas
B) so-called hidden or missing matter since its absorbing properties render it invisible in the Galaxy
C) molecules such as H₂ and CO, which are strong absorbers, in molecular clouds
D) dust

Question 27

What was the implication of the observation by Edwin Hubble that the Cepheid variable stars he measured in the Andromeda "nebula" appeared to be very faint compared with what was expected?

A) The observed period was affected by the intense gravitational field of the "nebula" as predicted by general relativity, leading to incorrect luminosity determination.
B) The Andromeda "nebula" was very far away and was in fact a galaxy.
C) Hubble had discovered a new class of intrinsically faint Cepheid variable stars.
D) Dust and gas in the "nebula" had severely reduced the light from these stars.

Question 28

A Cepheid variable star with a pulsation period of a few days is seen in the spiral arm of a galaxy.Its apparent brightness is measured as 10⁴ times fainter than an equivalent star 1000 ly away from the Sun in the Milky Way Galaxy.Assuming no light absorption between galaxies,what is the distance to the far Cepheid and hence to the galaxy?

A) 10⁷ ly (10⁴ times farther away)
B) 10,000 ly (10 times farther away)
C) 10 ly (100 times closer)
D) 100,000 ly (100 times farther away)

Question 29

Cepheid variable stars are invaluable in astronomy because of the close relationship between:

A) the peak wavelength of their spectra and their surface temperatures.
B) their luminosity, or absolute magnitude, and their pulsation period.
C) the redshift of their spectra and their distance from the Sun.
D) their apparent magnitude and their pulsation period.

Question 30

The observation by Hubble that demonstrated for the first time that the Andromeda "nebula" was at a very large distance from the Sun and outside the Milky Way Galaxy was that:

A) Cepheid variable stars appeared to be very faint in the "nebula."
B) the "nebula" appeared to be rotating night by night around a center that was not the center of the Milky Way.
C) stars with characteristics similar to those of the Sun appeared to be absent in the "nebula."
D) globular clusters appeared to be distributed in a halo around the "nebula," a sure sign of a separate galaxy.

Question 31

How has the maximum intrinsic brightness (minimum absolute magnitude)of Type Ia supernovae been measured?

A) The distances to nearby Type Ia supernovae have been measured using trigonometric parallax.
B) Type Ia supernovae have been observed in distant galaxies in which Cepheid variables have also been observed.
C) The brightnesses of Type Ia supernovae have been measured at several different wavelengths, from which their temperature and hence their intrinsic brightnesses can be calculated.
D) The brightnesses of several Type Ia supernovae have been accurately measured relative to the brightnesses of the original stars before they exploded.

Question 32

A classical Cepheid variable star is seen to vary regularly with a period of 25 days.How many times brighter than the Sun would this star appear to be if it were to replace the Sun in our solar system? (See Figure 13-3 of Comins,Discovering the Essential Universe,6th ed.)

A) 1000
B) 4
C) 10,000
D) 10²

Question 33

If interstellar dust obscures our view of distant regions of space at optical wavelengths,which statement is TRUE?

A) The obscuration is very clumpy and random over the whole sky; the individual absorbing dust clouds show no preference for one particular direction or plane.
B) The obscuration is roughly uniform over the whole sky.
C) The obscuration is the least in the plane of the Milky Way Galaxy and is strongest when we look out into the galactic halo, at right angles in this plane.
D) The obscuration is severe only in the plane of the Milky Way Galaxy.

Question 34

The significant feature of a Cepheid variable is that there is a relationship between two intrinsic parameters,one of which can be easily measured,while knowledge of the other parameter is required.These parameters are:

A) period of brightness variation and spectral color.
B) variation of spectral color and distance to the star.
C) period of brightness variation and luminosity.
D) amplitude of brightness variation and luminosity.

Question 35

In the eighteenth century,Sir William Herschel used star counts in different regions of the sky along the Milky Way to estimate the position of the center of the Milky Way.He incorrectly concluded that the Sun was close to the center.The reason for this erroneous conclusion was that:

A) the redshift of the more distant stars made them invisible to Herschel.
B) Herschel counted all "stars" in each star field and included many galaxies that were outside the Milky Way, thus confusing the distribution.
C) the large quantity of absorbing dust between stars obscured the more distant regions of the Galaxy.
D) emissions from hot hydrogen gas clouds served to hide the more distant stars, localizing his search.

Question 36

Hubble carried out the determination of the distance to the Andromeda "nebula" (which finally resolved the Curtis-Shapley debate on the nature of "spiral nebulae")by observing:

A) pulsars.
B) Cepheid variable stars.
C) the Doppler shift of stars in the "nebula."
D) the main sequence of stars in the "nebula."

Question 37

The method Hubble used to determine the distance to the Andromeda Galaxy (M31),thereby establishing the concept of separate and individual galaxies throughout the universe,was the:

A) observation of Cepheid variable stars.
B) measurement of the redshift of the whole galaxy.
C) measurement of stellar parallax, or apparent motion, of stars because of Earth's orbital motion.
D) observation of the brightnesses of novae.

Question 38

In 1785,William Herschel proposed that the solar system was at the center of the Milky Way Galaxy.How did he reach this conclusion?

A) Like the ancient Greeks and many of their successors for over a thousand years, Herschel based his conclusion on the philosophical importance of Earth in the cosmos.
B) Herschel's observations suggested that the stars in the Milky Way Galaxy were distributed uniformly around Earth.
C) Herschel measured distances to globular clusters and determined that they were distributed uniformly around Earth.
D) Herschel determined that Cepheid variable stars in the Milky Way Galaxy were distributed uniformly around Earth.

Question 39

The observational fact about a Cepheid variable star that leads to a measurement of its distance from us is that its period of variation is directly related to its:

A) surface temperature.
B) position along the spiral arms of a galaxy.
C) apparent magnitude.
D) absolute magnitude.

Question 40

What is it that makes the study of the structure of the Milky Way Galaxy more difficult than that of much more distant spiral galaxies?

A) The galactic center is visible only from the southern hemisphere, where, until recently, no major telescopes were available for the study of galactic structure.
B) The Milky Way appears to be unique, with a structure quite unlike any other galaxy.
C) The Sun is within the Galaxy, and its motion confuses the interpretation of the motion of other parts of the Galaxy.
D) Most of the Milky Way is hidden behind dense gas and dust clouds in the galactic plane.

Question 41

Suppose we could examine a star and determine its apparent magnitude m without any intervening gas and dust.Compared with its apparent magnitude measured with gas and dust between us and the star,this "clean" measurement of m would be:

A) a larger number.
B) a smaller number.
C) the same number.
D) either larger or smaller depending on the star's speed (toward or away).

Question 42

The method used by Harlow Shapley in 1917 to estimate the Sun's location in the Milky Way Galaxy was the measurement of the:

A) locations of globular clusters around the Galaxy.
B) density of stars in different directions along the Milky Way.
C) distances to open star clusters and H II regions in the disk of the Galaxy.
D) structure of the Andromeda Galaxy and a comparison of it with the structure of the Milky Way.

Question 43

Where are many of the older,metal-poor stars found in the Milky Way Galaxy?

A) throughout the whole Galaxy
B) at the galactic center
C) in the disk and spiral arms
D) in globular clusters and in the galactic halo

Question 44

In 1917,Harlow Shapley plotted the positions of almost a hundred globular clusters.But the galactic plane is cloaked by gas and dust,and the best way to "see" through this is with long-wavelength radio waves.However,in 1917 radio astronomy did not exist.How did Shapley accomplish his task?

A) He could only see globular clusters that were relatively close to the Sun.
B) He concentrated on using the long-wavelength end of the visible spectrum: red. This worked well because globular clusters are old and are dominated by red stars.
C) The globular clusters are mostly in a halo up off the galactic plane, so the dust was not a great problem.
D) In order to form a globular cluster, a great deal of interstellar dust is used up. Thus, globular clusters create holes in the dust and are relatively easy to see.

Question 45

Radio waves of 21-cm wavelength originate from which component of the interstellar medium?

A) cool, neutral atomic hydrogen
B) cold molecular hydrogen, H₂
C) cool carbon monoxide, CO
D) hot, ionized atomic hydrogen

Question 46

Trumpler discovered the effect of interstellar dust by comparing two different distance measurements of distant clusters.The first was based on appearance (concentration of stars toward the center,etc.),which was unaffected by dust.The other was spectroscopic parallax (use of the H-R diagram),which was affected by dust.What should such a comparison show?

A) The H-R diagram should always give larger distances.
B) The H-R diagram should always give smaller distances.
C) There should be no difference in the distances.
D) The distances should be different, but sometimes the H-R diagram should give the larger distance and sometimes the smaller depending on the actual distance to the cluster.

Question 47

Why is it that we rely upon the 21-cm line associated with the spin flip in hydrogen and not the more familiar hydrogen Balmer lines to gather information from the far side of the galactic plane?

A) The Balmer lines are associated with neutral hydrogen, and most of the hydrogen we need to observe is molecular hydrogen.
B) The Balmer lines are associated with molecular hydrogen, and most of the hydrogen we need to observe is neutral hydrogen.
C) The Balmer lines are in the visible and ultraviolet regions of the spectrum, and these do not penetrate the gas and dust clouds easily.
D) Photons released in the spin flip transition have much more energy than those that come from the Balmer transitions.

Question 48

The one component of the material of the Milky Way Galaxy that prevents us from seeing and photographing the galactic center at optical wavelengths is:

A) very cold hydrogen gas.
B) interstellar dust.
C) the glare of light from nearby stars.
D) hot hydrogen gas.

Question 49

Suppose we could examine a star and determine its absolute magnitude M without any intervening gas and dust.Compared with its absolute magnitude measured with gas and dust between us and the star,this "clean" measurement of M would be:

A) a larger number.
B) a smaller number.
C) the same number.
D) either larger or smaller depending on the star's speed (toward or away).

Question 50

What useful purpose did variable stars serve for Harlow Shapley in locating the galactic center?

A) Variable stars are important spiral arm tracers and thus defined the shape of the Milky Way Galaxy.
B) The brightness variations of variable stars allowed accurate distances to be measured.
C) Variable stars emit copious amounts of infrared radiation and are thus visible through interstellar dust that obscures visible light.
D) Variable stars are concentrated in the galactic center and so defined its direction.

Question 51

Harlow Shapley's original estimate of the distance from the Sun to the galactic center was incorrect because it did not take scattering by interstellar dust into account.Shapley's original estimate was:

A) too large because light scattering and absorption by dust made nearer stars look dim-as though they were more distant.
B) too large because scattering by dust made stars look redder-as though they were highly Doppler-shifted and thus more distant.
C) too small because gas and dust tend to reflect light into the path of the beam, thus making stars look brighter and nearer.
D) too large because scattering by dust made stars look redder-as though they were red giants of larger luminosity and thus farther away.

Question 52

When distances were carefully measured from Earth to globular clusters above and below the Milky Way plane (where our view of them is not obscured by interstellar dust and gas),the distribution of the clusters was found to be:

A) spherically symmetric about a point in the constellation Sagittarius and concentrated in that direction.
B) in a relatively flat disk almost perpendicular to the plane of the Galaxy, with a relatively higher density of clusters toward its center.
C) concentrated in the plane of the Milky Way and clustered around the Sun's position, indicating that the Sun is close to the Galaxy's center.
D) uniformly distributed throughout space, with no concentration in any area of the Milky Way.

Question 53

What would you expect to be the overall color of a globular cluster of stars? Explain.

A) blue because of the contribution from young and very hot stars in the cluster
B) red because of the emission of light by the hydrogen gas in H II regions surrounding the stars in the cluster
C) red because of the older population of stars in the cluster
D) blue because of the scattering of starlight from the dust surrounding the stars in the cluster

Question 54

The spiral-arm structure of the Milky Way Galaxy has been measured and evaluated MOST effectively by observations of:

A) globular clusters in the halo of the Galaxy.
B) Balmer emission lines of visible radiation from hydrogen.
C) 21-cm radiation from interstellar hydrogen and the distribution of young stars.
D) Lyman UV radiation from hot hydrogen gas.

Question 55

In order to emit a 21-cm photon,an atom of neutral hydrogen in interstellar space must absorb energy to place it in the excited state (the parallel spin configuration).What is the MOST likely mechanism to achieve this?

A) thermal energy
B) collisions
C) "falling" from an even higher energy state
D) cosmic rays

Question 56

The plane of our Galaxy is filled with clouds of gas and dust.To receive information from objects embedded in these clouds,astronomers use receivers sensitive in the following ranges EXCEPT:

A) ultraviolet.
B) X-ray.
C) infrared.
D) radio.

Question 57

Which type of radiation has been MOST effective in evaluating the spiral arm structure of our Galaxy?

A) 21-cm radio emission from electron "spin-flip" transitions in cool hydrogen gas
B) Lyman-α ultraviolet emission from hot hydrogen gas
C) synchrotron radiation from electrons spiraling in magnetic fields in the spiral arms
D) neutrinos from exploding stars in the spiral arms since they can easily penetrate dust and gas

Question 58

In which constellation in Earth's sky is the center of the Milky Way Galaxy located?

A) Sagittarius
B) Lyra
C) Hercules
D) Ursa Major

Question 59

What happens when the electron in a hydrogen atom flips its direction of spin from parallel to antiparallel to that of the proton?

A) Nothing. This event is a forbidden transition that never occurs.
B) The atom emits a photon of 656.3-nm wavelength (Hα) in the red region of the spectrum.
C) The atom emits a photon of 21-cm wavelength in the radio region of the spectrum.
D) The atom emits a photon of 121.5-nm wavelength (Lα) in the UV region of the spectrum.

Question 60

Harlow Shapley first located the center of the Milky Way Galaxy in 1917 by:

A) measuring the positions of supernova explosions throughout the Galaxy.
B) observing the distribution of globular clusters in the galactic halo.
C) measuring redshifts of stars in the galactic plane and disk.
D) observing the distribution of hydrogen gas, measured by 21-cm radio emission.

Question 61

When a hydrogen atom is excited by some addition of energy and it responds with electron transitions between electron energy levels,the radiation it emits is usually in which regions of the electromagnetic spectrum?

A) visible and infrared
B) visible and ultraviolet
C) radio
D) microwave

Question 62

Which component of the Milky Way Galaxy outlines the spiral arms of the Galaxy?

A) young O and B stars, dust, and gas
B) globular clusters
C) predominantly solar-type stars
D) white dwarf stars

Question 63

The 21-cm line is one of the most important wavelengths in radio astronomy.It is produced mainly in:

A) emission nebulae.
B) neutral hydrogen.
C) ionized hydrogen.
D) hot O and B stars.

Question 64

What is the motion of stars in the bar across the inner part of the Milky Way Galaxy?

A) The stars move along one side of the bar and then come back along the other side.
B) The stars oscillate back and forth along the bar, retracing the same path again and again.
C) The stars are part of a density wave which moves around the galactic center. The bar simply represents the place where stars and dust are most concentrated.
D) The stars basically remain stationary within the bar. The bar itself moves slowly around the galactic center, requiring more than 200 million years to orbit once.

Question 65

The ratio of thickness to diameter of the Milky Way Galaxy is:

A) 1/500.
B) 1/5.
C) 1/50.
D) 1/1000.

Question 66

If we aim a radio telescope at a distant spiral arm of the Milky Way Galaxy,we will probably observe a 21-cm line.If we point a large optical telescope at this same region,we will probably not be able to detect the neutral hydrogen that gives rise to the 21-cm radio signal.Why not?

A) Neutral hydrogen is incapable of emitting visible radiation.
B) The Hα line-the primary emission of neutral hydrogen-has a much smaller energy than the 21-cm radio wave and thus is harder to detect.
C) There is little energy in the depths of space to excite visible radiation from neutral hydrogen.
D) The visible light emitted by neutral hydrogen is all absorbed by H II gas regions before reaching the solar system.

Question 67

What atomic transition occurs in the atoms of hydrogen gas in the spiral arms of the Milky Way Galaxy to produce the 21-cm radio emission?

A) transition from n = 2 level to n = 1 level in atomic hydrogen
B) change in rotation of the molecule H₂ about an axis perpendicular to the molecular axis
C) change in the vibrational state of the H atoms in the H₂ molecule
D) inversion of the electron spin relative to the proton spin, from parallel to antiparallel

Question 68

When we measure the narrow line emissions of hydrogen at 21-cm radio wavelengths along a particular line of sight through the disk of the Milky Way Galaxy,we can tell the distances to different hydrogen clouds because:

A) absorption of extragalactic radiation at this wavelength is greater the farther away the absorber is from the Sun.
B) clouds that are farther away have smaller angular sizes.
C) the emission is weaker from clouds that are farther away.
D) clouds at different distances have different Doppler shifts because of the rotation of the Galaxy.

Question 69

What is the distribution of giant molecular clouds in the Milky Way Galaxy and other similar galaxies?

A) Giant molecular clouds occur primarily in the spiral arms.
B) Giant molecular clouds are distributed uniformly throughout the disk.
C) Giant molecular clouds are concentrated close to the galactic center.
D) Giant molecular clouds are distributed throughout the halo, with greater density toward the center.

Question 70

The major advantages of the 21-cm radio emission from hydrogen gas for investigating the spiral structure of our Galaxy are that:

A) it is relatively easily absorbed by hydrogen gas in the Milky Way, so measurements are not confused by emission of this radiation from other galaxies beyond the Milky Way. It originates only from cold hydrogen gas and can be used to map this important component.
B) radio waves easily penetrate the Milky Way dust and gas, and they are a very narrow-wavelength line emission; thus, its Doppler shift can be used to measure gas motions.
C) Doppler shift of this narrow-wavelength line emission is caused by the temperature of the hot hydrogen gas and therefore can be used to measure the distribution and temperature of this important component of the Milky Way.
D) this emission can easily penetrate the Milky Way gas and dust and comes only from hot gas, hence its ability to be used to map the distribution of hot hydrogen gas.

Question 71

What quantum transition occurs inside a hydrogen atom to produce a 21-cm radio photon?

A) An electron falls from the n = 100 level to the n = 99 level in the atom.
B) An electron in the ground atomic state reverses its direction of spin with respect to that of the proton.
C) An electron reverses the direction of its motion in orbit around the proton.
D) The electron combines with the proton in the nucleus to become a neutron, producing energy.

Question 72

When we look out into the plane of the Milky Way Galaxy,how far can we see?

A) infinitely far
B) about 100,000 ly
C) less than 1000 ly
D) about 10,000 ly

Question 73

A neutral hydrogen atom consists of a proton and an electron.The energy of the atom is:

A) the same, regardless of the relative spin orientations of the two particles.
B) highest when the proton spin and the electron spin are perpendicular to each other.
C) highest when the proton spin and the electron spin directions are parallel.
D) highest when the proton spin and the electron spin directions are antiparallel.

Question 74

Recent observations seem to indicate that,rather than being a spiral galaxy,the Milky Way may be:

A) a barred spiral with a definite, straight bar across its center.
B) an irregular galaxy in which matter is distributed chaotically.
C) an elliptical galaxy with little structure.
D) two elliptical galaxies colliding with each other, in view of the very active star formation within the galactic plane, brought about by the vastly increased density during the collision.

Question 75

The stellar components of the Milky Way Galaxy that act as tracers for the mapping of spiral-arm structure in the Galaxy are:

A) bright Population I stars and emission nebulae surrounding them.
B) globular clusters.
C) supernova explosions since they are very luminous and can be seen through considerable dust and gas.
D) old red giant stars and white dwarfs.

Question 76

The 21-cm line is one of the most important wavelengths in radio astronomy.The fact that it is a relatively long wavelength is important because long wavelengths:

A) are more likely to interact with clouds of gas and dust.
B) are more likely to pass through clouds of gas and dust.
C) carry more energy than short wavelengths of the same energy.
D) can be detected with greater resolution than can short wavelengths.

Question 77

We aim our radio telescope at a distant region of the Milky Way Galaxy and detect 21-cm radio waves with no Doppler shift.Which possible explanation is NOT correct?

A) The neutral hydrogen in this region is not moving relative to Earth.
B) The neutral hydrogen in this region is moving perpendicularly across our line of sight.
C) The neutral hydrogen in this region is in a circular orbit around the galactic center at the same radius as the solar system.
D) The neutral hydrogen in this region is moving away from Earth instead of toward Earth.

Question 78

Where are bright,young O and B stars MOST likely to be found in the Milky Way Galaxy?

A) inside the spiral arms
B) in globular clusters
C) in the outermost regions of the disk, where much of the "dark matter" is located
D) between spiral arms, where there is less absorbing material

Question 79

The dimensions of the disk of the Milky Way Galaxy are diameter _____,thickness _____.

A) 100,000 ly; 2000 ly
B) 10,000 ly; 28,000 ly
C) 2000 ly; 100,000 ly
D) 28,000 ly; 2000 ly

Question 80

Which statistic is NOT useful for mapping the locations and shapes of the spiral arms of our Galaxy?

A) the distribution of globular clusters
B) the distribution of O and B stars
C) the distribution of emission nebulae (H II regions)
D) the distribution of giant molecular clouds