EXAM ANSWERS FOR ASTRO 101
2) What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?
A) the galaxy's disk
B) the galaxy's bulge
C) a globular cluster
D) the galaxy's halo
3) The Sun's location in the Milky Way Galaxy is
A) very near the galactic center.
B) in the halo of the galaxy, about 28,000 light-years above the galactic disk.
C) at the very outer edge of the galactic disk.
D) in the galactic disk, roughly halfway between the center and the outer edge of the disk.
4) What do we mean by the interstellar medium?
A) the dust that fills the halo of the Milky Way Galaxy
B) the middle section of the Milky Way Galaxy
C) the gas and dust that lies in between the stars in the Milky Way Galaxy
D) the name of an oracle who can channel messages from beings that live near the star called Vega
5) What are the Magellanic Clouds?
A) two small galaxies that probably orbit the Milky Way Galaxy
B) two nebulae located in the disk of the Milky Way Galaxy and visible only from the Southern Hemisphere
C) star-forming clouds found in the constellation Orion
D) the clouds of dust and gas found interspersed in many places throughout the Milky Way Galaxy
6) How do disk stars orbit the center of the galaxy?
A) They all orbit in roughly the same plane and in the same direction.
B) They have orbits randomly inclined and in different directions relative to the galactic center.
C) They follow spiral paths along the spiral arms.
D) They follow orbits that move up and down through the disk, typically taking them about 50,000 light-years above and below the disk on each orbit.
7) How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?
A) by counting the number of stars visible in this region of the galaxy
B) by estimating the amount of gas and dust in between the stars
C) by using the law of conservation of angular momentum to calculate the orbital speeds of nearby stars
D) by applying Newton's version of Kepler's third law to the orbits of the Sun or other nearby stars around the center of the Galaxy
8) Elements heavier than hydrogen and helium constitute about ________ of the mass of the interstellar medium.
9) What do we mean by the star-gas-star cycle?
A) It is the idea that stars in close binary systems can exchange gas with one another.
B) It is the set of nuclear reactions by which heavy elements are produced in the cores of massive stars.
C) It describes the orbits of the stars and interstellar medium around the center of the galaxy.
D) It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.
10) What are cosmic rays?
A) another name for gamma rays and X-rays
B) fast moving dust particles in the interstellar medium
C) subatomic particles that travel close to the speed of light
D) lasers used as weapons by extraterrestrials
11) The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with
A) ground-based visible-light telescopes.
B) space-based ultraviolet telescopes.
C) X-ray telescopes.
D) radio telescopes observing at a wavelength of 21 centimeters.
12) Which of the following analogies best describes how the structure of the galaxy's spiral arms is maintained?
A) Like military jets flying in formation above a football stadium, the stars in the spiral arms keep a spiral-shaped formation as they orbit the galaxy.
B) Like cars slowing in traffic to look at an accident, stars slow as they pass through the spiral arms.
C) Like a coiling rope, the spiral arms wind up tighter with every galactic rotation.
D) Like the fins of a giant pinwheel toy, the spiral arms carry a set of bright stars around as they sweep through the galaxy.
13) What do we mean by a protogalactic cloud?
A) a cloud of hydrogen and helium that contracts to become a galaxy
B) a term once used historically to refer to any galaxy
C) the cloud-like halo that surrounds the disks of spiral galaxies
D) a cloud of gas that was once a galaxy
14) Most stars in the Milky Way's halo are
A) very old.
B) found inside molecular clouds.
C) very young.
D) blue or white in color.
15) What is an ionization nebula?
A) a region of very hot, low-density gas surrounding a recent supernova
B) a clump of gas that will soon give birth to a new star
C) a colorful cloud of gas that glows because it is heated by light from nearby hot stars
D) a name sometimes used to describe spiral galaxies besides the Milky Way
16) What do halo stars do differently from disk stars?
A) They remain stationary, quite unlike disk stars that orbit the galactic center.
B) They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.
C) Halo stars explode as supernovae much more frequently than disk stars.
D) They orbit the center of the galaxy at much lower speeds than disk star.
17) Where does most star formation occur in the Milky Way Galaxy?
A) everywhere throughout the galactic disk
B) in the central bulge
C) within the halo
D) in the spiral arms
18) Based on observations, which of the following statements about stars in the Milky Way is generally true?
A) The older the star, the bluer its color.
B) The older the star, the faster its orbital speed.
C) The older the star, the lower its abundance of heavy elements.
D) The younger the star, the higher its mass.
19) What kind of object do we think lies in the center of the Milky Way Galaxy?
A) a 3- to 4-million-solar-mass black hole
B) a gigantic X-ray binary system
C) a dense cluster of young, hot stars
D) an enormous collection of dark matter, which explains why we detect no light at all from the galactic center
19.5 Mastering Astronomy Concept Quiz
1) If we could see our own galaxy from 2 million light-years away, it would appear
A) as a flattened disk with a central bulge and barred-spiral arms.
B) as a faintly glowing band of light stretching all the way around the sky.
C) to fill the sky with widely spaced stars.
D) like a single, dim star.
2) How does the interstellar medium affect our view of most of the galaxy?
A) It prevents us from seeing most of the galactic disk with visible and ultraviolet light.
B) It absorbs all wavelengths of light.
C) It produces so much visible light that it blocks our view of anything beyond it.
D) It has no effect on visible-light observations, but prevents us from studying the galactic center with radio waves or X-rays.
3) Applying the Newton's version of Kepler's third law (or the orbital velocity law) to a star orbiting 40,000 light-years from the center of the Milky Way Galaxy allows us to determine
A) the total mass of the entire Milky Way Galaxy.
B) the mass of the black hole thought to reside in the center of the galaxy.
C) the percentage of the galaxy's mass that is made of dark matter.
D) the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center.
4) How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?
A) It should be higher in mass.
B) It should have a higher fraction of elements heavier than hydrogen and helium.
C) It should be much brighter.
D) It should orbit the galactic center at a much higher rate of speed.
E) All of the above would be expected.
5) Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?
A) It is reasonable, because we already know that interstellar dust obscures our view through the disk of the galaxy.
B) The 10% figure is too low, because most of the mass of the galaxy is in the form of interstellar dust.
C) The 10% figure is too high because there are not enough heavy elements to make that much dust.
D) It seems reasonable as long as we assume that red giant stars—which produce dust grains in their stellar winds—are more common than we thought.
6) The most common form of gas in the disk of the Milky Way Galaxy is
A) molecular hydrogen.
B) gas in hot bubbles.
C) atomic hydrogen gas.
D) gas in stellar winds.
7) How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?
A) The total amount of gas will be about the same, but it will contain a much higher percentage of elements heavier than hydrogen and helium.
B) The total amount of gas will be much less than it is today.
C) The total amount of gas will be much greater, since many stars will undergo supernovae between now and then.
D) Thanks to the recycling of the star-gas-star cycle, the interstellar medium should look about the same in 50 billion years as it does today.
8) Over time, the star-gas-star cycle leads the gas in the Milky Way to
A) have a greater abundance of heavy elements.
B) have a lower abundance of heavy elements.
C) become denser and have a greater abundance of heavy elements.
D) become denser and hotter.
9) Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?
A) the Chandra X-ray Observatory
B) the Keck I telescope on the summit of Mauna Kea
C) the SOFIA airborne infrared observatory
D) the Hubble Space Telescope
10) If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
A) Gas that changes only in very slow and steady ways, so that the movie would in fact be quite boring.
B) The entire disk of the Milky Way would pulsate in and out as it contracts to form stars and then blows out in supernovae and then contracts to form stars again and so on.
C) The movie would alternate back and forth between being very bright when there is a lot of gas and very dark when there is very little gas.
D) Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.
11) What observational evidence supports the galactic fountain model (which describes how gas cycles between the disk of the galaxy and regions high above the disk)?
A) We have discovered a jet of ionized gas shooting out of the bulge of our galaxy.
B) We have discovered that the entire galactic disk is being uniformly "rained on" by cool gas coming from the halo.
C) We see hot gas high above the region of the disk near our solar system, along with cool gas that appears to be raining down from the halo.
D) We have observed a lot of water molecules in the interstellar medium.
12) All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except
A) young stars.
B) globular clusters.
C) X-ray binaries.
D) high-mass, red supergiant stars.
13) Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found
A) in the halo.
B) only in or near star-forming clouds.
C) only in the central bulge.
D) evenly spread throughout the galactic disk.
14) Which of the following statements comparing halo stars to our Sun is not true?
A) Most stars in the halo have cooler surface temperatures than the Sun.
B) Most stars in the halo are less luminous than the Sun.
C) Most stars in the halo contain a much lower percentage of heavy elements than the Sun.
D) Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.
15) Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?
A) It is probably a halo star that is currently passing through the disk.
B) It is a very young star, recently formed.
C) It has been pushed to high speed by the shock wave from a nearby supernova.
D) It is a very high mass star.
16) Why do we believe that most of the mass of the Milky Way is in the form of dark matter?
A) Although dark matter emits no visible light, we have detected its radio emissions.
B) The orbital speeds of stars far from the galactic center are surprisingly high.
C) Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk.
D) Our view of distant galaxies is often obscured by dark blotches, which are presumably made of dark matter.
17) Spiral arms appear bright because
A) they contain more hot young stars than other parts of the disk.
B) they contain far more stars than other parts of the galactic disk.
C) they contain more molecular clouds than other parts of the disk.
D) they are the only places where we find stars within the disk of the galaxy.
18) How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
A) The stars that formed first eventually settled into a galactic disk, circling the center of the galaxy.
B) The protogalactic cloud gradually formed stars, starting from the center of the galaxy working outwards.
C) The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
D) The protogalactic cloud gradually formed stars, starting from the outer edges of the spiral arms and working inward.
19) If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?
A) The spiral arms will seem to "wind up," to wrap more and more tightly around the center of the Galaxy.
B) The spiral arms will eventually dissipate and fade away, since they are a temporary phenomenon that should only last for a million years or so.
C) Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.
D) The spiral arms will eventually unwind, as centripetal forces send the stars flying outwards into intergalactic space.
20) What is the best evidence for an extremely massive black hole in the center of the Milky Way?
A) Huge amounts of X-rays are pouring out of the center of the galaxy.
B) The center of our galaxy hosts a pulsar that is spinning so fast that it could only be a black hole.
C) We observe stars vanishing in the center of the Galaxy as they are sucked into the black hole.
D) The orbits of stars in the center of the galaxy indicate that the presence of 3- to 4-million-solar-mass object in a region no larger than our Solar System.
21) Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?
A) It is by far the brightest source of visible light lying in the direction of the galactic center.
B) It is thought to harbor a black hole of more than 3 million solar masses.
C) It is a source of X-ray emission that we have observed with telescopes in space.
D) It is a source of bright radio emission.
The Cosmic Perspective, 8e (Bennett)
Chapter 20 Galaxies and the Foundations of Modern Cosmology
20.1 Multiple-Choice Questions
1) 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.
2) 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
3) Which of the following types of galaxies are most spherical in shape?
4) Which of the following types of galaxies are reddest in color?
5) Which of the following statements about galaxies is true?
A) Small galaxies outnumber large galaxies and produce most of the light in the universe.
B) Small galaxies outnumber large galaxies but large galaxies produce most of the light in the universe.
C) There is an approximately equal number of small and large galaxies in the universe and together they each contribute an equal amount of light.
D) Most galaxies in the universe are about the same size as the Milky Way.
E) Galaxies come in a wide variety of shapes and sizes but are all very blue in color.
6) Which types of galaxies have a clearly defined halo component?
A) spirals only
B) ellipticals only
C) lenticulars only
D) irregulars only
E) all but irregulars
7) 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
8) 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.
9) The disk component of a spiral galaxy includes which of the following parts?
C) spiral arms
D) globular clusters
E) all of the above
10) 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.
11) 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.
12) Most large galaxies in the universe are
B) spiral or lenticular.
13) Which of the following types of galaxies are most commonly found in large clusters?
14) 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
15) Which of the following is true about irregular galaxies?
A) They are composed solely of old stars.
B) They generally have significant bulge populations.
C) They were more common when the universe was younger.
D) They have reddish colors.
E) They have well defined spiral arms.
16) Why are Cepheid variables important?
A) Cepheid variables are stars that vary in brightness because they harbor a black hole.
B) Cepheids are pulsating variable stars, and their pulsation periods are directly related to their true luminosities. Hence, we can use Cepheids as "standard candles" for distance measurements.
C) Cepheids are a type of young galaxy that helps us understand how galaxies form.
D) Cepheids are supermassive stars that are on the verge of becoming supernovae and therefore allow us to choose candidates to watch if we hope to observe a supernova in the near future.
17) What is a standard candle?
A) an object for which we are likely to know the true luminosity
B) an object for which we can easily measure the apparent brightness
C) a class of objects in astronomy that all have exactly the same luminosity
D) any star for which we know the exact apparent brightness
E) a long, tapered candle that lights easily
18) How did Edwin Hubble measure the distance to the Andromeda Galaxy?
A) He measured its parallax.
B) He applied the period-luminosity relation to Cepheid variables.
C) He deduced it from its redshift.
D) He used white dwarf supernovae.
19) How was Edwin Hubble able to use his discovery of a Cepheid in Andromeda to prove that the "spiral nebulae" were actually entire galaxies?
A) There are no Cepheids in the Milky Way, so his discovery proved that it had to be in another galaxy.
B) He measured the stellar parallax of the Cepheid in Andromeda, was able to determine the distance to it, and showed that it was far outside the Milky Way Galaxy.
C) From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.
D) Since a Cepheid is a type of luminous galaxy, when he found it in Andromeda he was able to prove that Andromeda was a separate galaxy from the Milky Way.
20) What two quantities did Edwin Hubble plot against each other 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
21) What is Hubble's law?
A) The longer the time period between peaks in brightness, the greater the luminosity of the Cepheid variable star.
B) The recession velocity of a galaxy is directly proportional to its distance from us.
C) The recession velocity of a galaxy is inversely proportional to its distance from us.
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.
22) 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
23) What is the primary practical difficulty that limits the use of Hubble's law for measuring distances?
A) Redshifts of galaxies are difficult to measure.
B) The recession velocities of distant galaxies are so great that they are hard to measure.
C) We do not know Hubble's constant very accurately yet.
D) Hubble's law is only useful theoretically; it is difficult to use in practice.
E) The motion of Earth relative to the Milky Way is difficult to account for.
24) White-dwarf supernovae are good standard candles for distance measurements for all the following reasons except which?
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.
25) What makes white-dwarf supernovae good standard candles?
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
26) What is the most accurate way to determine the distance to a nearby star?
A) radar ranging
B) stellar parallax
C) using Cepheid variables
D) Hubble's law
27) What is the most accurate way to determine the distance to a nearby galaxy?
A) radar ranging
B) stellar parallax
C) using Cepheid variables
D) Hubble's law
28) What is the most accurate way to determine the distance to a very distant irregular galaxy?
A) using Cepheid variables
B) using a white-dwarf supernova as a standard candle
C) main sequence fitting
D) Hubble's law
29) Which of the following sequences lists the methods for determining distance in the correct order from nearest to farthest?
A) parallax, Cepheid variables, Hubble's law
B) parallax, Hubble's law, Cepheid variables
C) parallax, white-dwarf supernovae, Hubble's law
D) parallax, Hubble's law, white-dwarf supernovae
30) Dr. X believes that the Hubble constant is H0 = 55 km/s/Mpc. Dr. Y believes it is H0 = 80 km/s/Mpc. 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.
31) Dr. Smith believes that the Hubble constant is H0 = 70 km/s/Mpc. Dr. Jones believes it is H0 = 50 km/s/Mpc. 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.
32) 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 = 65 km/s/Mpc. How fast would a galaxy located 500 megaparsecs distant be receding from us?
A) 65 km/s
B) 65 Mpc/s
C) 32,500 km/s
D) 9 km/s
E) 0.65 times the speed of light
33) Hubble's "constant" is constant in
C) space and time.
D) our Galaxy but different in others.
34) Based on current estimates of the value of Hubble's constant, how old is the universe?
A) between 4 and 6 billion years old
B) between 8 and 12 billion years old
C) between 12 and 16 billion years old
D) between 16 and 20 billion years old
E) between 20 and 40 billion years old
35) 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 sensitive enough detectors.
E) The cosmological horizon is infinitely far away, and we can't see to infinity.
36) What does the equivalent of an H-R diagram for galaxies, plotting luminosity versus color, show?
A) galaxies fill the diagram showing that there is no correlation between luminosity and color
B) two clumps, one blue with relatively low luminosity, one red with relatively high luminosity, and a valley in between with few galaxies
C) a continuum from faint, blue galaxies to bright, red galaxies
D) a continuum from faint, red galaxies to bright, blue galaxies
E) a main sequence, just as for stars
37) Why are galaxies in the blue cloud blue?
A) They contain hot, young stars that signify active star formation.
B) They are moving slower than stars in the red cloud.
C) They contain lots of dust that cause the bluish color.
D) They contain cold, dying stars that are fading to black.
20.2 True/False Questions
1) Although it is difficult to tell from our vantage point inside the galaxy, astronomers suspect that the Milky Way is a barred spiral.
2) Spiral galaxies have more gas, dust, and younger stars than elliptical galaxies do.
3) Stars are continually forming in the halo of our Galaxy today.
4) A lenticular galaxy is another name for an elongated elliptical galaxy.
5) There are more large spiral galaxies than there are large elliptical galaxies.
6) Elliptical galaxies are more likely to be found in clusters than are spiral galaxies.
7) Massive-star supernovae and white-dwarf supernovae work equally well as standard candles for measuring cosmic distances.
8) The larger the value of Hubble's constant, the more rapid the expansion of the universe and hence the younger the universe.
9) All galaxies start out as members of the blue cloud, but some transition to the red sequence.
10) If we were to measure Hubble's constant 10 billion years in the future, we would find it exactly the same as it is today.
20.3 Short Answer Questions
1) Explain how we estimate that there are about 50-100 billion galaxies in the observable universe.
Answer: Obviously it's impossible to count so many galaxies one by one, but by observing a small part in detail, we can extrapolate to get the total number. As an example, the Hubble deep field shows thousands of galaxies in a very small angular area of the sky (about equal to the size of a grain of sand held at arm's length). Multiplying the number of galaxies by the ratio of the angular area of the entire sky to the angular area of the deep field then gives an estimate of the total number of galaxies in the observable universe.
2) Summarize the links in the distance chain that allow us to estimate distances to the farthest reaches of the universe.
Answer: We can determine the distance to solar system objects through radar ranging. This gives us an accurate distance to the Sun so that we can use stellar parallax to measure the distances to the nearest stars. Since there is a period-luminosity relationship for Cepheids, we can measure the distances to other Cepheids by just measuring the time period between peaks of brightness. Cepheids are bright enough to be observed in galaxies other than the Milky Way. Therefore, they serve as a bridge between us and standard candles in other galaxies. Once we determine the distance to another galaxy for which we have observed a white-dwarf supernova, we then can calibrate the average luminosity of white-dwarf supernovae. White-dwarf supernovae and Cepheids get us far enough away from the Milky Way that we then can calibrate Hubble's law by determining the Hubble constant. Once we know the Hubble constant, we will be able to determine a galaxy's distance from its redshift.
3) List at least three qualities that would tend to make a type of astronomical object useful as a standard candle.
Answer: Answers will vary, but some of the key properties of good standard candles include little statistical scatter in brightness; high luminosity, so they can be seen at a great distance; and relative commonness, so they can be found in many objects.
4) Explain why we observe distant galaxies to be moving away from us and therefore believe that the universe is expanding, but we don't see individual galaxies or clusters expanding.
Answer: The pull of gravity works to slow the expansion rate of the universe. Therefore, in dense regions such as galaxies and clusters of galaxies, gravity is able to overcome the expansion and keep these objects from expanding. However, on larger scales, where the average density is lower, gravity does not have as strong an effect, and therefore the expansion continues between clusters of galaxies.
5) Briefly explain why we think white-dwarf supernovae are useful for measuring cosmic distances.
Answer: They all come from explosions of white dwarfs that reach the white-dwarf limit, so we expect them all to have the same luminosity; observations of white-dwarf supernovae for which we can measure distance independently confirm that they all have the same luminosity. Since we can assume we know their luminosity, we can use their apparent brightnesses to determine distance from the luminosity-distance formula.