Deck 13: Exploring Our Galaxy

Question 1

Because of the severe interstellar dust interference blocking visual wavelengths,most of our information about the galactic center comes form infrared and radio observations.

Answer 1

True

Question 2

Stars are evenly distributed across the spherical region enclosing our Galaxy.

Answer 2

False

Question 3

Only about 10% of the mass of the Galaxy is in the form of visible stars,gas,and dust.

Answer 3

True

Question 4

X-ray flares coming from Sagittarius A* dramatically brighten in just 10 minutes,which shows that the source can be no larger than 10 light minutes across.

Question 5

Approximately 10% of the mass of the Galaxy is made up of dark matter.

Question 6

All of the stars that have been observed near the supermassive black hole are being ripped apart and pulled into the event horizon to fall into the singularity.

Question 7

The Milky Way is an irregular galaxy.

Question 8

Halo stars and globular clusters move in orbits that are oriented at random angles to the plane of the disk of a galaxy.

Question 9

One proposal for the mysterious dark matter in the Galaxy is that it is made of massive compact halo objects (MACHOs).

Question 10

Dark streaks across the Milky Way from the viewpoint of Earth are caused by interstellar dust in the plane of our Galaxy.

Question 11

In a spin-flip transition of an electron,a photon is emitted.

Question 12

The halo of the Galaxy is a spherical distribution that is traced by globular clusters.

Question 13

The Galaxy rotates like a solid disk,like a CD or DVD.

Question 14

Flocculent spiral galaxies have spiral arms that are broad,fuzzy,chaotic,and poorly defined and grand-design spiral galaxies have thin,graceful,well-defined spiral arms.

Question 15

Stars in the disk of the Galaxy are mostly old,metal-poor,Population II stars.

Question 16

The 21-cm emission is used mainly to map out globular clusters around the Galaxy.

Question 17

When protons and electrons in neutral hydrogen spin,they act like tiny magnets.

Question 18

Visible light passes more easily though dust and gas than infrared light in our Galaxy,revealing structures.

Question 19

The Sun takes about 220 million years to complete one trip around the Galaxy.

Question 20

RR Lyrae variable stars are bigger,brighter and have longer periods of pulsation than Cepheid variables.

Question 21

In the 1790s,Sir William Herschel tried to measure the Sun's position in our Galaxy by

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

Question 22

"Standard candles," which are important for finding distances to remote galaxies,are

A) standard laboratory light sources with which the brightness of a galaxy can be compared.
B) heat sources used for calibrating infrared observations of galaxies.
C) stars and other objects of known intrinsic brightness.
D) laboratory light sources with a well-known spectrum for calibration of stellar and galactic spectra.

Question 23

Which component of our Galaxy accounts for interstellar extinction-the dimming of light from distant objects?

A) Molecules such as H₂ and CO,which are strong absorbers,in molecular clouds
B) Dust
C) The so-called dark matter,because its absorbing properties render it invisible in the Galaxy
D) Cool hydrogen gas

Question 24

Who was the first to look at the Milky Way with a telescope?

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

Question 25

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 26

How would astronomers,ancient or modern,use a "standard candle"?

A) They would use its faint light to illuminate the setting dials of their telescopes.
B) They would use it to measure time,using its slow but steady melting.
C) They would use it to generate a known and extremely reproducible standard spectrum for comparison with stellar spectra.
D) They would measure distance by comparing its apparent brightness with its known absolute brightness.

Question 27

Why do the spiral arms show up so clearly in spiral galaxies?

A) Stars are spread uniformly over the galaxy but the dust forms a spiral pattern,absorbing starlight;the spiral arms are the dust-free regions between the dust lanes.
B) Stars are spread almost uniformly throughout the disk of the galaxy outside the nuclear bulge,but the brightest stars occur only in the spiral arms,making the arms stand out.
C) There are many more stars in the arms than in the regions between the arms,so the arms stand out distinctly.
D) Stars occur only in the spiral arms (and the nuclear bulge),with essentially none between the arms,making the arms stand out brightly.

Question 28

To an astronomer,what is a "standard candle"?

A) A standard light source that can be placed in a telescope,to which the brightness of stars and other objects can be compared
B) Any type of object whose absolute magnitude is known
C) Any galaxy whose redshift has been measured accurately
D) An accurately defined brightness scale for stars and galaxies,such as the magnitude scale

Question 29

Why are we able to see only a small part of our Galaxy,the Milky Way Galaxy?

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

Question 30

The Milky Way,in which the Sun resides,is an example of which type of galaxy?

A) An elliptical galaxy
B) A normal spiral galaxy
C) A barred spiral galaxy
D) An irregular galaxy

Question 31

What does a spiral galaxy look like when seen edge-on?

A) Round,but without spiral arms because they are hidden
B) A thick line curved into a spiral shape
C) A thick,flat line with a bulge in the center
D) A thick,flat line

Question 32

The significant feature of a Cepheid variable is that there is a relationship between two intrinsic parameters,the first being easily measured,the second being the parameter that is required.These parameters are

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

Question 33

The Milky Way is one of myriad

A) solar systems.
B) galaxies.
C) planets.
D) stars.

Question 34

In determining the distance to a galaxy by using observations of a Cepheid variable star,which of the following is NOT needed?

A) The star's velocity via the Doppler effect
B) The Cepheid's period of variability
C) The type of spectrum of the Cepheid: metal-rich or metal-poor
D) The star's average brightness or apparent magnitude

Question 35

Interstellar dust obscures our view of distant regions of space at optical wavelengths.Which of the following statements is TRUE concerning this dust obscuration?

A) Distant regions are severely obscured only in the plane of the Galaxy.
B) The obscuration of distant regions of space is roughly uniformly over the whole sky.
C) Distant regions are obscured randomly over the whole sky;individual,absorbing dust clouds show no preference for one particular direction or plane.
D) Distant regions are obscured the least in the plane of the Galaxy,and are strongest when we look out into the galactic halo,at right angles to this plane.

Question 36

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),their distribution was found to be

A) in a relatively flat disk almost perpendicular to the plane of the Galaxy,with relatively higher density of clusters toward its center.
B) spherically symmetric about a point in the constellation Sagittarius and concentrated in that direction.
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 Galaxy.

Question 37

What is the basic shape of a spiral galaxy?

A) A round,thin disk of uniform brightness with its edges bent up and down into a spiral shape
B) A round,flat disk with long lanes of stars that curve outward in a spiral shape from the very center of the galaxy
C) Approximately spherical with long lanes of dark dust clouds curving through it in a spiral pattern
D) A round,flat disk containing long lanes of stars that curve outward in a spiral shape from the edge of a round,nuclear region of uniform brightness

Question 38

Cepheid stars are useful to astronomers as indicators of

A) white dwarf star behavior.
B) distance,particularly to stars in our Galaxy and to nearby galaxies.
C) stars with very high-speed motion.
D) the mechanics of eclipsing variable stars.

Question 39

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

A) Keeping of accurate time
B) Measurement of the gravitational fields of supermassive black holes by their effect upon the star's pulsation rate
C) Measurement of the surface temperatures of stars
D) Measurement of distances to galaxies

Question 40

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

A) Herschel counted all stars in each star field,including many that were outside our Galaxy,thus confusing the distribution.
B) the redshift of the more distant stars made them invisible to Herschel.
C) he had no knowledge of the large quantity of dust between stars,which obscured the more distant regions of the Galaxy.
D) he mistook globular clusters for stars,and those are distributed uniformly around the Sun.

Question 41

In which spin configuration is the energy of the hydrogen atom GREATEST?

A) When the proton spin and the electron spin are in the same direction
B) When the proton spin and the electron spin are perpendicular to each other
C) When the proton spin and the electron spin are in opposite directions
D) All of these spin configurations have the same energy;none is greatest.

Question 42

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

Question 43

Interstellar matter blocks our view of the disk of our Galaxy

A) not at all at any wavelength.
B) more-or-less equally at all wavelengths from radio waves to light waves.
C) most at radio wavelengths,where hydrogen absorbs radio waves efficiently,and least at optical wavelengths.
D) more at optical wavelengths,less in the infrared,and not at all at radio wavelengths.

Question 44

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 45

Where in space would you look for a globular cluster?

A) In the Milky Way halo,orbiting the galactic center in a long elliptical orbit
B) In the galactic disk,moving in a circular orbit around the galactic center
C) In the asteroid belt
D) Only in elliptical galaxies,because they are composed of old stars

Question 46

Harlow Shapley's original estimate of the distance from the Sun to the galactic center was incorrect because it did not take interstellar extinction into account.This 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 back 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 47

The 21-centimeter line is one of the most important wavelengths in radio astronomy.Its 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 information than short wavelengths of the same energy.
D) can be detected with greater resolution than short wavelengths.

Question 48

The Milky Way is an example of which type of galaxy?

A) Irregular
B) Elliptical
C) Lenticular (S0)
D) Spiral

Question 49

If you were able to view the entire Milky Way Galaxy through a filter,which passed only near-infrared wavelengths,what would you see?

A) Billions of points of light representing the near-infrared emissions of the stars
B) Relatively few points of light representing the near-infrared emission of newly formed stars
C) A bright glow from the dust in the plane of the Galaxy
D) Almost nothing,because the Galaxy emits almost no near-infrared radiation

Question 50

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

A) Their brightness variations allowed accurate distances to be measured.
B) They emit copious amounts of infrared radiation and are thus visible through interstellar dust,which obscured visible light.
C) They are concentrated in the galactic center and so defined its direction.
D) They are important spiral arm tracers and thus defined the shape of the Galaxy.

Question 51

The method used by Harlow Shapley in 1917 to estimate the Sun's location in our 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 comparing it with the structure of our Galaxy.

Question 52

In our Galaxy,young metal-rich stars are found

A) in the disk and spiral arms.
B) everywhere in the Galaxy.
C) only at the galactic center.
D) in the globular clusters,in the galactic halo.

Question 53

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

A) Ionized atomic hydrogen
B) Neutral atomic hydrogen
C) Carbon monoxide,CO
D) Molecular hydrogen,H₂

Question 54

The dimensions of the disk of our Milky Way Galaxy are

A) diameter,80,000 light-years;thickness,6500 light-years.
B) diameter,6500 light-years;thickness 2000 light-years.
C) diameter,2000 light-years;thickness,160,000 light-years.
D) diameter,160,000 light-years;thickness,2000 light-years.

Question 55

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

A) 1/1000.
B) 1/3.
C) 1/80.
D) 1/40.

Question 56

Where would you look in our Galaxy to find older,metal-poor stars?

A) In the disk and spiral arms
B) Everywhere in the Galaxy
C) In the globular clusters in the galactic halo
D) Only at the galactic center

Question 57

Which of the following types of stars are not found in the halo of our Galaxy?

A) O-type stars
B) Population II stars
C) K-type stars
D) M-type stars

Question 58

The 21-centimeter 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 59

Where in the Milky Way would you expect most star formation to be taking place?

A) Globular clusters
B) The halo
C) The disk and spiral arms
D) The bulge

Question 60

Where is the solar system located in our Galaxy?

A) It is not in a galaxy,but in intergalactic space.
B) In the galactic halo
C) In the galactic disk
D) In the galactic nucleus

Question 61

What do we know about the geometry of the dark matter distribution in the vicinity of the Milky Way Galaxy?

A) It appears to be a lens-shaped disk like the Galaxy's disk of visible stars,only larger.
B) It appears to be a spherical halo.
C) Dark matter,including that near the Milky Way Galaxy,appears to be part of a uniform density distribution,which fills all of space.
D) The geometry of the dark matter distribution,like its constituents,is totally unknown at the present time.

Question 62

Which kind of stars are the major source of energy for the heating of the dust clouds and the H II emission nebulae within the planes of the Milky Way and other galaxies?

A) Hot,young O and B stars,via their UV radiation
B) The numerous old,red-giant K and M stars,via their IR heat radiation
C) Very hot white dwarf stars,the remnants of planetary nebulae in the gas clouds
D) The very many nova and supernova explosions of stars within the gas and dust clouds

Question 63

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

A) clouds at different distances have different Doppler shifts due to the rotation of the Galaxy.
B) the farther away the gas cloud,the greater the delay in the arrival time of the 21-cm emission.
C) clouds that are farther away have smaller angular sizes.
D) the emission is weaker from clouds that are farther away.

Question 64

The major advantage of the measurement of the 21-cm radio emission from hydrogen gas for investigating the spiral structure of our Galaxy is that

A) 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.
B) it is relatively easily absorbed by hydrogen gas in the Milky Way so that 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.
C) this emission can easily penetrate the Milky Way gas and dust and comes only from hot gas;hence,it can be used to map the distribution of hot hydrogen gas.
D) radio waves easily penetrate the Milky Way dust and gas;it is a very narrow wavelength,thus its Doppler shift can be used to measure gas motions.

Question 65

The density wave that produces the spiral arm in the Milky Way Galaxy is similar in properties to a

A) light wave.
B) wave on a stretched string.
C) compression wave.
D) gravitational wave.

Question 66

Which of the following has NOT been proposed as a candidate for dark matter?

A) Massive compact halo objects (MACHOs)
B) Dark nebulae
C) Weakly interacting massive particles (WIMPS)
D) Neutrinos

Question 67

What happens when the electron in a hydrogen atom flips its direction of spin,starting parallel to the spin of the proton and ending antiparallel to the spin of the proton?

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

Question 68

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

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

Question 69

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

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

Question 70

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

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

Question 71

How is the mass of the Galaxy estimated?

A) By observing the bending of light from distant galaxies as it passes near the Milky Way center
B) By observing its movement toward neighboring galaxies due to gravitational attraction
C) By counting stars and assuming an average stellar mass
D) By applying Newton's extension of Kepler's laws to the motion of the Sun and other stars

Question 72

The possible presence of a very large amount of unseen ("dark")matter in the halo of our Galaxy is deduced from the

A) unexpected absence of luminous matter (stars,etc. )beyond a certain distance.
B) rotation curve of our Galaxy,which indicates higher than expected orbital speeds in the outer regions of the Galaxy.
C) rotation curve of our Galaxy,which shows that orbital speeds in the outer parts of the Galaxy decrease in a way that follows Kepler's law.
D) unexpected high amount of interstellar absorption in certain directions.

Question 73

Which of the following 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 molecular clouds

Question 74

Astronomers have found the existence of spiral arms in galaxies difficult to account for because the

A) inner part of a galaxy rotates in a shorter time than the outer parts,so the arms should have wound up so tightly that they would have disappeared over the lifetime of the galaxy.
B) arms should have been destroyed by collisions with other galaxies over the galaxy's lifetime.
C) outer regions of a galaxy including the spiral arms have no significant rotation,so the arms should have fallen into the center of the galaxy.
D) outer parts of a galaxy rotate faster than the inner parts,so the arms should have straightened out into spokes like those of a bicycle wheel.

Question 75

In the self-propagating star formation theory,

A) spiral arms are formed and sustained primarily by the actions of solar mass stars because there are so many more solar mass stars than high-mass stars.
B) spiral arms are poorly defined because they are formed and sustained primarily by the actions of short-lived massive stars.
C) the passage of dust clouds through the spiral arms triggers star formation.
D) spiral structure results with thin,well-defined arms.

Question 76

Recent observations suggest that the Milky Way may

A) be an irregular galaxy,with chaotic distribution of matter within it.
B) have vast stellar streams in a plane perpendicular to the galaxy's disk,formed by a collision of the Milky Way with another large spiral galaxy.
C) be an elliptical galaxy,with little structure.
D) have a central bar,formed by an elongation of the central bulge.

Question 77

Which of the following components of the Galaxy best outline 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 78

What fraction of the mass of our Galaxy is in a form that we have been able to see?

A) About 10%
B) 100%-all matter can be seen.
C) About 50%
D) About 90%

Question 79

Much of the mass of our Galaxy appears to be in the form of "dark matter" of unknown composition.At present,this matter can be detected only because

A) it absorbs light from distant galaxies and quasars and obscures them.
B) it emits synchrotron radiation.
C) its gravitational pull affects orbital motions of matter in the Galaxy.
D) it blocks out the light from distant stars in the plane of our Galaxy.

Question 80

Which two parameters of star motion in the Milky Way are represented by its rotation curve?

A) Orbital speed of the stars as a function of their age
B) Orbital period of the stars as a function of their distance from the galactic center
C) Orbital speed as a function of star distance from the galactic center
D) Star position above or below the galactic plane as a function of distance from the galactic center