Astronomy HW/Practice Exam Questions Unit 3
The mass of a supermassive black hole thought to power a quasar is typically __________.
1 billion solar masses
Which of the four numbered galaxies have ongoing star formation? Select all that apply.
1,2
Which of the four numbered galaxies likely to be spiral or irregular in shape? Select all that apply.
1,2
Rank the four numbered galaxies on the graph at left according to their overall color, from bluest on the left to reddest on the right.
1,2,3,4
Match the terms in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each term only once.
1. The globular cluster M13 belongs to the halo component of the Milky Way Galaxy. 2. A(n) elliptical galaxy contains hot, ionized gas but very little cool gas or dust. 3. The type of galaxy known as a(n)irregular galaxy was more common in the universe 10 billion years ago. 4. The Milky Way is a(n) spiral galaxy. 5. Our Sun belongs to the disk component of the Milky Way Galaxy. 6. Scientists investigating cosmology study how the distribution of galaxies changes with time.
The Large Magellanic Cloud is a small galaxy that orbits the Milky Way. It is currently orbiting the Milky Way at a distance of roughly 160000 light-years from the galactic center at a velocity of about 300 km/skm/s. Use these values in the orbital velocity law to get an estimate of the Milky Way's mass within 160000 light-years from the center. (The value you obtain is a fairly rough estimate because the orbit of the Large Magellanic Cloud is not circular.) Express your answer as a multiple of Sun's mass to two significant figures.
1.0×10^12 MSun
The kinetic energy EkineticEkinetic of an amount of mass MM traveling at a velocity vv is given by the formula Ekinetic=Mv2/2.Ekinetic=Mv2/2. The total kinetic energy of the matter ejected from a supernova explosion is about 10441044 joules. Determine the typical speed at which that matter is ejected from a supernova with a mass of 28 MSunMSun. Express your answer to two significant figures and include the appropriate units.
1.9×10^6 m/s
How do the three distance computations agree? Based on your results, estimate the uncertainty in the distance you have found.
10%
Estimate the luminosity of the Cepheid with a period of 35 days.
10,000 solar luminosities
Roughly what percentage of the mass-energy of in-falling matter can be converted to other forms of energy as the matter falls into a black hole?
10-40%
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 _____.
1150AU
Suppose that you measure a galaxy's redshift, and from the redshift you determine that its recession velocity is 30,000 (3×1043×104) kilometers per second. According to Hubble's law, approximately how far away is the galaxy?
1.4 billion light-years
Where are most of the Milky Way's globular clusters found?
in the halo
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 dots farthest apart
Cosmological redshift is the result of __________.
the expansion of the universe
We can study how galaxies evolve because ______.
the farther away we look, the further back in time we see
What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?
the galaxy's bulge
What do we mean by the interstellar medium?
the gas and dust that lies in between the stars in the Milky Way galaxy
Which part of the galaxy has gas with the hottest average temperature?
the halo
Which of the following observations provides direct evidence that galaxies containing active galactic nuclei have central, supermassive black holes?
the high orbital velocities of stars and gas clouds at the very centers of these galaxies
Estimate the value of H0H0 corresponding to the solid line in the figure. Express your answer kilometers per second per million light-years to two significant figures.
160
Elements heavier than hydrogen and helium constitute about _____ of the mass of the Milky Way's interstellar medium.
2 %
What is the typical percentage (by mass) of elements other than hydrogen and helium in stars that are forming right now in the vicinity of the Sun?
2%
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?
20 yr
Estimate the luminosity of the Cepheid with a period of 8 days.
2000 solar luminosities
What would your estimate be for the age of the universe if you measured Hubble's constant to be 11 kilometers per second per million light-years?
28 billion years
What is the approximate age of the universe indicated by that erroneous value of H0H0? Express your answer in years to one significant figure.
2x10^9
Rank the four numbered galaxies on the graph at left according to their total luminosities, from dimmest on the left to most luminous on the right.
3,1,2,4
Suppose you observe a star orbiting the galactic center at a speed of 1200 km/skm/s in a circular orbit with a radius of 14 light-days. Calculate the mass of the object that the star is orbiting. Express your answer in solar masses to two significant figures.
3.9x10^6
When we observe a distant galaxy whose photons have traveled for 10 billion years before reaching Earth, we are seeing that galaxy as it was when the universe was
4 billion years old.
Part complete The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass (MMM) of a central object, in solar masses, from an orbiting object's period (ppp) in years and semimajor axis (aaa) in astronomical units: M=a3p2M=a3p2 Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object?
4 million solar masses
Consider these actual data for three Cepheids in the galaxy M100M100; the luminosities were determined with Leavitt's law from the observed periods (measured with observations from the Hubble Space Telescope). Cepheid 1: luminosity = 3.9×1030W3.9×1030W, apparent brightness = 9.3×10−19W/m29.3×10−19W/m2 Cepheid 2: luminosity = 1.2×1030W1.2×1030W, apparent brightness = 3.8×10−19W/m23.8×10−19W/m2 Cepheid 3: luminosity = 2.5×1030W2.5×1030W, apparent brightness = 8.7×10−19W/m2 Compute the distance to M100 with data from each of the three Cepheids. Express your answers in light-years to two significant figures separated by commas.
6.1×10^7,5.3×10&7,5.1×10^7
Suppose that Hubble's constant were H0H0 = 45 km/s/Mlykm/s/Mly (which is not its actual value). What would the approximate age of the universe be in that case? Express your answer in years to two significant figures.
6.7x10^9 yr
Suppose you observe a star orbiting the galactic center at a speed of 1400 km/skm/s in a circular orbit with a radius of 26 light-days. Calculate the mass of the object that the star is orbiting. Express your answer in solar masses to two significant figures.
9.9x10^6
What is a radio galaxy? How can radio galaxies affect the gas surrounding them? Drag the items on the left to the appropriate blanks on the right to complete the sentences. (Not all terms will be used.)
A radio galaxy is a galaxy that has powerful radioluminosity compared to its visible and infraredluminosity from stars. Radio galaxies can emit powerful jets of fast-moving particles. They inject high amounts of kinetic energy into their surroundings .
Choose the correct definition of starburst galaxy.
A starburst galaxy is a galaxy that is forming new stars at a very high rate.
Why do astronomers hypothesize that a massive black hole lies at the center of M87?
A very small region at the center of M87 releases an enormous amount of energy.
Select the correct definitions of active galactic nuclei and quasars.
Active galactic nuclei are centers of the galaxies which are unusually bright, with luminosity produced by sources other than stars. Quasars are the brightest of them and have luminosities of hundreds times greater than our galaxy has.
The patterns on the graph of galaxy colors and luminosities allow scientists to make inferences about galaxy evolution. Which of the following statements accurately reflect current scientific thinking about galaxy evolution based on these data?
All galaxies start their lives as members of the blue cloud. Very large galaxies tend to use up the gas available for star formation more rapidly than smaller galaxies. Some large red galaxies arose from mergers of smaller blue galaxies.
What are we seeing in this Hubble Space Telescope photograph?
Almost every object in the photo is a galaxy, and these galaxies are at many different distances from us.
The three cubes represent three stages in a computer simulation of the formation of protogalactic clouds; they represent a region about 500 million light-years across. Which of the following statements about this model is true?
Although the gas looks uniform in the first frame, the model assumes there were small regions with slightly higher densities than other regions
As you learned in Part E, galaxy M82 has unusually active star formation. Why do astronomers think that this is a "burst" of star formation, rather than thinking that this galaxy always has such a high rate of star formation?
At its current rate of star formation, the galaxy would use up all its dust and gas in just a few hundred million years.
Why can't we see past the cosmological horizon?
Beyond the cosmological horizon, we would be looking back to a time before the universe was born.
Choose the correct explanation of how studies of absorption lines in quasar spectra allow us to study the gas outside of galaxies.
Clouds of intergalactic gas absorb certain wavelengths of the emission spectra of quasars. Because of the redshift, it is possible to distinguish spectra from more distant and correspondingly younger parts of the universe.
What are the dark blobs (indicated by the red arrows) in this photograph from the Hubble Space Telescope?
Cold, dense molecular clouds in which stars are forming.
Choose the correct statement explaining the evidence that supermassive black holes really exist. Drag the items on the left to the appropriate blanks on the right to complete the sentences. (Not all terms will be used.)
Evidence can be found in the orbits of gas clouds and accretion disks in galaxies like M87 and NGC 4258. Their orbital speeds show that they are orbiting extremely massive objects. The central objects must also be quite small in size for their masses in these cases. We know no objects that could be so massive and so small apart from a supermassive black hole.
Why do virtually all the galaxies in the universe appear to be moving away from our own?
Expansion causes all galaxies to move away from nearly all others.
Which of the following statements correctly explains why galaxy collisions should have been more common in the past than they are today?
Galaxies were closer together in the past, because the universe was smaller.
Spectral lines from Galaxy B are redshifted from their rest wavelengths twice as much as the spectral lines from Galaxy A. According to Hubble's law, what can you say about their approximate relative distances?
Galaxy B is twice as far as Galaxy A
Which of the following statements is not an assumption used in models of galaxy formation?
Gas contracted to form the disks of galaxies before any stars were born.
If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
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.
Assuming that the bright core of M87 is powered by a supermassive black hole, which of the following best describes the source of energy that makes the core appear so bright?
Gravitational potential energy is converted to thermal energy as matter from the surrounding gas disk spirals into the central black hole.
Choose the correct explanation of why is that gas important to galaxy evolution.
Halo gas and intergalactic gas supply cold gas to the galaxies for star formation.
In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thereby putting to rest the idea that it might have been a cloud within our own galaxy. What key observation enabled him to prove this?
He observed individual Cepheid variable stars in Andromeda.
Listed following are several stars found in the disk and halo of the Milky Way Galaxy. Assume that both the blue and yellow disk stars are members of the same open cluster. Rank the stars based on the abundance of elements heavier than carbon that you would expect to find in each of the stars, from highest to lowest. If you expect two (or more) stars to have approximately the same abundance, rank them as equal by dragging one on top of the other(s).
Highest Abundance: Yellow main-sequence star in open cluster in disk AND hot, blue main-sequence star in diskLowest Abundance: red giant in globular cluster M13 AND red main-sequence star in globular cluster M13
How can we use variations in luminosity to set limits on the size of an active galactic nuclei? Drag the items on the left to the appropriate blanks on the right to complete the sentences. Not all terms will be used.
If an object changes its brightness in 1 hour, that the object cannot be more than 1 light-hour in size. If it were, the light from the far side of the object would reach our telescopes more than 1 hour after the light from the front side and the change of brightness would take more than an hour to occur. Therefore, the size of the variable object must be less than the product of the speed of light and the period of brightness variations. The hourly changes of brightness of active galaxy nuclei and quasars show that their radiation is generated in a volume no bigger than our solar system.
Click the icon "Spiral arm and star motion" in the interactive figure and watch the animation of the galaxy's rotation over several hundred million years. Which of the following statements accurately describe the motion? Select all that apply.
Individual stars orbit around the center of the galaxy. Individual stars move in and out of spiral arms over time.
The top panorama shows our view of the Milky Way in all directions as it appears in visible light. The bottom panorama shows the same view, but in a different wavelength of light. What wavelength band are we seeing in the bottom photo, and how do you know?
Infrared light, because the dust that appears dark in the visible light photo glows in infrared light.
Different regions of the galaxy tend to contain stars of different ages. Place labels for the ages of stars in the correct regions of the galaxy painting. Drag the labels to the appropriate blanks on the diagram. You may use labels more than once.
Inside both young and old stars outside mostly very old stars
Based on what you have learned, which of the following best describes the meaning of Hubble's constant (H0H0 )?
It describes the expansion rate of the universe, with higher values meaning more rapid expansion.
Is this a photo of a spiral galaxy or an elliptical galaxy, and how do you know?
It is a spiral galaxy because we can see a dusty disk going across the center.
This visible-light photo shows a starburst galaxy. What is going on with the reddish gas that seems to be emerging on both sides of it?
It is gas being blown out in a galactic wind.
Which of the following best describes the status of the Milky Way in our Local Group of galaxies?
It is one of the two largest galaxies in the group.
How does the interstellar medium affect our view of most of the galaxy?
It prevents us from seeing most of the galactic disk with visible and ultraviolet light
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?
It should have a higher fraction of elements heavier than hydrogen and helium
How would you classify the listed galaxies using the system of Hubble's galaxy classes? Drag the items on the left to the appropriate blanks on the right.
M87is an elliptical galaxy. It is pretty round so it is probably an E0galaxy. is an elliptical galaxy. It is pretty round so it is probably an E0E0 galaxy.NGC 1300 NGC 1300is obviously a barred spiral. It is an SBbor SBcgalaxy, given how tightly its spiral arms are wrapped. is obviously a barred spiral. It is an SBbSBb or SBcSBc galaxy, given how tightly its spiral arms are wrapped.NGC 4594 NGC 4594is an edge-on spiral with a large bulge. It does not show the bar and its arms are tightly wrapped, therefore it is an Sagalaxy. is an edge-on spiral with a large bulge. It does not show the bar and its arms are tightly wrapped, therefore it is an SaSagalaxy.NGC 4414 NGC 4414is a tilted disk galaxy with a flocculent, discontinuous spiral arms. It does not have a bar, and its bulge is not very large. It should be Scor Sbgalaxy. is a tilted disk galaxy with a flocculent, discontinuous spiral arms. It does not have a bar, and its bulge is not very large. It should be ScSc or SbSb galaxy.M101 M101is a face-on spiral galaxy. It does not have a bar, its bulge is not very large, and its spiral arms are not very tight. It should be Scor Sbgalaxy. is a face-on spiral galaxy. It does not have a bar, its bulge is not very large, and its spiral arms are not very tight. It should be ScSc or SbSb galaxy.
Sort each item into the appropriate bin according to whether it is more common in spiral arms or about equally common within and between the spiral arms in a spiral galaxy's disk. Drag items into the appropriate bin.
More common in spiral arms: dense, dusty gas clouds star formation young stars ionization nebulae Equally common within and between spiral arms: old stars
Which of the following statements comparing halo stars to our Sun is not true?
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.
Does Hubble's law work well for galaxies within the Local Group? Why or why not?
No, because galaxies in the Local Group are gravitationally bound together.
The following figures show several stars found in the disk and halo of the Milky Way Galaxy. Rank the stars based on their current age, from oldest to youngest. If two (or more) stars have approximately the same age (that is, ages within a few million years), rank them as equal by dragging one on top of the other(s).
Oldest: Red Giant in globular cluster M13 AND red main-sequence star in globular cluster M13Middle: the sunYoungest: Hot, blue main-sequence star in disk
Why are collisions between galaxies more likely than collisions of stars within galaxies?
Relative to their sizes, galaxies are much closer together than stars
Which of the following is an important starting assumption in models of galaxy formation?
Some regions in the universe start out denser than others.
What are the starting assumptions for models of galaxy formation?
Some regions of gas were slightly denser than others. The universe was initially filled more or less uniformly with hydrogen and helium gas.
Watch the video that comes up when you click the icon "Star formation in spiral arms" in the interactive figure. Which of the following best describes what spiral arms are?
Spiral arms are waves of higher density that move outward through a galaxy, triggering star formation as they pass.
Following are a number of distinguishing characteristics of spiral and elliptical galaxies. Match each characteristic to the appropriate galaxy type.
Spiral galaxies:1. have a flattened disk of stars2. are rare in central regions of galaxy clusters3. contain many bright, hot stars4. contain abundant clouds of cool gas and dust5. have significant, ongoing star formation Elliptical galaxies:1. are more reddish in color2. contain primarily old, low-mass stars
Study all of the individual images and the Multi image that combines all of them. Which of the following statements provides the best interpretation of what we see in the Multi image?
Stars in the central region of this galaxy are exploding as supernovae, generating a galactic wind of hot gas and warm dust gains.
If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?
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.
Compare that speed with the Sun's orbital speed around our galaxy. Based on your comparison, do you think the galaxy's gravity would be strong enough to retain the supernova debris if there were no interstellar medium to slow it down?
Supernova debris moves about 100 times faster than the Sun. The gravity of galaxy alone would not retain the supernova debris.
Which is not a general characteristic of starburst galaxies?
The "starburst" is thought to be caused by the presence of a supermassive black hole in the galaxy's center.
The third image in the video (with the most detailed view of the galactic center) is labeled "gas disk." Which of the following best describes what we are seeing in this photo?
The black hole is located deep within the bright central region, and around this region we see gas that is orbiting the central black hole.
Suppose that Hubble's constant were 11 km/s/million light-years instead of 22 km/s/million ligh-years. How would this graph look different?
The data would follow a more shallow line, closer to horizontal than the current line.
How does the diameter of the disk of the Milky Way Galaxy compare to its thickness?
The diameter is about 100 times as great as the thickness.
Match the words in the left column to the appropriate blanks in the sentences on the right. Match the words in the left column to the appropriate blanks in the sentences on the right. Words can be used more than once.
The disk population of stars contains both young and old stars, all of which have heavy elementals abundances of about 2% heavy elements. The halo stars are all very old and have low masses. The halo stars have about 100 times less heavy elements than the disk stars.
Describe how a spiral galaxy is thought to form. Drag the items on the left to the appropriate blanks on the right to complete the sentences. (Items can be used once or not at all.)
The extra density in some areas increased the gravitational pull, slowing and then reversing the expansion that occurred due to the cosmological expansion, forming protogalactic gas clouds. The gas cloud begins to form stars, but the star formation does not use up all the gas. The angular momentum of the original gas cloud causes the gas cloud to spin and flatten out as it collapses, gas particles collide, and the gas cools, thereby forming the disk of a spiral galaxy out of the leftover gas.
Interactions among galaxies also are thought to influence a galaxy's type in at least some cases. Which of the following does not support the idea that interactions can shape galaxies?
The fact that more distant galaxies have larger redshifts.
Which statement below correctly describes the relationship between expansion rate and age for the universe?
The faster the rate of expansion, the younger the age of the universe.
Choose the correct statement explaining what do starbursts tell us about the star-gas-star cycle within galaxies.
The large number of supernovae after the first starburst stage creates a galactic wind that blows most of the gas out of the galaxy, ending star formation until the gas cools down.
What does Hubble's law tell us?
The more distant a galaxy, the faster it is moving away from us.
All of the following observations are real. Which one does not support the model in which active galactic nuclei are powered by accretion disks around massive black holes?
The most luminous active galactic nuclei have huge redshifts.
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
This photo shows the central region of a cluster of galaxies. Based on what you have learned from your text, which of the following statements about this photo is not true?
The oldest stars in these galaxies are much older than the oldest stars in our own Milky Way.
What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?
The orbital speeds of stars far from the galactic center are surprisingly high.
What is the best evidence for an extremely massive black hole in the center of the Milky Way?
The orbits of stars in the center of the galaxy indicate that the presence of an approximately 4 million solar-mass object in a region no larger than our solar system.
Notice the well-defined spiral arms in this photograph of the galaxy M51. What makes the spiral arms so much brighter than regions between the arms?
The presence of many massive young stars in the spiral arms.
The infrared image of M82 shows a whitish-blue region running down the center, along with a much larger red region. What is emitting the light that is shown in red, and what is emitting the light that is shown in whitish-blue?
The red is emission from warm dust grains; the whitish-blue is emission from stars.
Which of the following models best explains why our galaxy has spiral arms?
The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy.
How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?
The total amount of gas will be much less than it is today.
Suppose we observe a source of x-rays that varies substantially in brightness over a period of a few days. What can we conclude?
The x-ray source is no more than a few light-days in diameter.
You've now found that the central object has a mass of about 4 million solar masses but is no more than about 70 AUAU 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?
There is no known way to pack so much mass into such a small volume without it collapsing into a black hole.
How do disk stars orbit the center of the galaxy?
They all orbit in roughly the same plane and in the same direction.
Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?
They are rare events, so we have observed them in only a tiny fraction of all galaxies.
Which of the following is not a feature of central dominant galaxies?
They are spiral galaxies.
Imagine that radar had never been invented and that we instead had to rely on a less reliable method of measuring distances in our solar system. If that method led us to underestimate the Earth-Sun distance by 10%, how would it affect other measurements in the distance chain?
They would all be off by the same 10%.
What can we conclude about this galaxy from the fact that there are so many supernovae occurring?
This galaxy contains an unusually large number of young stars.
This painting represents the Milky Way Galaxy as it might appear from a distance. Identify the indicated features. Drag the labels to the appropriate blanks on the diagram.
Top to bottom -spiral arm -bulge -disk
This diagram shows several stages in a computer simulation of a collision between two galaxies. What is being simulated?
Two spiral galaxies merge to become an elliptical galaxy surrounded by debris.
Why do we use Hubble's law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases?
We have not observed white dwarf supernovae in most galaxies.
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?
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
Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. Which statement would be true?
We would not be able to measure the distances of distant galaxies.
What is the shape of the Milky Way's halo?
What is the shape of the Milky Way's halo?
Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.
What is this a picture of?
a cluster of galaxies
Which of these galaxies is most likely to be oldest?
a galaxy in the Local Group
In which of these galaxies would you be least likely to find an ionization nebula heated by hot young stars?
a large elliptical galaxy
Which of these galaxies would you most likely find at the center of a large cluster of galaxies?
a large elliptical galaxy
A collision and merger of two large elliptical galaxies will eventually produce
a large elliptical galaxy.
A standard candle is __________.
a light source of known luminosity
One possible explanation for a galaxy's type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed as a result of other interactions, we'd expect this galaxy to be ______.
a spiral galaxy
Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?
a white dwarf supernova
The rate at which supernovae explode in a starburst galaxy that is forming stars 10 times as fast as the Milky Way is
about 10 times the rate in the Milky Way.
Intergalactic hydrogen clouds are easiest to study by looking at __________.
absorption lines in quasar spectra
Which kind of star is most likely to be found in the halo?
an M star
According to current understanding, what is a quasar?
an active galactic nucleus that is particularly luminous
The most common form of gas in the disk of the Milky Way galaxy is _________.
atomic hydrogen gas
Assume that Hubble's constant is 22 kilometers per second per million light-years. Then we would expect a galaxy 100 million light-years away to be moving __________. (Assume the motion is due only to Hubble's law.)
away from us at 2,200 km/s
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
The fact that the universe is expanding means that space itself is growing __________.
between clusters of galaxies
How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?
by applying Newton's version of Kepler's third law (or the equivalent orbital velocity formula) to the Sun's orbit around the center of the Galaxy
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years awa
cepheids
Which of the following statements correctly summarize key differences between the disk and the halo? Check all that apply.
check all
Below is a sketch of our galaxy as it would appear edge-on. Identify the disk , bulge, and halo. Drag the appropriate labels to their respective targets.
disk, bulge, halo
The most basic difference between elliptical galaxies and spiral galaxies is that ______.
elliptical galaxies lack anything resembling the disk of a spiral galaxy
How do we determine the Milky Way's mass outside the Sun's orbit?
from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun
The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that __________.
galaxy formation and supermassive black hole formation must be related
All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________.
globular clusters
Which of the following is not one of the three major categories of galaxies?
globular galaxies
The primary source of a quasar's energy is
gravitational potential energy
According to the model in which that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from __________.
gravitational potential energy released by matter that is falling toward the black hole
Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.
have a greater abundance of heavy elements
To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________.
if they observed for many more years, the dots would trace out ellipses
Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in __________.
infrared light
What type of galaxy is M82 based on its appearance in the visible-light view?
irregular
According to observations, which type of galaxy was much more common when the universe was 2 billion years old than it is today?
irregular galaxies
If we say that a galaxy has a lookback time of 1 billion years, we mean that __________.
its light traveled through space for 1 billion years to reach us
This figure shows observations that have allowed astronomers to determine the orbits of several stars around Sgr A* (the central object). What can we learn about Sgr A* by analyzing these orbits?
its mass
On a graph plotting galaxy luminosities against galaxy colors, the red sequence represents galaxies that are __________ than galaxies of the blue cloud.
larger and more elliptical
This painting represents the Milky Way Galaxy as it would appear edge-on from a distance. Label the indicated features; be sure to pay attention to where the leader lines are pointing. Drag the labels to the appropriate blanks on the diagram. You may use a label more than once.
left to right -global clusters -bulge -halo -disk -disk
Galaxies with disks but no evident spiral arms are called __________.
lenticular galaxies
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.
longest time 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 fromtop to bottom shortest time
Based on this graph, if you can measure the mass of a galaxy's central bulge, then you will also know the approximate __________.
mass of the galaxy's supermassive black hole
According to Leavitt's law, we can determine the luminosity of a Cepheid in a distant galaxy by
measuring its period of brightening and dimming.
We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble's law, we must first calibrate it by __________.
measuring the distances to many distant galaxies with a standard candle technique
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
Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found _________.
only in or near star-forming clouds
The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with __________.
radio telescopes observing at a wavelength of 21 centimeters
Based on this diagram, "red sequence" galaxies are __________ than "blue cloud" galaxies.
redder and more luminous
Hubble's galaxy classification diagram (the "tuning fork") ______.
relates galaxies according to their shapes
Two ways in which the starting conditions in a protogalactic cloud might cause it to become an elliptical (rather than spiral) galaxy are if the cloud begins with either _________________.
relatively little angular momentum or relatively high density
Observations indicate that over billions of years, galaxies in general tend to change from _________.
smaller and bluer to larger and redder
Look again at the visible-light view of M82. What is the source of the white and blue light that dominates the image?
stars
What does cosmological redshift do to light?
stretches its wavelength
Suppose that Hubble's constant were H0H0 = 42 km/s/Mlykm/s/Mly (which is not its actual value). What would the approximate age of the universe be in that case? Express your answer in years to two significant figures.
t0 = 7.1×10^9yr
Radar, the first link in the cosmic distance chain, is used to establish the baseline distance necessary for the second link, parallax. What baseline distance must we know before we can measure parallax?
the Earth-Sun distance
The luminosity of a quasar is generated in a region the size of
the color system
Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called __________.
the cosmological horizon
Hubble's law expresses a relationship between __________.
the distance of a galaxy and the speed at which it is moving away from us
A quasar's spectrum is hugely redshifted. What does this large redshift tells us about the quasar?
the distance to the quasar
What law of nature explains why the galaxy began to rotate rapidly and flatten out as it shrunk in size?
the law of conservation of angular momentum
Applying Newton's version of Kepler's third law (or the orbital velocity law) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine ______.
the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center
Observations have shown that when the mass of the central black hole is large, then __________.
the mass of the bulge of the host galaxy is also large
We measure the mass of the black hole at the galactic center from:
the orbits of stars in the galactic center.
This image combines a visible-light view of the galaxy Hercules A with an image showing radio wave emission (the reddish colors) in its vicinity. The likely explanation for the two huge clumps of radio emission far to either side of the visible galaxy is __________.
the radio emission comes from gas shot out into space by jets powered by a supermassive black hole in the galactic center
If you observed the redshifts of galaxies at a given distance to be twice as large as they are now, then you would determine a value for Hubble's constant that is
twice as large as its current value.
Most stars in the Milky Way's halo are _________.
very old
When the ultraviolet light from hot stars in very distant galaxies finally reaches us, it arrives at Earth in the form of
visible light
Which cosmic distance measurement techniques are considered standard candle techniques?
white dwarf supernovae (distant standards) main-sequence fitting cepheids