chapter 19, 20, 22, and 23 Astronomy

Models of the Big Bang that include inflation predict that the overall geometry of the universe should be "flat" (in spacetime). This prediction _________.

agrees with data found by studying the cosmic microwave background

Which types of galaxies have a clearly defined halo component? spirals only irregulars only ellipticals only lenticulars only all but irregulars

all but irregulars

Based on current estimates of the value of Hubble's constant, how old is the universe? between 20 and 40 billion years old between 12 and 16 billion years old between 16 and 20 billion years old between 8 and 12 billion years old between 4 and 6 billion years old

between 12 and 16 billion years old

In terms of the eras that scientists use to describe different times in the history of the universe, we live in the __________.

era of galaxies

According to current understanding, if the universe continues to expand forever, the last major source of light will come from _________.

evaporation of black holes

What kinds of objects lie in the halo of our galaxy? O and B stars gas and dust globular clusters open clusters all of the above

globular clusters

If observations had shown that the cosmic microwave background was perfectly smooth (rather than having slight variations in temperature), then we would have no way to account for _________.

how galaxies came to exist

Where would you be most likely to find an ionization nebula? in the disk between spiral arms in the halo in a spiral arm

in a spiral arm

The Sun's location in the Milky Way Galaxy is _________. very near the galactic center in the galactic disk, roughly halfway between the center and the outer edge of the disk at the very outer edge of the galactic disk in the halo of the galaxy, about 28,000 light-years above the galactic disk

in the galactic disk, roughly halfway between the center and the outer edge of the disk

Where does most star formation occur in the Milky Way today? in the Galactic center in the halo in the spiral arms uniformly throughout the Galaxy in the bulge

in the spiral arms

Which of the following observations cannot be explained by the Big Bang theory, unless we assume that an episode of inflation occurred?

the fact that the temperature of the cosmic microwave background is almost the same everywhere

Which part of the galaxy has gas with the hottest average temperature? the disk the halo the bulge

the halo

The Big Bang theory is widely accepted today because it successfully predicts and explains several key observed features of the universe. Two of its important predictions that have been verified are ___________.

(1) the existence and specific characteristics of the observed cosmic microwave background and (2) the observed overall chemical composition of the universe

Why do disk stars bob up and down as they orbit the galaxy? - because the gravitational pull of other disk stars always pulls them toward the disk -because of friction with the interstellar medium -because the halo stars keep knocking them back into the disk

- because the gravitational pull of other disk stars always pulls them toward the disk

How do we determine the Milky Way's mass outside the Sun's orbit? -from the Sun's orbital velocity and its distance from the center of our galaxy -from the orbits of halo stars near the Sun -from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun

-from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun

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 10 billion. 1 trillion. 1 billion. 100 million. 100 billion.

100 billion

What is the diameter of the disk of the Milky Way? 1,000,000 light-years 10,000 light-years 1,000 light-years 100 light-years 100,000 light-years

100,000 light-years

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? 0.65 times the speed of light 9 km/s 32,500 km/s 65 km/s 65 Mpc/s

32,500 km/s

What is the approximate temperature of the universe (as a whole) today?

3K

Based on evidence from measurements of the acceleration of the expansion rate and from careful study of the cosmic microwave background, about what percentage of the universe's total mass and energy takes the form of ordinary atomic matter (protons, neutrons, and electrons)?

5%

Suppose you observe a distant galaxy with a lookback time of 8 billion years. What was the maximum possible age for that galaxy when the light we are now observing from it began its journey to Earth? (Hint: Assume the galaxy was born less than a billion years after the Big Bang.)

6 billion years how i got the answer: 13.7 - 8= 5.7 round up 5.7 which will then be 6 billion years.

Which of the following statements correctly describes current understanding of the eventual fate of the universe?

Although the accelerating expansion suggests that the universe will continue to expand forever, it is possible that future discoveries will indicate a different fate.

What is the major difference between an elliptical galaxy and a spiral galaxy? An elliptical galaxy lacks a disk component. Elliptical galaxies are not as big as spiral galaxies. A spiral galaxy has a spherical halo. There are no dwarf spiral galaxies, but there are dwarf ellipticals. A spiral galaxy contains mostly younger stars.

An elliptical galaxy lacks a disk component.

Which of the following statements can not be tested by science today?

Before Planck time, our universe sprouted from another universe.

What is the significance of Planck time?

Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.

In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times before about 380,000 years after the Big Bang. Why?

Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.

In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn't the same fusion processes produce carbon and heavier elements in the early universe?

By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.

Scientists using the Hubble Space Telescope have observed Cepheids in the galaxy M 100. Here are the actual data for three Cepheids in M 100: Cepheid 1: luminosity = 3.9×10^30watts brightness 9.3×10^−19watt/m^2 Cepheid 2: luminosity = 1.2×10^30watts brightness 3.8×10^−19watt/m^2 Cepheid 3: luminosity = 2.5×10^30watts brightness 8.7×10^−19watt/m2 Compute the distance to M 100 with data from each of the three Cepheids

Cepheid 1: 6.1×10^7 ly Cepheid 2: 5.3×10^7ly Cepheid 3: 5.1×10^7 ly how I got the answer: Solve for d d = sqrt(L/4πb) Cepheid 1: d = sqrt(3.9x10^30/ (4π x9.3 x 10^-19) = 5.77 x 10^23 meters = 60,990,250.8 ly = 6.1×10^7 Cepheid 2: d = sqrt(1.2x10^30/ (4π x 3.8 x 10^-19) = 5.012 x 10^23 meters = 52,978,013.3 ly = 5.3×10^7 Cepheid 3: d = sqrt (2.5 x 10^30/ (4π x 8.7 x 10^-19) = 4.78 x 10^23 meters = 50,525,719 ly = 5.1×10^7

Chapter 20

Chapter 20

Chapter 23 reading and concept quiz

Chapter 23 reading and concept quiz

Which of the following statements about large-scale structure is probably not true?

Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.

Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. Based on current understanding, which of the following would have to be true for this to be the case?

Dark energy does not exist and there is much more matter than current evidence suggests.

Which of the following best sums up current scientific thinking about the nature of dark energy?

Dark energy probably exists, but we have little (if any) idea what it is

Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?

Dark matter is the dominant form of mass in both clusters and in individual galaxies.

Each item below belongs either with the population of disk stars or the population of halo stars of the Milky Way Galaxy. Match each item to the appropriate population stars with the smallest abundance of heavy elements high-mass stars youngest stars globular clusters stars whose orbits can be inclined at any angle the Sun stars that all orbit in nearly the same plane oldest stars

Disk Stars: the Sun youngest stars stars that all orbit in nearly the same plane high-mass stars ~~~~~~~~~~~~~~~~~ Halo Stars: -oldest stars -globular clusters -stars whose orbits can be inclined at any angle -stars with the smallest abundance of heavy elements

Which of the following statements about the disk of the Milky Way is false? The length of the disk is about 100 times its thickness. The average age of disk stars is less than that of halo stars. Disk stars are all younger than 5 billion years. Disk stars have a higher proportion of heavy elements, on average, than halo stars. Disk stars orbit in the same direction around the Galactic center.

Disk stars are all younger than 5 billion years.

According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?

During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the slight excess of matter particles.

Which analogy best explains why inflation predicts that the overall geometry of the observable universe should appear to be flat?

Earth is so big that it the part explored by an ant appears flat

It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?

Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.

Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?

Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.

The Milky Way looks the same in X rays as it does at infrared wavelengths. True False

False

The Sun is located at the edge of the galaxy, approximately 50,000 light-years from the galactic center. True False

False

What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?

Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.

What evidence suggests that the protogalactic cloud that formed the Milky Way resulted from several collisions among smaller clouds? 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. The Milky Way is the central galaxy of a cluster of galaxies. The Milky Way resembles an elliptical galaxy more than other spirals do. The stars in the halo of the Milky Way are organized into several dense clusters arranged throughout the halo. The bulge of the Milky Way is surrounded by many globular clusters, just as elliptical galaxies are.

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.

How does the hypothesis of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?

Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.

How does the interstellar medium obscure our view of most of the galaxy? It produces so much visible light that it is opaque and blocks our view of anything beyond it. It absorbs visible, ultraviolet, and some infrared light. It absorbs all wavelengths of light. It reflects most light from far distances of the galaxy away from our line of sight. all of the above

It absorbs visible, ultraviolet, and some infrared light.

When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?

It bends or distorts the light coming from galaxies located behind it.

What is the distinguishing characteristic of what we call ordinary (or baryonic) matter?

It consists of atoms or ions with nuclei made from protons and neutrons.

What is antimatter?

It consists of particles that are just like those of ordinary matter except with opposite properties, such as opposite charge.

Which of the following is not an observed characteristic of the cosmic microwave background?

It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.

Why do we call dark matter "dark"?

It emits no radiation that we have been able to detect.

Which of the following best summarizes what we mean by the term dark energy?

It is a name given to whatever is causing the expansion of the universe to accelerate with time.

Which statement about the cosmic microwave background is not true?

It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.

Which of the following best sums up current scientific thinking about the nature of dark matter?

Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.

Is space expanding within clusters of galaxies?

No, because their gravity is strong enough to hold them together even while the universe as a whole expands.

Part A: Consider the following hypothetical observations, some of which are real and some of which are fictional. For each observation, your job is to answer this question: If the observation were real, would it provide evidence for or against the idea that the universe is expanding? Sort each observation into the appropriate bin as follows: Place an observation in the "Supports the expanding universe" bin if it would provide evidence that the universe is expanding. Place an observation in the "Contradicts the expanding universe" bin if it would provide evidence that would force us to reconsider the idea of an expanding universe. Place an observation in the "Neither supports nor contradicts" bin if it does not allow us to distinguish between a universe that is expanding and a universe that is not expanding. ~~~~~~~~~~~~~~~~~~~~~~~~ Part B: Consider the hypothetical observation "Irregular galaxies outside the Local Group are moving toward us." From Part A, this observation would contradict the idea of an expanding universe. Why? Because while irregular galaxies are expected to move with irregular speeds, they should still be moving away from us. Because Hubble's law predicts that all galaxies outside our Local Group should be moving away from us. Because irregular galaxies are expected to show a different pattern of expansion than other galaxy types. Because the idea that the universe is expanding should be observable only within our Local Group. ~~~~~~~~~~~~~~~~~~~~~~ Part C: Consider the observation "The Andromeda Galaxy, a member of our Local Group, is moving toward us." Why doesn't this observation contradict the idea that the universe is expanding? Because expansion is expected to cause some galaxies to move toward us. Because the galaxies of the Local Group are gravitationally bound together. Because the Andromeda Galaxy is a spiral galaxy. Because the Local Group is located at the center of the expansion. ~~~~~~~~~~~~~~~~~~~~~ Part D: We can in principle measure the expansion rate by studying galaxies in many different directions in space and at different times of year. If we compare such observations, we would find that the expansion rate is __________. the same no matter when or in which direction we measure it varies with both the time of year and the direction in which we look the same in all directions, but varies with the time of year the same at all times of year, but varies in different directions

Part A: Supports the expanding universe= All galaxies in the Coma cluster of galaxies have redshifted spectra. Galaxies 200 million light-years away move away from us twice as fast as galaxies 100 million light-years away. The measured rate of expansion is the same in all directions. ----------- Contradicts the expanding universe= Irregular galaxies outside the Local Group are moving toward us. Galaxy speeds are faster in summer than in winter. Spiral galaxies move away from us 10% faster than elliptical galaxies at the same distances. ----------- Neither supports nor contradicts= The Andromeda Galaxy, a member of our Local Group, is moving toward us. ~~~~~~~~~~~~~~~~~~~~~~ Part B: Because Hubble's law predicts that all galaxies outside our Local Group should be moving away from us. ~~~~~~~~~~~~~~~~~~~~~ Part C: Because the galaxies of the Local Group are gravitationally bound together. ~~~~~~~~~~~~~~~~~~~~~ Part D: the same no matter when or in which direction we measure it

Learning Goal: To calculate the approximate mass of the central object in the Milky Way using real data. Introduction: The diagram in (Figure 1) shows orbits of stars around a compact object at the center of the Milky Way Galaxy. The colored dots represent stellar positions at approximately one-year intervals, with each color representing a different star; the background photo is an infrared image showing the stars at one particular time. The dashed ellipses represent the orbits calculated for the stars based on the observed stellar motions. (These data are courtesy of Andrea Ghez at UCLA; her team continues to collect these data.) Part A: To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________. the central object has a mass of about 4 million solar masses if they observed for many more years, the dots would trace out ellipses if they observed for many more years, the dots would trace out parabolas the stars are all main-sequence stars ~~~~~~~~~~~~ Part B: Notice that some of the stars on the diagram are represented by a series of dots that are very close together, while others have their dots farther apart. Keeping in mind that all the stellar positions were measured at approximately one-year intervals, which stars are moving the fastest in their orbits during the time period indicated by the dots? The fastest stars are the ones with: the dots farthest apart the dots closest together the most dots ~~~~~~~~~~~~ Part C: To determine the mass of the central object, we must apply Newton's version of Kepler's third law, which requires knowing the orbital period and average orbital distance (semimajor axis) for at least one star. We could consider any of the stars shown in the figure, so let's consider the star with the highlighted orbit (chosen because its dots are relatively easy to distinguish). What is the approximate orbital period of this star? 5 yr 20 yr 45 yr 100 yr ~~~~~~~ Part D: Look again at the orbit of the star with the highlighted orbit. By comparing the orbit to the scale bar shown on the diagram, you can estimate that this orbit has a semimajor axis of about _____. 12 AU 250 AU 1150 AU 2300 AU ~~~~~~~~~~ Part E: The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass (M) of a central object, in solar masses, from an orbiting object's period (p) in years and semimajor axis (a) in astronomical units: M=a^3/p^2 Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object? 0.004 solar masses 40 solar masses 400 solar masses 200,000 solar masses 4 million solar masses 40 million solar masses ~~~~~~~ Part F: From Part E you know the mass of the central object. Now consider its size. Based on what you can see in the diagram, you can conclude that the diameter of the central mass is __________. no more than about 70 AU approximately 200 AU approximately 1150 AU at least 2300 AU ~~~~~~~~~ Part G: You've now found that the central object has a mass of about 4 million solar masses but is no more than about 70 in diameter—which means it cannot be much larger than the size of our planetary system. Why do these facts lead astronomers to conclude that the central object is a black hole? The central object does not show up in the infrared photo, so we conclude that it must be black. The mass is far above the maximum possible mass for a neutron star, so the object must be a black hole. There is no known way to pack so much mass into such a small volume without it collapsing into a black hole. Observations of other black holes have shown that this size and mass is typical, so this object must also be a black hole.

Part A: if they observed for many more years, the dots would trace out ellipses ~~~~~~~~~~~~~~ Part B: the dots farthest apart ~~~~~~~~~~~ Part C: 20 yr ~~~~~~~~~ Part D: 1150 AU ~~~~~~~~~~~ Part E: 4 million solar masses ~~~~~~~~~ Part F: no more than about 70 AU ~~~~~~~~ Part G: There is no known way to pack so much mass into such a small volume without it collapsing into a black hole.

Part A: How do spiral and elliptical galaxies differ in terms of the presence or absence of disk and halo components? The major difference between spiral and elliptical galaxies is that both types of galaxies lack a significant disk component, although elliptical have the halo component. The major difference between spiral and elliptical galaxies is that spiral galaxies lack a significant disk component, although both types have the halo component. The major difference between spiral and elliptical galaxies is that spiral galaxies lack a significant disk component, although elliptical have the halo component. The major difference between spiral and elliptical galaxies is that elliptical galaxies lack a significant disk component, although both types have the halo component. ~~~~~~~~~ Part B: How does this difference explain the lack of hot, young stars in elliptical galaxies? Like the halos of spiral galaxies, elliptical galaxies lack cool gas and so have much star formation. As a result, we see many hot, young stars in these galaxies. Like the halos of spiral galaxies, elliptical galaxies lack hot gas and so have much star formation. As a result, we see many hot, young stars in these galaxies. Like the halos of spiral galaxies, elliptical galaxies lack hot gas and so do not have much star formation. As a result, we see few hot, young stars in these galaxies. Like the halos of spiral galaxies, elliptical galaxies lack cool gas and so do not have much star formation. As a result, we see few hot, young stars in these galaxies.

Part A: The major difference between spiral and elliptical galaxies is that elliptical galaxies lack a significant disk component, although both types have the halo component. ~~~~~~~~~~ Part B: Like the halos of spiral galaxies, elliptical galaxies lack cool gas and so do not have much star formation. As a result, we see few hot, young stars in these galaxies.

Part A: Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest. -the edge of the central bulge -a cloud of gas and dust in the outskirts of the disk -a globular cluster in the outskirts of the halo -our solar system ~~~~~~~~~~~~~~~~~~ Part B: Imagine a photon of light traveling the different paths in the Milky Way described in the following list. Rank the paths based on how much time the photon takes to complete each journey, from longest to shortest - across the diameter of the central bulge -across the diameter of the galactic halo -across the diameter of the galactic disk -from the Sun to the center of the galaxy -through the disk from top to bottom

Part A: Farthest to Closest -A globular cluster in the outskirts of the halo -A cloud of gas and dust in the outskirts of the disk -Our solar system -The edge of the central bulge ~~~~~~~~~~~~~~ Part B: Longest to Shortest -across the diameter of the galactic halo -across the diameter of the galactic disk -from the Sun to the center of the galaxy -across the diameter of the central bulge -through the disk from top to bottom

You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only one-quarter as bright as Supernova B. What can you say about their relative distances? Supernova B is four times as far away as Supernova A. Supernova A is four times as far away as Supernova B. Supernova A is twice as far away as Supernova B. Supernova B is twice as far away as Supernova A. We can't say anything about their relative distances because we do not have enough information.

Supernova A is twice as far away as Supernova B.

The story of how we came to learn the structure of the Milky Way is an excellent example of how science progresses. Select the correct statements. Check all that apply. A) The Milky Way is much wider than it's thickness. So it is a narrow band with many stars in it, while the sky outside the band has much fewer stars. B) Shapley's 20th-century observations of globular cluster orbits, which center on a point about 30,000 light years from our Sun, showed we weren't in the center of the galaxy. C) The Milky Way is not wider than it's thickness. We see stars distributed more or less evenly across the sky. D) Shapley's 20th-century observations of globular cluster orbits, which center on a point about 30,000 light years from our Sun, showed we were in the center of the galaxy.

The Milky Way is much wider than it's thickness. So it is a narrow band with many stars in it, while the sky outside the band has much fewer stars. Shapley's 20th-century observations of globular cluster orbits, which center on a point about 30,000 light years from our Sun, showed we weren't in the center of the galaxy.

Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?

The actual average density of matter, even with dark matter included, is only about a quarter of the critical density.

What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?

The basic chemical composition of the universe had been determined.

What happens when a particle of matter meets its corresponding antiparticle of antimatter?

The combined mass of the two particles is completely transformed into energy (photons).

The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation (these are not real observations) would call our current theory into question?

The discovery of a galaxy with a helium abundance of only 10% by mass.

how did the early universe change with time?

The universe has progressed through a series of eras, each marked by unique physical conditions. We know little about the Planck era, when the four forces may have all behaved as one. Gravity became distinct at the start of the GUT era, and electromagnetism and the weak force became distinct at the end of the electroweak era. Matter particles annihilated all the antimatter particles by the end of the particle era. Fusion of protons and neutrons into helium ceased at the end of the era of nucleosynthesis. Hydrogen nuclei captured all the free electrons, forming hydrogen atoms at the end of the era of nuclei. Galaxies began to form at the end of the era of atoms. The era of galaxies continues to this day.

What are basic characteristics of stars' orbits in the disk, halo, and bulge of our galaxy? The disk stars have nearly circular orbits. They have vertical motions out of the plane, making them appear to bob up and down, but they never get "too far" from the disk. Orbits of stars in the bulge and the halo of the galaxy are much less orderly, traveling around the galactic center on elliptical orbits with more or less random orientations. The bulge stars have nearly circular orbits. They have vertical motions out of the plane, making them appear to bob up and down, but they never get "too far" from the bulge. Orbits of stars in the disk and the halo of the galaxy are much less orderly, traveling around the galactic center on elliptical orbits with more or less random orientations. The halo stars have nearly circular orbits. They have vertical motions out of the plane, making them appear to bob up and down, but they never get "too far" from the halo. Orbits of stars in the bulge and the disk of the galaxy are much less orderly, traveling around the galactic center on elliptical orbits with more or less random orientations.

The disk stars have nearly circular orbits. They have vertical motions out of the plane, making them appear to bob up and down, but they never get "too far" from the disk. Orbits of stars in the bulge and the halo of the galaxy are much less orderly, traveling around the galactic center on elliptical orbits with more or less random orientations.

Why do scientists assume that the early universe was much hotter and denser than the universe of today?

The fact that the universe is expanding implies that objects were closer together in the past, and compressing material makes it hotter and denser.

Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter and that all of it suddenly decays tomorrow into photons or other forms of energy. Based on current understanding, which of the following would begin to occur?

The galaxies in clusters would begin to fly apart.

Based on observations, which of the following statements about stars in the Milky Way is generally true? The older the star, the lower its abundance of heavy elements. The less massive the star, the older it is. The older the star, the bluer its color. The older the star, the faster its orbital speed. The younger the star, the higher its mass.

The older the star, the lower its abundance of heavy elements.

Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?

The orbital speeds would fall off sharply with increasing distance from the galactic center.

Which of the following models best explains why our galaxy has spiral arms? The spiral arms were imprinted on the galaxy at its birth. Ever since, like a coiling rope, the spiral arms have been wound tighter with each galactic rotation. The spiral arms are composed of groups of stars that are bound together by gravity and therefore always stay together as the galaxy rotates. No model can explain the existence of the arms, which rotate with the galaxy like the fins of a giant pinwheel toy. The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy.

The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy

Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?

The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass

Which of the following statements correctly summarizes the events in the early universe according to the Big Bang theory?

The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe.

Scientists do not know what dark energy is, yet they still claim that it is the dominant constituent of the mass-energy of the universe. What (if any) evidence supports this claim?

There are two lines of evidence: measurements of the accelerating expansion and the universal geometry inferred from the cosmic microwave background.

Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?

There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales

Which of the following statements best explains what we mean when we say that the electroweak and strong forces "froze out" at 10^-38 second after the Big Bang?

These two forces first became distinct at this time.

How do disk stars orbit the center of the galaxy? They have orbits randomly inclined and in different directions relative to the galactic center. They all orbit in roughly the same plane and in the same direction. 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. They follow spiral paths along the spiral arms.

They all orbit in roughly the same plane and in the same direction.

What do halo stars do differently from disk stars? Halo stars explode as supernovae much more frequently than disk stars. They remain stationary, quite unlike disk stars that orbit the galactic center. They orbit the center of the galaxy at much lower speeds than disk stars. They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.

They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.

What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?

They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.

If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?

They travel at speeds close to the speed of light.

According to the Big Bang theory, how many forces, and which ones, operated in the universe during the GUT era?

Two forces: gravity and a single force that later became the strong, weak, and electromagnetic forces

How does gravitational lensing tell us about the mass of a galaxy cluster?

Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.

What is the primary way in which we determine the mass distribution of a spiral galaxy?

We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.

How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?

We detect this gas with X-ray telescopes.

What is the primary practical difficulty that limits the use of Hubble's law for measuring distances? Redshifts of galaxies are difficult to measure. The motion of Earth relative to the Milky Way is difficult to account for. We do not know Hubble's constant very accurately yet. The recession velocities of distant galaxies are so great that they are hard to measure. Hubble's law is only useful theoretically; it is difficult to use in practice.

We do not know Hubble's constant very accurately yet.

Why can't current theories describe what happened during the Planck era?

We do not yet have a theory that links quantum mechanics and general relativity.

How do we determine the conditions that existed in the early universe?

We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.

Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?

Why is the sky dark at night?

The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?

Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.

What is a rotation curve?

a graph showing how orbital velocity depends on distance from the center for a spiral galaxy

In which of these galaxies would you be least likely to find an ionization nebula heated by hot young stars? a small irregular galaxy a large elliptical galaxy a large spiral galaxy

a large elliptical galaxy

What do we mean by the term inflation?

a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence

Which of the following statements correctly summarize key differences between the disk and the halo? Check all that apply. -Gas and dust are abundant in the disk but not in the halo. -Disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red. -Clusters of young stars are found only in the disk. -Stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits.

all of them

What produces the 21-cm line that we use to map out the Milky Way Galaxy? ionized hydrogen helium carbon monoxide molecular hydrogen atomic hydrogen

atomic hydrogen

chapter 22 reading and concept quiz

chapter 22 reading and concept quiz

The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that _________.

dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way

We determine the distance of a Cepheid by ________ measuring its parallax. determining its luminosity from the period-luminosity relation and then applying the inverse square law for light. knowing that all Cepheids have about the same luminosity and then applying the inverse square law for light.

determining its luminosity from the period-luminosity relation and then applying the inverse square law for light.

Hubble's constant is a "constant" in that its value __________.

is the same across all of space and does not change on human time scales

Approximately how many stars does a dwarf elliptical galaxy have? 100 billion 10 billion less than a million less than a billion 1 trillion

less than a billion

Which of the following best summarizes what we mean by the term dark matter?

matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light

Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?

measuring the temperatures of stars in the halos of the galaxies

The primary evidence that has led astronomers to conclude that the expansion of the universe is accelerating comes from __________.

observations of white dwarf supernovae

Experiments allow physicists today to reproduce (on small scales) energy and temperature conditions thought to have prevailed in the early universe as far back in time as about _________.

one ten-billionth (10-10) of a second after the Big Bang

The disk component of a spiral galaxy includes which of the following parts? spiral arms halo globular clusters bulge all of the above

spiral arms

Most large galaxies in the universe are ______ elliptical. spiral or lenticular. abnormal. irregular.

spiral or lenticular.

What is the most accurate way to determine the distance to a nearby star? using Cepheid variables Hubble's law radar ranging stellar parallax

stellar parallax

The four fundamental forces that operate in the universe today are _________.

strong force, weak force, electromagnetic force, and gravity

What are cosmic rays? gamma rays and X rays any light waves from space subatomic particles that travel close to the speed of light fast-moving dust particles in the interstellar medium lasers used as weapons by extraterrestrials

subatomic particles that travel close to the speed of light

Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered ___________.

surprising because those locations would never have been close enough to have any light or matter exchanged between them

Based on current evidence, a supercluster is most likely to have formed in regions of space where _________.

the density of dark matter was slightly higher than average when the universe was young

Measuring the amount of deuterium in the universe allows us to set a limit on _________.

the density of ordinary (baryonic) matter the universe

Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.

the electromagnetic and weak forces into the electroweak force

Hubble's constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble's constant (such as 22 km/s/Mly), the age of the universe is oldest if __________.

the expansion rate has been increasing with time (an accelerating universe)

In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at its mass-to-light ratio), we expect that ______.

the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter

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? the one that is bluest in color the one that is closest to us the one that is farthest away the one that appears smallest in size the one that is reddest in color

the one that is farthest away

When we speak of the large-scale structure of the universe, we mean _________.

the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe

The critical density of the universe is the _______.

the total density of matter and energy needed to give the universe a "flat" geometry (in space time).

Sound waves in space_____ travel much faster than sound on Earth and are therefore very loud. travel much faster than sound on Earth but have such low density that they are inaudible. do not exist. travel so slowly that they are unnoticeable. can travel through the halo but not the disk of the galaxy.

travel much faster than sound on Earth but have such low density that they are inaudible.

Spiral galaxies have more gas, dust, and younger stars than elliptical galaxies do. True False

true

Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be ___________.

uniformly bright

A grand unified theory (GUT) refers to theories that _________.

unify the strong force with the electromagnetic and weak forces

What is the most accurate way to determine the distance to a nearby galaxy? stellar parallax radar ranging Hubble's law using Cepheid variables

using Cepheid variables

What two quantities did Edwin Hubble plot against each other to discover the expansion of the Universe? velocity and temperature velocity and distance luminosity and distance luminosity and temperature age and distance

velocity and distance