Deck 15: Galaxies and the Foundation of Modern Cosmology

Question 1

Which of the following types of galaxies have a disk and spheroidal component but lack spiral arms?

A) ellipticals
B) lenticulars
C) irregulars
D) all of the above

Answer 1

B

Question 2

Which of the following types of galaxies appear reddest in color?

A) spirals
B) ellipticals
C) lenticulars
D) irregulars

Answer 2

B

Question 3

What is a standard candle?

A) an object for which we are likely to know its true luminosity
B) an object for which we can easily measure its apparent brightness
C) a class of objects that we know all have exactly the same luminosity
D) any star for which we know its exact apparent brightness
E) a unit of luminosity used by astronomers

Answer 3

A

Question 4

Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately

A) 100 million.
B) 1 billion.
C) 10 billion.
D) 100 billion.
E) 1 trillion.

Question 5

Suppose that we look at a photograph of many galaxies. Assuming that all galaxies formed at about the same time, which galaxy in the picture is the youngest?

A) the one that is reddest in color
B) the one that is bluest in color
C) the one that is farthest away
D) the one that is closest to us
E) the one that appears smallest in size

Question 6

What two quantities did Edwin Hubble compare for a sample of galaxies to discover the expansion of the Universe?

A) velocity and distance
B) luminosity and distance
C) velocity and temperature
D) luminosity and temperature
E) age and distance

Question 7

Which types of galaxies have a clearly defined spheroidal component?

A) spirals only
B) ellipticals only
C) lenticulars only
D) irregulars only
E) all but irregulars

Question 8

How was Edwin Hubble able to use his discovery of Cepheids in Andromeda to prove that the "spiral nebulae" were actually galaxies external to the Milky Way?

A) There are no Cepheids in the Milky Way, so his discovery proved that Andromeda had to be in another galaxy.
B) He measured the parallaxes of the Cepheids in Andromeda to determine their distances. He showed that Andromeda was far outside the Milky Way Galaxy.
C) He used main-sequence fitting to determine the distances to the Cepheids. He showed that Andromeda was far outside the Milky Way Galaxy.
D) From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda. He showed that it was far outside the Milky Way Galaxy.
E) As a Cepheid is a type of luminous galaxy, Hubble's discovery of Cepheids in Andromeda proved that it was a separate galaxy from the Milky Way.

Question 9

Which of the following is true about irregular galaxies?

A) They are composed solely of old stars.
B) They have significant spheroidal component.
C) They were more common when the universe was younger.
D) They have reddish colors.
E) They have well defined spiral arms.

Question 10

Why are Cepheid variables important?

A) Cepheid variables are stars that vary in brightness because they harbor a black hole. Therefore, they provide direct evidence for black holes.
B) Cepheids variables are pulsating stars whose pulsation periods are directly related to their true luminosities. Therefore they can be used as distance indicators.
C) Cepheids variables are a type of irregular galaxy, much more common in the early universe. Therefore they help to understand how galaxies formed.
D) Cepheids variables are supermassive stars that are on the verge of becoming supernovae. Therefore they allow us to choose candidates to watch if we hope to observe a supernova.

Question 11

How did Edwin Hubble measure the distance to the Andromeda Galaxy?

A) He measured its parallax.
B) He applied main-sequence fitting to star clusters in Andromeda.
C) He applied the period-luminosity relation to Cepheid variables in Andromeda.
D) He measured its redshift and applied Hubble's Law.
E) He detected white dwarf supernovae in Andromeda.

Question 12

What is the major difference between an elliptical galaxy and a spiral galaxy?

A) A spiral galaxy contains mostly younger stars.
B) A spiral galaxy has a spherical halo.
C) An elliptical galaxy lacks a disk component.
D) Elliptical galaxies are not as big as spiral galaxies.
E) There are no dwarf spiral galaxies, but there are dwarf ellipticals.

Question 13

Why is the Hyades Cluster important for building up a catalog of the true luminosities of main-sequence stars?

A) It is an extremely bright star cluster.
B) It is close enough to us that the distance to its stars can be found by stellar parallax.
C) It is an old globular cluster whose age we can determine very accurately by measuring its main-sequence turn-off point.
D) It's a cluster that has been photographed over many decades, so we know how the stars vary in brightness.
E) It contains many Cepheid variable stars that can be used to determine its distance.

Question 14

The disk component of a spiral galaxy includes which of the following parts?

A) halo
B) bulge
C) spiral arms
D) globular clusters
E) all of the above

Question 15

What is Hubble's law?

A) The luminosity of the Cepheid variable star is directly proportional to its pulsation period.
B) The recession velocity of a galaxy is directly proportional to its distance.
C) The recession velocity of a galaxy is inversely proportional to its distance.
D) The faster a spiral galaxy's rotation speed, the more luminous it is.
E) The faster a spiral galaxy's rotation speed, the less luminous it is.

Question 16

Most large galaxies in the universe are

A) elliptical.
B) spiral or lenticular.
C) irregular.
D) abnormal.

Question 17

How does a lenticular galaxy differ from a normal spiral galaxy?

A) It has no bulge.
B) It has an elongated bulge resembling a bar more than a sphere.
C) It is flatter in shape.
D) It has no gas or dust.
E) It has no spiral arms.

Question 18

Which types of galaxies have a clearly defined disk component?

A) spirals only
B) ellipticals only
C) lenticulars only
D) irregulars only
E) spirals and lenticulars

Question 19

Compared to spiral galaxies, elliptical galaxies are

A) redder and rounder.
B) redder and flattened.
C) bluer and rounder.
D) bluer and flattened.
E) always much smaller.

Question 20

Approximately how many stars does a dwarf elliptical galaxy have?

A) 1 trillion
B) 100 billion
C) 10 billion
D) less than a billion
E) less than a million

Question 21

What is the most accurate way to determine the distance to a nearby star?

A) radar ranging
B) stellar parallax
C) main-sequence fitting
D) Cepheid variables
E) Hubble's law

Question 22

Hubble's "constant" is constant in

A) time.
B) space.
C) space and time.
D) our Galaxy, but it is different in others.

Question 23

Which of the following gives the two main assumptions of theoretical models of galaxy evolution?

A) The beginning of the universe is modeled after a supernova explosion, and all elements were produced by this exploding star.
B) Hydrogen and helium filled all of space, and certain regions of the universe were slightly denser than others.
C) Hydrogen and helium filled all of space, and the entire universe had exactly the same density everywhere.
D) The universe has always been expanding, and denser areas contracted to form the first stars.
E) The universe was composed originally only of hydrogen, and all the other elements came from stars.

Question 24

Dr. Smith believes that the Hubble constant is H₀ = 25 km/s/Mly while Dr. Jones believes it is H₀ = 19 km/s/Mly. Which statement below automatically follows?

A) Dr. Smith believes that the universe is expanding, but Dr. Jones does not.
B) Dr. Smith believes that the Andromeda Galaxy (a member of our Local Group) is moving away from us at a faster speed than Dr. Jones believes.
C) Dr. Smith believes that the universe is older than Dr. Jones believes.
D) Dr. Smith believes that the universe is younger than Dr. Jones believes.
E) Dr. Smith believes that the universe will someday stop expanding, while Dr. Jones believes it will expand forever.

Question 25

Why can't we see past the cosmological horizon?

A) The universe extends only to this horizon.
B) Beyond the cosmological horizon, we are looking back to a time before the universe had formed.
C) We do not have telescopes big enough.
D) We do not have detectors sensitive enough.
E) The cosmological horizon is infinitely far away, and we can't see to infinity.

Question 26

If we represent the Milky Way Galaxy as the size of a grapefruit, the distance to the Andromeda Galaxy would be about

A) 3 cm.
B) 3 m.
C) 30 m.
D) 3 km.
E) 300 km.

Question 27

Why is a dense cloud more likely to produce an elliptical galaxy than a spiral galaxy?

A) The higher density of gas has a stronger force of gravity, and therefore the cloud collapses more quickly.
B) The force of gravity can pull the material into a more spherical shape.
C) The more frequent collisions between particles randomize the particle orbits.
D) The thickness of the dense cloud prevents a disk from forming.
E) The higher gas density forms stars more efficiently, so all the gas is converted into stars before a disk can form.

Question 28

What is the most accurate way to determine the distance to a very distant galaxy?

A) main-sequence fitting
B) Stellar parallax
C) Cepheid variables
D) white dwarf supernova
E) Hubble's law

Question 29

Which of the following types of protogalactic clouds is most likely to form an elliptical galaxy?

A) a very low-density cloud with very little angular momentum
B) a dense cloud with very little angular momentum
C) a low-density cloud with quite a bit of angular momentum
D) a dense cloud with quite a bit of angular momentum
E) a very massive cloud with any density and a lot of angular momentum

Question 30

Which of the following is a consequence of Hubble's Law?

A) the Big Bang
B) all galaxies are moving away from us equally fast
C) the more distant a galaxy is from us, the faster it moves away from us
D) the closer a galaxy is to us, the faster it moves away from us
E) more distant galaxies appear younger

Question 31

What makes white-dwarf supernovae very good standard candles for distance measurements?

A) They are very bright, so they can be used to determine the distances to galaxies billions of light-years away.
B) They should all have approximately the same luminosity.
C) They occur so frequently that we can use them to measure the distances to virtually all galaxies.
D) We have had several occur close to us in the Milky Way, so we have been able to determine their luminosities very accurately.
E) both A and B

Question 32

What is the most accurate way to determine the distance to a nearby galaxy?

A) radar ranging
B) stellar parallax
C) main sequence fitting
D) Cepheid variables
E) Hubble's law

Question 33

Dr. X believes that the Hubble constant is H₀ = 20 km/s/Mly while Dr. Y believes it is H₀ = 24 km/s/Mly. Which statement below automatically follows?

A) Dr. X believes that the universe is expanding, but Dr. Y does not.
B) Dr. X believes that the Andromeda Galaxy (a member of our Local Group) is moving away from us at a slower speed than Dr. Y believes.
C) Dr. X believes that the universe is older than Dr. Y believes.
D) Dr. X believes that the universe will someday stop expanding, while Dr. Y believes it will expand forever.
E) Dr. X believes that the universe has a much higher density than Dr. Y believes.

Question 34

Recall that Hubble's law is written v = H₀d, where v is the recession velocity of a galaxy located a distance d away from us, and H₀ is Hubble's constant. Suppose H₀ = 20 km/s/Mly. How fast would a galaxy 1000 Mly distant be receding from us?

A) 20 km/s
B) 20 Mly/s
C) 20,000 km/s
D) 50 km/s
E) 0.20 times the speed of light

Question 35

Which of the following processes slowed the collapse of protogalactic clouds?

A) the formation of the first generation of stars
B) the conversion of gravitational potential energy into kinetic and thermal energy as the cloud collapsed
C) the shock waves from the exploding supernovae of the earliest stars
D) the pull of gravity of the mass of the cloud material
E) the radiating away of thermal energy

Question 36

Based on current estimates of the value of Hubble's constant, how old is the universe?

A) 4 to 8 billion years old
B) 8 to 12 billion years old
C) 12 to 15 billion years old
D) 15 to 20 billion years old
E) More than 20 billion years old

Question 37

Scale the Milky Way down to the size of a compact disc. Which of the following best describes the size, shape, and distance of the Andromeda Galaxy on the same scale?

A) a compact disk a few meters away
B) a grapefruit a few meters away
C) a compact disk about the length of a football field away
D) a pecan nut about 1 centimeter away
E) a grapefruit about 1 kilometer away

Question 38

I observe a galaxy that is 100 million light years away. Which of the following do I see?

A) the light from the galaxy as it is today, and it is blueshifted
B) the light from the galaxy as it is today, and it is redshifted
C) the light from the galaxy as it was 100 million years ago, and it is blueshifted
D) the light from the galaxy as it was 100 million years ago, and it is redshifted
E) Nothing: the galaxy lies beyond the cosmological horizon.

Question 39

How do observations of galaxies at different distances help us learn about galaxy evolution?

A) Observations of different distances show galaxies of different ages and therefore different stages of evolution.
B) Observations of the most distant galaxies will allow us to observe the birth of galaxies.
C) We can observe the evolution of a single galaxy over time.
D) We can observe galaxies at different distances merge, helping us learn how mergers affect evolution.
E) We can see what our galaxy used to look like over time, helping us theorize about the physical processes that led to its current appearance.

Question 40

Which of the following is not a good reason why white-dwarf supernovae are good standard candles for distance measurements?

A) All white-dwarf supernovae involve the explosion of stars of nearly the same mass.
B) White-dwarf supernovae are so bright that they can be detected even in very distant galaxies.
C) White-dwarf supernovae are common enough that we detect several every year.
D) White-dwarf supernovae occur only among young and extremely bright stars.
E) All white-dwarf supernovae have similar light curves, which makes them easy to distinguish from massive-star supernovae.

Question 41

What evidence suggests that the protogalactic cloud that formed the Milky Way resulted from several collisions among smaller clouds?

A) The stars in the halo of the Milky Way are organized into several dense clusters arranged throughout the halo.
B) The Milky Way resembles an elliptical galaxy more than other spirals do.
C) Halo stars differ in age and heavy-element content, but these variations do not seem to depend on the stars' distance from the galactic center.
D) The bulge of the Milky Way is surrounded by many globular clusters, just as elliptical galaxies are.
E) The Milky Way is the central galaxy of a dense cluster of galaxies.

Question 42

Why should galaxy collisions have been more common in the past than they are today?

A) Galaxies were more active in the past and therefore would have collided with each other more frequently.
B) Galaxies were much bigger in the past because they had not contracted completely.
C) Galaxies were closer together in the past because the universe was smaller.
D) Galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars and light.
E) Galaxy collisions shouldn't have been more common in the past than they are now.

Question 43

What is a quasar?

A) a star-like object that actually represents a bright patch of gas in the Milky Way
B) a very large galaxy thought to be formed by the merger of several smaller galaxies, resulting in a quick burst of star formation
C) The name given to the largest objects in the Kuiper Belt
D) the extremely bright center of a distant galaxy, thought to be powered by a supermassive black hole
E) another name for very bright stars of spectral type O

Question 44

Which of the following is not a piece of evidence supporting the conclusion that active galactic nuclei are powered by accretion disks around massive black holes?

A) Observed radiation from the galactic center can vary significantly in brightness in times as short as a few days.
B) Infrared observations show that many stars are forming near the centers of active galaxies.
C) Radio observations sometimes show long jets of material that can extend millions of light-years out from the galactic center.
D) The total amount of radiation coming from the galactic center can, in some cases, exceed 100 times the total luminosity of the Milky Way Galaxy.
E) Spectral lines from the galactic center indicate that clouds of gas are orbiting a central object at very high speed.

Question 45

Suppose we observe a source of X-rays that varies substantially in brightness over a period of a few days. What can we conclude?

A) The X-ray source must be a quasar.
B) The X-ray source must contain a black hole with an accretion disk.
C) The X-ray source can be no more than a few light-days in diameter.
D) The X-ray source must have a strong, rapidly varying magnetic field.
E) We must seeing the rapid orbit of two stars in a binary system.

Question 46

Spiral galaxies have more gas, dust, and young stars than elliptical galaxies.

Question 47

If an object doubles its luminosity in 10 hours, how large can the emitting source of light be?

A) about 10 light-years across
B) about 10 parsecs across
C) about 10 light-hours across
D) varies depending on how luminous the object is
E) varies depending on how far away the object is

Question 48

Which of the following is evidence for supermassive black holes in active galaxies?

A) the discovery of powerful jets coming from a compact core
B) rapid changes in the luminosity of the galaxy nucleus
C) quasars emit approximately equal power at all wavelengths from infrared to gamma rays
D) the very high speeds of gas orbiting around the galactic nucleus
E) all of the above

Question 49

Why do we believe that starburst galaxies represent a temporary stage in galaxy evolution?

A) We observe starbursts to last only a few years at a time.
B) Such galaxies form stars at such a high rate that they would have consumed all their gas long ago if they had always been forming stars at this high rate.
C) We don't see any nearby starburst galaxies.
D) All starburst galaxies look like normal spiral galaxies, aside from the starbursts.

Question 50

Which of the following cannot be true of the very first stars formed in the Universe?

A) Some may have all exploded as supernovae by now.
B) Some may have formed in large clusters.
C) Some may have formed singly, in isolation from one another.
D) Some may still exist in the Milky Way today.
E) Some may have had rocky planets around them.

Question 51

All spiral galaxies have both a disk and spheroidal component to their stellar populations.

Question 52

Which of the following is not a strong argument for the theory that some large elliptical galaxies formed as the result of galaxy collisions?

A) Elliptical galaxies dominate the population in dense galaxy clusters.
B) Some ellipticals have stars and gas that rotate opposite to the rest of the galaxy.
C) Some elliptical galaxies are surrounded by shells of stars.
D) Computer simulations predict that the product of a galaxy collision is generally an elliptical galaxy.
E) Galaxy collisions are common and most galaxies in the universe are elliptical.

Question 53

Which of the following is not true of quasars?

A) Some quasars are more than a thousand times more luminous than the Milky Way.
B) Quasars were more common in the past.
C) The light produced by quasars comes from an accretion disk surrounding a supermassive black hole.
D) Quasars are powered by the intense production of large numbers of stars that can only be sustained for a relatively short time.
E) Some quasars can change their brightness on the timescale of hours.

Question 54

In the 1960s, Maarten Schmidt determined that quasars were very distant objects by

A) determining how luminous they were.
B) determining how small the source of light was from its luminosity variations.
C) discovering that they were embedded in distant galaxies.
D) determining their redshifts.
E) determining their parallax angles.

Question 55

How is the energy that powers radio galaxies, quasars, and other active galactic nuclei produced?

A) by nuclear fusion near a central black hole
B) by magnetic fields that trap and accelerate charged particles, which then radiate large amounts of energy
C) by matter that has been converted to pure energy by interacting with the jets shot out by a central black hole
D) by gravity, which converts the potential energy of matter falling toward a central black hole into kinetic energy, which is then converted to thermal energy by collisions among the particles of matter
E) by matter-antimatter annihilation near a central black hole

Question 56

How many more stars does a starburst galaxy form, in one year, than the Milky Way?

A) a few
B) about ten
C) about a hundred
D) about a thousand
E) about the same, but it does so for much longer

Question 57

Most active galactic nuclei are at large distances from us; relatively few nearby galaxies harbor active galactic nuclei. What does this imply?

A) Massive black holes existed only when the universe was young and no longer exist today.
B) Active galactic nuclei tend to become less active as they age.
C) Active galactic nuclei can form only at large distances from the Milky Way.
D) The jets seen in many active galactic nuclei must cause them to move far away from us.

Question 58

What is a central dominant galaxy?

A) a galaxy around which many other smaller galaxies orbit
B) a giant spiral galaxy that exerts large tidal forces on other nearby galaxies
C) a spiral galaxy from which many smaller galaxies form when it is stripped apart by tidal forces
D) a giant elliptical galaxy at the center of a dense cluster
E) a hypothesized galaxy type that no longer exists but once dominated the structure of the universe

Question 59

What evidence supports the idea that a collision between two spiral galaxies might lead to the formation of a single elliptical galaxy?

A) observations of some elliptical galaxies surrounded by shells of stars that probably formed from stars stripped out of smaller galaxies
B) the fact that elliptical galaxies dominate the galaxy populations at the cores of dense clusters of galaxies
C) observations of some elliptical galaxies with stars and gas clouds in their cores that orbit differently from the other stars in the galaxy
D) observations of giant elliptical galaxies at the center of dense clusters that may have grown by consuming other galaxies
E) all of the above

Question 60

Where are the X-rays produced that are emitted by quasars and other active galactic nuclei?

A) in the hot gas in an accretion disk around a central black hole
B) in an ionization nebulae of interstellar gas surrounding the accretion disk
C) in dust grains in molecular clouds that encircle the active galactic nucleus
D) in fast-moving jets of electrons that are ejected from the active galactic nucleus
E) all of the above

Question 61

Does It Matter What the Universe Expands Into? A natural question of anyone who meets Hubble's Law for the first time is to ask what exactly the universe is expanding into. This is often further motivated by the analogy of an inflating balloon universe, where two-dimensional galaxies ride along on the surface of the balloon. Imagine, that you live in one of these balloon-borne galaxies. Your universe is the surface of the balloon. You can move along the surface, but you can never exist above or below it. Now one of your two-dimensional friends asks what your (balloon) space is expanding into. Do you need to answer this question, or can the expansion you experience be completely described without reference to an unseen, third-dimension? Imagine someone argues that question "what is the universe expanding into?" does not even need an answer! Would you agree or disagree? Explain. If you disagree, would there be any way to gather scientific evidence to answer this question?

Question 62

Quasars radiate most of their energy as radio emission.

Question 63

Summarize the links in the distance chain that allow us to estimate distances to the farthest reaches of the universe.

Question 64

The larger the value of Hubble's constant, the more rapid the expansion of the universe and the smaller (and hence younger) our estimate for the age of the universe.

Question 65

List at least three qualities that make an astronomical object a good standard candle.

Question 66

There are more large spiral galaxies than there are large elliptical galaxies.

Question 67

A protogalactic cloud with very little angular momentum is more likely to form an elliptical galaxy than a spiral galaxy.

Question 68

Galaxy mergers take hundreds of millions of years to complete.

Question 69

Galaxy collisions were more common in the past.

Question 70

Elliptical galaxies are more likely to be found in clusters than spiral galaxies.

Question 71

Massive-star supernovae and white-dwarf supernovae work equally well as standard candles for measuring cosmic distances.

Question 72

The Redshift Controversy: The early debate on whether or not quasar redshifts were correctly interpreted by Hubble's Law (to indicate that quasars lay at enormous distances) is an excellent example of how a scientific debate was (reasonably) quickly settled by appeals to the observational evidence. Outline all of the observational evidence that quasars are indeed associated with the nuclei of distant galaxies and, therefore, their redshifts can be correctly interpreted using Hubble's Law.

Question 73

Stars continue to form in the halo of our Galaxy today.

Question 74

A protogalactic cloud with slow star formation is more likely to form a spiral galaxy than an elliptical galaxy.

Question 75

Supermassive Black Holes: A Scientific Theory? Summarize all of the available evidence that active galaxies are powered by supermassive black holes. Do your best to assemble enough evidence that this explanation should be considered a scientific theory in the sense outlined at the end of Section 3.4 (The Nature of Science) of Chapter 3. Do you think astronomers have met this challenge?

Question 76

Distant galaxies are more likely to be irregular in shape than galaxies closer to the Milky Way.

Question 77

The collision of two spiral galaxies will likely result in a single giant spiral galaxy.

Question 78

A lenticular galaxy is another name for an elongated elliptical galaxy.

Question 79

Explain how we can estimate that there are about 50-100 billion galaxies in the observable universe.

Question 80

The black hole mass at the center of a galaxy is generally larger in galaxies with larger central bulges.