ASTRO WK 7 mastering astronomy chp 10 hw
The following figures give the approximate speeds at which five galaxies are moving away from Earth due to the expansion of the universe. Rank the galaxies based on their distance from Earth, from farthest to closest.
(Farthest) -130,000 km/s -45,000 km/s -18,730 km/s -5,264 km/s -1,577 km/s (Closest)
The following figures give the approximate distances of five galaxies from Earth. Rank the galaxies based on the speed with which each should be moving away from Earth due to the expansion of the universe, from fastest to slowest.
(Fastest) -5 billion light-years -2 billion light-years -800 million light-years -230 million light-years -70 million light-years (Slowest)
Hubble's law is a relationship between galaxy speeds and galaxy distances. This relationship can be shown as a line on a graph of speed versus distance. Each of the following four graphs shows a possible relationship expressing Hubble's law. Rank the graphs based on their predictions of the speed, from fastest to slowest, for a galaxy located 400 million light-years away from Earth.
(Fastest) -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 400 -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 600 -Speed away from Earth (km/s) 20000, Distance from Earth (millions of light-years) 1000 -Speed away from Earth (km/s) 15000, Distance from Earth (millions of light-years) 1200 (Slowest)
The following figures give the approximate speeds at which five galaxies are moving away from Earth due to the expansion of the universe. Rank the galaxies based on the amount of redshift that would be observed in each galaxy's spectrum, from largest to smallest.
(Largest redshift) -130,000 km/s -45,000 km/s -18,730 km/s -5,264 km/s -1,577 km/s (Smallest redshift)
The following four graphs are the same as those from Part D, with each showing a possible relationship for Hubble's law. Rank the graphs based on the prediction they each would make for the current age of the universe, from oldest to youngest.
(Oldest universe) -Speed away from Earth (km/s) 15000, Distance from Earth (millions of light-years) 1200 -Speed away from Earth (km/s) 20000, Distance from Earth (millions of light-years) 1000 -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 600 -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 400 (Youngest universe)
Which cosmic distance measurement techniques are considered standard candle techniques?
* white dwarf supernovae (distant standards) *cepheids *main-sequence fitting
Suppose that you measure a galaxy's redshift, and from the redshift you determine that its recession velocity is 30,000 (3×104) kilometers per second. According to Hubble's law, approximately how far away is the galaxy?
1.4 billion light-years
A Cepheid with a period of 30 days has an average luminosity that is about __________ times the luminosity of the Sun. (You will have to click on the blue Cepheids link on the main screen of the Interactive Figure to get back to the relevant plot.)
10,000
Current estimates place the age of the universe at about ______.
14 billion years
Click on the blue Cepheid label, then click the label on the bottom of the Interactive Figure that reads Cepheids as Standard Candles to bring up the next screen. Then click the Next button in the Interactive Figure to bring up the screen that shows a Cepheid light curve in the upper left. Notice that the window with the light curve also has a box that tells you the apparent brightness of the Cepheid, which shows a value of 1.3×104. (The units are shown in the box, but you only need to focus on the numerical value.) Based on this apparent brightness and the luminosity you found in Part D, what is the approximate distance to this Cepheid?
2.5 million light-years
Click on the blue Cepheid label, then click the label on the bottom of the Interactive Figure that reads Cepheids as Standard Candles to bring up the next screen. Then click the Next button in the Interactive Figure to bring up the screen that shows a Cepheid light curve in the upper left. What is the approximate luminosity of the Cepheid whose light curve is shown in the graph?
9000 LSun
Imagine that when we looked out into the universe we found that the light from all galaxies was blueshifted (rather than redshifted) and that the light from the most distant galaxies was blueshifted by the greatest amount. Which statement best describes what we would conclude about the motions of galaxies in this case?
All are moving toward Earth, with distant galaxies moving faster than nearby galaxies.
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 Hubble's law predicts that all galaxies outside our Local Group should be moving away from us.
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 the galaxies of the Local Group are gravitationally bound together.
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
Cepheids
To get started, click on the blue Cepheids link on the main screen of the Interactive Figure. The Interactive Figure (in red) shows a graph of the Cepheid period-luminosity relation. This graph indicates that __________.
Cepheids with longer periods have higher luminosities
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.
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.
Based on what you have learned, which of the following best describes the meaning of Hubble's constant (H0 )?
It describes the expansion rate of the universe, with higher values meaning more rapid expansion.
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.
The following diagrams are similar to those in Part A, except this time each one shows two galaxies: "your galaxy" and a second galaxy. Rank the diagrams based on how far the second galaxy has moved away from your galaxy due to expansion of the universe between the early time and the later time, starting at the left with the one that has moved the largest distance and moving to the one that has moved the smallest distance at the right.
Largest distance are the dots farthest apart to dots closest together (smallest)
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 do not have much star formation. As a result, we see few hot, young stars in these galaxies.
The following diagrams are similar to those from Part B, with each diagram showing the position of "your galaxy" and another galaxy at an early and later time in the history of the universe. Rank the diagrams based on the speed at which the other galaxy is moving away from your galaxy as the universe expands, from fastest to slowest.
Same as part B: fastest are the dots farthest apart to dots closest together (slowest)
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 stars 2. are rare in central regions of galaxy clusters 3. contain many bright, hot stars 4. contain abundant clouds of cool gas and dust 5. have significant, ongoing star formation Elliptical galaxies: 1. are more reddish in color 2. contain primarily old, low-mass stars
Why do you think astronomers would want to devote so much precious telescope time to observing totally ordinary regions of the sky in such great detail?
Statistics tells something about objects that are not in the statistical sample.
Which of the following three-step processes correctly describes how we use Cepheids as a tool to make cosmic distance measurements? In all cases, assume that the Cepheid's apparent brightness has been carefully measured through observations.
Step 1: Measure the period of the Cepheid's brightness variations. Step 2: Use the period-luminosity relation to determine the Cepheid's luminosity. Step 3: Calculate the Cepheid's distance from its luminosity and apparent brightness.
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.
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.
Click on the blue Cepheid label, then click the label on the bottom of the Interactive Figure that reads Cepheids as Standard Candles to bring up the next screen. Read the instructions that appear in the upper left hand corner and study the animation. What actually causes a Cepheid to vary in apparent brightness?
The Cepheid varies in radius, and its luminosity is greater when its radius is larger.
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 elliptical galaxies lack a significant disk component, although both types have the halo component.
What does Hubble's law tell us?
The more distant a galaxy, the faster it is moving away from us.
The following diagrams are similar to those from parts B and C, with each diagram showing the position of "your galaxy" and another galaxy at an early and later time in the history of the universe. (But note that this set of diagrams differs from the prior sets.) Rank the diagrams based on the speed at which the other galaxy is moving away from your galaxy as the universe expands, from fastest to slowest. If two (or more) of the diagrams show galaxies moving at the same speed, show this equality by dragging one diagram on top of the other(s). (Hint: Notice that two of the diagrams show the galaxies separated by one distance and two of the diagrams show them separated by a different distance; that is, there are only two different distances shown among the four diagrams.)
The two with points farthest apart are fastest (on top of each other); and the two with points closest together are slowest (on top of each other)
By looking at the graph of Hubble's law, what can we say about the galaxies that have the lowest speeds?
They are moving away from Earth and are closer to Earth than galaxies with high speeds.
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%.
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.
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.
About how many galaxies are there in a typical cluster of galaxies?
a few dozen
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 standard candle is _________.
a light source of known luminosity
What is main-sequence fitting?
a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram
What is a Cepheid variable?
a type of very luminous star that makes an excellent standard candle
Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?
a white dwarf supernova
The following diagrams represent a balloon analogy for the expansion of the universe. Each diagram shows two balloons: The small pink balloon represents the universe at an early time and the large red balloon represents the universe at some later time. The black dot on each balloon represents a galaxy. Rank the diagrams based on how much the galaxy has expanded in size (due to the overall universal expansion) from the early time to the later time in each case, from most expansion to least expansion. If you think that two (or more) of the diagrams show galaxies that have expanded by the same proportion, indicate this equality by dragging one diagram on top of the other(s).
all stacked The universe as a whole is expanding, but gravitationally bound objects such as galaxies are not expanding with time. Because no galaxy expands at all, all the diagrams are ranked equal.
Scientists investigating _______ study how the distribution of galaxies changes with time.
cosmology
We determine the distance of a Cepheid by
determining its luminosity from the period-luminosity relation and then applying the inverse square law for light.
Our Sun belongs to the _______ of the Milky Way Galaxy.
disk component
A(n) ______ contains hot, ionized gas but very little cool gas or dust.
elliptical galaxy
The globular cluster M13 belongs to the ______ of the Milky Way Galaxy.
halo component
The type of galaxy known as a(n) ______ was more common in the universe 10 billion years ago.
irregular galaxy
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
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
The graph of Hubble's law shows that galaxies with high speeds as measured from Earth are __________.
moving away from Earth and are farther from Earth than galaxies with lower speeds
The Milky Way is a(n) _____
spiral galaxy
Overall, what is our most accurate technique for measuring the distance to a nearby star?
stellar parallax
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
Hubble's law expresses a relationship between __________.
the distance of a galaxy and the speed at which it is moving away from us
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
Suppose that galaxy B is twice as far from Earth as galaxy A. Hubble's law predicts that galaxy B will be moving away from Earth with approximately _____.
twice the velocity of galaxy A
When the ultraviolet light from hot stars in very distant galaxies finally reaches us, it arrives at Earth in the form of
visible light
The age of the universe is related to the slope of the graph of Hubble's law, and current data put the age of the universe at about 14 billion years. Suppose that future observations showed that the slope of Hubble's law on the graph is actually steeper than that shown. In that case, the age of the universe would be _________ than 14 billion years because the universe is expanding ______ than current data suggest. (Each choice gives words to fill in the two blanks, separated by a slash.)
younger / more rapidly
