If the orbital distance of Earth was greater than its actual value, the parallax angle to nearby stars would be ___________.
answer
higher
question
If the orbital distance of Earth was greater than its actual value, the maximum distance a star could be away from Earth for its distance to be determined using the parallax method (with current telescopes) would be _________.
answer
higher than the actual upper limit
question
Stars that have the highest apparent brightness are ________.
answer
nearby and very luminous
question
Massive-star supernovae are not good standard candles because ___________.
answer
they have different luminosities (since they have different masses)
question
What two things are needed to determine a standard candle's distance from Earth?
answer
apparent brightness and luminosity
question
About how long would it take for a radar pulse to travel round trip from Earth to Neptune and back again to Earth?
answer
8 hours
question
A supernova is observed in a galaxy 2,000 million light-years away. How long ago did the supernova occur?
answer
2 billion (giga) years ago
question
Which type of standard candle is the most appropriate for determining the distance to nearby galaxies (like the Andromeda Galaxy)?
answer
Cepheid variables
question
Which of the following is the least accurate method for determining cosmic distances?
answer
Hubble's Law
question
Astronomers believe that nearly all galaxies are moving away from us because _________.
answer
the space in between the galaxies is expanding
question
Current estimates place the value of Hubble's constant (H0) near 22 km/s/Mly, giving 14 billion years as the age of the universe. If H0 were in fact 44 km/s/Mly, the approximate age of the universe would be _____________.
answer
half as much
question
Suppose Hubble's constant is H0 = 22 km/s/Mly.
Then we would expect a galaxy located 10 million light-years (Mly) away to be moving away from us at a speed of about _______________.
answer
220 km/s
question
An astronomer looking out into space will observe that _______.
answer
the farther out a galaxy is from Earth, the faster it is moving away
question
From our vantage point in the Milky Way, we see distant galaxies all moving away from us and more distant ones moving faster. Suppose you lived in a distant galaxy, what would you observe?
answer
The same rule that we observe: all galaxies moving away from you, and more distant ones moving faster.
question
One reason Hubble's constant isn't exactly known today is ______.
answer
the exact distance of a galaxy is difficult to measure
question
Hubble's law shows that galaxies with high speeds as measured from Earth are __________.
answer
moving away from Earth and are farther from Earth than galaxies with lower speeds
question
What can we say about the galaxies that have the lowest speeds?
answer
They are moving away from Earth and are closer to Earth than galaxies with high speeds.
question
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 _____.
answer
twice the velocity of galaxy A
Hubble's law (the straight-line fit to the data) predicts that a galaxy's recession velocity is proportional to its distance from Earth. Hubble's law therefore predicts that a galaxy twice as far away moves at twice the speed, a galaxy three times as far away moves at three times the speed, and so on.
question
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?
answer
All are moving toward Earth, with distant galaxies moving faster than nearby galaxies.
In this hypothetical case, all the galaxies would be rushing toward Earth, with the more distant galaxies approaching at the fastest pace. This would tell us we lived in a collapsing universe rather than an expanding one.
question
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.
answer
younger/more rapidly
A more steeply sloped line for Hubble's law would indicate faster speeds for galaxies at all distances, which would mean a faster rate of expansion. And a faster rate of expansion means it has taken less time since the Big Bang for the universe to reach its present size. In other words, a steeper slope (which means a larger value of Hubble's constant) means a younger and more rapidly expanding universe.
question
[Note]: Recall that a redshift tells us that an object is moving away from us, and the larger the redshift, the higher the speed. Note that expansion applies to the whole universe, so astronomers tend to think of galaxies being carried along with the expansion rather than moving "through" the universe. For this reason, redshifts due to expansion are often called "cosmological redshifts."
answer
***
question
[Note]: If you look at the speed corresponding to a distance of 400 million light-years on each graph, you will see that the speed declines in the order shown. Note also that this ranking puts the graphs in order of declining steepness. In other words, a steeper slope for Hubble's law would predict faster speeds for galaxies at particular distances.
answer
***
question
Because galaxies all started out close together (early in the history of the universe), higher speeds would mean that they would have reached their current distances in less time — and less time means a younger universe. In other words, the steeper the graph for Hubble's law, the more rapid the rate of expansion (a higher value of Hubble's constant), and the more rapid the rate of expansion the younger the universe.
answer
***
question
Some things to know about Universe Expansion
answer
(1) The universe as a whole is expanding, but gravitationally bound objects such as galaxies are not expanding with time.
(2) Notice that the galaxies that started farther away (on the small balloon) traveled greater distances as the balloon expanded
(3) The farther away a galaxy is from your galaxy, the faster it will be moving away from you. This simple statement embodies the idea behind Hubble's law.
(4) The location of your galaxy does not matter: The speed at which the other galaxy is moving away from you depends only on its distance, with more distant galaxies moving faster in accord with Hubble's law.
question
Hubble's law expresses a relationship between __________.
answer
the distance of a galaxy and the speed at which it is moving away from us
question
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 __________.
answer
measuring the distances to many distant galaxies with a standard candle technique
question
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?
answer
1.4 billion light years
question
Based on what you have learned, which of the following best describes the meaning of Hubble's constant (H0 )?
answer
It describes the expansion rate of the universe, with higher values meaning more rapid expansion.
The larger the value of Hubble's constant, the faster galaxies are being carried away from us with the expansion of the universe. For example, if a galaxy's velocity depended only on the expansion of the universe, then a galaxy that is 1 million light-years away would have a recession velocity of 22 kilometers per second, a galaxy 10 million light-years away would have a recession velocity of 220 kilometers per second, and a galaxy 100 million light-years away would have a recession velocity of 2200 kilometers per second. In other words, Hubble's constant essentially tells us the expansion rate of the universe, since it tells us how fast galaxies are moving at different distances away from us. A higher value of Hubble's constant would mean higher recession speeds—which would mean a more rapid expansion rate for the universe.
question
The Interactive Figure (in red) shows a graph of the Cepheid period-luminosity relation. This graph indicates that __________.
answer
Cepheids with longer periods have higher luminosities
question
A Cepheid with a period of 30 days has an average luminosity that is about __________ times the luminosity of the Sun.
answer
10,000
question
What actually causes a Cepheid to vary in apparent brightness?
answer
The Cepheid varies in radius, and its luminosity is greater when its radius is larger.
question
What is the approximate luminosity of the Cepheid whose light curve is shown in the graph?
answer
8000 Lsun
question
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?
answer
the Earth-Sun distance
question
What is the three-step processes that describes how we use Cepheids as a tool to make cosmic distance measurements?
answer
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.
question
Which cosmic distance measurement techniques are considered standard candle techniques?
answer
(1) Main-sequence fitting
-known luminosities
(2) White dwarf supernovae (distant standards)
- all thought to have the same luminosites
(3) Cepheids
- we know their luminosities from the period-luminosity relation
question
Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. What would be true?
answer
We would not be able to measure the distances of distant galaxies.
Explanation:
(1) Cepheids are used to measure the distances of nearby galaxies in which white dwarf supernovae have been detected, which allows us to determine the luminosities of these supernovae.
(2) White dwarf supernovae, in turn, allow us to calibrate Hubble's law.
(3) Therefore, without Cepheids, we would not be able to use either white dwarf supernovae or Hubble's law, and we would be unable to measure the distances of distant galaxies.
question
Why do we use Hubble's law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases?
answer
We have not observed white dwarf supernovae in most galaxies.
question
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?
answer
They would all be off by the same 10%.
question
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
answer
Cepheids
question
Which distance measurement technique is most suited to measuring objects at a distance of 10 million light-years?
answer
Cepheids
question
Which distance measurement technique is most suited to measuring objects at a distance of 10 light-years?
answer
parallax
question
Which standard-candle technique is best for measuring the distances of very distant galaxies?
answer
distant standards
question
What color(s) in the image represent(s) the radio emission?
answer
Purple and pinkish-white
question
What is the approximate distance from the far edge of one lobe to the far edge of the other?
answer
approximately 2,100,000 light-years
question
What is the approximate size of the visible galaxy producing the jets?
answer
about 200,000 light-years in diameter
question
What are some distinguishing characteristics of Spiral galaxies?
answer
(1) have flattened disk of stars
(2)contain many bright, hot stars
(3) have significant, ongoing star formation
(4) are rare in central regions of galaxy clusters
(5) contain abundant clouds of cool gas and dust
question
What are some distinguishing characteristics of Elliptical galaxies?
answer
(1) are more reddish in color
(2) contain primarily old, low-mass stars
question
A galaxy's type can be affected both by the conditions in the protogalactic cloud from which it forms ("initial conditions") and by later interactions with other galaxies.
What creates an elliptical galaxy?
answer
(1) stars form rapidly as the protogalactic cloud shrinks
(2) protogalactic cloud has high density
(3) a galaxy collision strips away gas
(4) protogalactic cloud rotates very slowly
Processes that do not allow gas to settle into a disk (or that strip gas away) lead to elliptical galaxies.
question
A galaxy's type can be affected both by the conditions in the protogalactic cloud from which it forms ("initial conditions") and by later interactions with other galaxies.
What creates a Spiral galaxy?
answer
(1) most protogalactic gas settles into a disk
(2) protogalactic cloud has high angular momentum
Processes that allow gas to settle into a disk lead to spiral galaxies
question
Suppose a collision strips gas out of a spiral galaxy. Why would this tend to change the spiral galaxy into an elliptical galaxy?
answer
a galaxy cannot have a disk if it does not have gas
question
High density tends to lead to more rapid star formation in a protogalactic cloud. Why does this rapid star formation tend to lead to an elliptical galaxy, rather than a spiral galaxy?
answer
Rapid star formation means that there may not be enough gas left to make a disk.
question
High angular momentum leads to faster rotation. Why does faster rotation tend to lead to a spiral galaxy, rather than an elliptical galaxy?
answer
Faster rotation leads to collisions among gas particles that cause the gas to settle into a spinning disk, rather than a more spread out cloud.
question
___________ Faster rotation leads to collisions among gas particles that cause the gas to settle into a spinning disk, rather than a more spread out cloud.
answer
starburst galaxies
question
The largest individual galaxies in the universe are known as ________.
answer
central dominant galaxies
question
The collective activity of many supernova events in a relatively small volume of a galaxy can create ________.
answer
galactic wind
question
The most luminous objects in the universe are __________.
answer
quasars
question
The energy for all active galactic nuclei is thought to comes from in-fall of matter into __________.
answer
supermassive black holes
question
___________ are often characterized by sources of immense energy located hundreds of thousands of light-years away on either side of their centers.
answer
radio galaxies
question
Why do astronomers hypothesize that a massive black hole lies at the center of M87?
answer
A very small region at the center of M87 releases an enormous amount of energy.
question
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?
answer
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.
question
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?
answer
Gravitational potential energy is converted to thermal energy as matter from the surrounding gas disk spirals into the central black hole.
question
Which of these galaxies would you most likely find at the center of a large cluster of galaxies?
answer
a large elliptical galaxy
question
We determine the distance of a Cepheid by ____________.
answer
determining its luminosity from the period-luminosity relation and then applying the inverse square law for light.
question
Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?
answer
a white dwarf supernovae
question
Why do virtually all the galaxies in the universe appear to be moving away from our own?
answer
Because expansion causes all galaxies to move away from nearly all others.
question
Which of these galaxies is most likely to be oldest?
(1) a galaxy in the Local Group
(2) a galaxy observed at a distance of 5 billion light-years
(3) a galaxy observed at a distance of 10 billion light-years
answer
(1) a galaxy in the Local Group
question
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 __________.
answer
4 billion (giga) years old
question
Which of these items is a key assumption in our most successful models for galaxy formation?
answer
Some regions of the universe were slightly denser than others.
question
Observations indicate that galaxies with more massive central black holes tend to also have _________________.
answer
a greater mass of stars in their central bulges
question
The luminosity of a quasar is generated in a region the size of _________.
answer
the solar system
question
The primary source of a quasar's energy is
answer
gravitational potential energy
question
The globular cluster M13 belongs to the _____________ of the Milky Way Galaxy.
answer
halo component
question
A(n) ___________ contains hot, ionized gas but very little cool gas or dust.
answer
elliptical galaxy
question
The type of galaxy known as a(n) ___________ was more common in the universe 10 billion years ago.
answer
irregular galaxy
question
The Milky Way is a(n) ________.
answer
spiral galaxy
question
Our Sun belongs to the _______ of the Milky Way Galaxy.
answer
disk component
question
Scientists investigating _________ study how the distribution of galaxies changes with time.
What two observable properties of a Cepheid variable are directly related to one another?
answer
the period between its peaks of brightness and its luminosity
question
What does Hubble's law tell us?
answer
The more distant a galaxy, the faster it is moving away from us.
question
If we say that a galaxy has a lookback time of 1 billion years, we mean that _________.
answer
its light traveled through space for 1 billion years to reach us
question
Current estimates place the age of the universe at about _________.
answer
14 billion (giga) years
question
What type of galaxy is M82 based on its appearance in the visible-light view?
answer
irregular
question
Look again at the visible-light view of M82. What is the source of the white and blue light that dominates the image?
answer
stars
question
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?
answer
The red is emission from warm dust grains; the whitish-blue is emission from stars.
question
What is seen in the images from the previous questions?
answer
Stars in the central region of this galaxy are exploding as supernovae, generating a galactic wind of hot gas and warm dust gains.
question
What can we conclude about this galaxy from the fact that there are so many supernovae occurring?
answer
This galaxy contains an unusually large number of young stars.
question
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?
answer
At its current rate of star formation, the galaxy would use up all its dust and gas in just a few hundred million years.
question
Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______.
answer
infrared light
question
Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______.
answer
may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies
question
Why should galaxy collisions have been more common in the past than they are today?
answer
Galaxies were closer together in the past because the universe was smaller.
question
Which of the following phenomena is probably NOT related to the presence of a supermassive black hole?
answer
the presence of globular clusters in the halos of galaxies
question
According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.
answer
light emitted by hot gas in an accretion disk that swirls around the black hole
question
What phenomena are related to the presence of a supermassive black hole?
answer
(1) quasars
(2) the radio emission from radio galaxies
(3) the huge jets seen emerging from the centers of some galaxies
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