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Mastering Astronomy Exam 3

Terms in this set (112)

Why are Cepheid variables important?
Cepheids are pulsating variable stars, and their pulsation periods are directly related to their true luminosities. Hence, we can use cepheids as "Standard Candles" for distant measurements
What is a "Standard Candle"?
An object for which we are likely to know the true luminosity
How did Edwin Hubble measure the distance to the Andromeda Galaxy?
He applied the period luminosity relation to cepheid variables.
How was Edwin Hubble able to use his discovery of a cepheid in Andromeda to prove that the "Spiral Nebulae" were actually entire galaxies?
From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.
White-dwarf supernovae are good standard candles for distance measurements for all the following reasons except which?
White-dwarf supernovae occur only among young and extremely bright stars.
What makes white-dwarf supernovae good standard candles?
They are very bright, so they can be used to determine the distances to galaxies billions of light-years away.

They should all have approximately the same luminosity.
What is the most accurate way to determine the distance to a nearby star?
stellar parallax ---- 1/arcsecond= parsecs
What is the most accurate way to determine the distance to a nearby galaxy?
Using Cepheid Variables
Which of the following sequences lists the methods for determining distance in the correct order from nearest to farthest?
parallax, Cepheid variables, Hubble's law
Massive-star supernovae and white-dwarf supernovae work equally well as standard candles for measuring cosmic distances.
False
Hubble's law expresses a relationship between __________.
the distance of a galaxy and the speed at which it is moving away from us (1/H0)
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
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 years
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.
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 A is twice as far away as Supernova B.
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.
5 billion years, 2 billion light years, 800 million light years, 230 million light years, 70 million light years.
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.
130,000 km/s, 45,000 km/s, 18,730 km/s, 5264 km/s, 1557 km/s
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.
130,000 km/s, 45,000 km/s, 18,730 km/s, 5,264 km/s, 1,577 km/s
Following are a number of distinguishing characteristics of spiral and elliptical galaxies. Match each characteristic to the appropriate galaxy type.
Spirals: contain abundant clouds of cool gas and dust, are rare in central regions of galaxy clusters,contain many bright hot stars, have significant ongoing star formation, have a flattened disk of stars

Elliptical: contain primarily old, low-mass stars, are more reddish in color
Which types of galaxies have a clearly defined halo component?
All but irregulars
Compared to spiral galaxies, elliptical galaxies are
redder and rounder.
The disk component of a spiral galaxy includes which of the following parts?
Spiral Arms
What is the diameter of the disk of the Milky Way?
100,000 light years
What is the thickness of the disk of the Milky Way?
1,000 light years
What kinds of objects lie in the halo of our galaxy?
globular clusters
What kinds of objects lie in the disk of our galaxy?
open clusters
gas and dust
old K and M stars
O and B stars
What makes up the interstellar medium?
gas and dust
If you were to take a voyage across the Milky Way, what kind of material would you spend most of your time in?
warm, rarefied clouds of atomic hydrogen
How does the interstellar medium obscure our view of most of the galaxy?
It absorbs visible, ultraviolet, and some infrared light.
Harlow Shapley concluded that the Sun was not in the center of the Milky Way Galaxy by
mapping the distribution of globular clusters in the galaxy.
Which of the following statements correctly summarize key differences between the disk and the halo?
Gas and dust are abundant in the disk but not in the halo.

Stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits.

Clusters of young stars are found only in the disk.

Disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red.
The Sun's location in the Milky Way Galaxy is _________.
in the galactic disk, roughly halfway between the center and the outer edge of the disk
Select the сorrect statements.
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.
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.
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
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 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
What do astronomers consider heavy elements?
all elements besides hydrogen and helium
Where are most heavy elements made?
in stars and supernovae
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.
More common in Spiral Arms:

ionization nebulae,
dense dusty gas clouds,
star formation,
young stars

More common within and between Spiral Arms:

Old Stars
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?
Individual stars orbit around the center of the galaxy.

Individual stars move in and out of spiral arms over time.
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.
Which of the following are you more likely to find within (or on the edges of) spiral arms of a spiral galaxy than in between these arms?
ionization nebulae, massive stars (spectral types O and B), dense, dusty gas clouds, young stars
What causes the glow of the reddish areas visible primarily in the spiral arms?
Clouds of gas are being heated by ultraviolet light from nearby, recently formed stars.
Consider the four features of spiral arms that you identified in Part A. What do they all have in common that explains why they are found together in spiral arms?
They all require rapid rotation ,
They are require very hot ionized gas,
They all require hydrogen gas.
Based on the association of star formation with spiral arms, we can conclude that the gas in spiral arms __________ than it does in regions between the arms.
has greater density
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.
Disk Stars:

youngest stars
The sun
stars that all orbit in nearly the same plane
high-mass stars

Halo Stars:

stars whose orbits can be inclined at any angle,
stars with the smallest abundance of heavy elements,
oldest stars,
globular clusters
How do we know that halo stars are older, on average, than disk stars?
There are no blue halo stars.
If we represent the Milky Way Galaxy as the size of a grapefruit (10-cm diameter), the distance to the Andromeda Galaxy would be about
3 m.
What evidence supports the idea that a collision between two spiral galaxies might lead to the creation of a single elliptical galaxy?
the fact that elliptical galaxies dominate the galaxy populations at the cores of dense clusters of galaxies.

observations of some elliptical galaxies surrounded by shells of stars that probably formed from stars stripped out of smaller galaxies.

observations of some elliptical galaxies with stars and gas clouds in their cores that orbit differently from the other stars in the galaxy.

observations of giant elliptical galaxies at the center of dense clusters that may have grown by consuming other galaxies
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.
Astronomers suspect that 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. Each item below describes either a condition in a protogalactic cloud or a later interaction. Match these items to the galaxy type you would expect to form as a result.
Elliptical Galaxies:

stars form rapidly as the protogalactic cloud shrinks.
protogalactic cloud has high density.
A galaxy collision strips away gas.
Protogalactic cloud rotates very slowly.

Spiral Galaxies: most protogalactic gas settles into a disk.
Protogalactic cloud has high angular momentum.
Suppose a collision strips gas out of a spiral galaxy. Why would this tend to change the spiral galaxy into an elliptical galaxy?
A galaxy cannot have a disk if it does not have gas.
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?
Rapid star formation means that there may not be enough gas left to make a disk.
High angular momentum leads to faster rotation. Why does faster rotation tend to lead to a spiral galaxy, rather than an elliptical galaxy?
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.
Einstein's general theory of relativity suggests that gravity is ______.
caused by curvature of spacetime
What do we mean by the straightest possible path between two points on Earth's surface?
the shortest path between the two points
As predicted by general relativity, time runs slightly slower at the peak of Mount Everest than it does at sea level.
False
What is spacetime?
the inseparable combination of space and time
If two straight lines start out parallel but eventually cross, then they must be in a _______.
Spherical Geometry
Listed below are some characteristics of flat, spherical, and saddle-shaped geometries. Match each of these characteristics to the corresponding geometry.
Flat Geometry:
sum of angles of a triangle is always 180∘
circumference of a circle is exactly 2πr

Spherical Geometry:
sum of angles of a triangle is greater than 180.
straightest possible path is a segment of a great circle.
lines that are initially parallel eventually converge

Saddle-Shaped Geometry:
sum of angles of a triangle is less than 180∘
lines that are initially parallel eventually diverge
What do we mean by the event horizon of a black hole?
It is the boundary within which events in the black hole cannot influence events in the outside universe.
What do we mean by gravitational time dilation?
It is the idea that time runs slower in places where gravity is stronger.
Of the following objects, on which one would time on its surface run most slowly?
an object with the same mass as the Sun but only half as large in radius
Which of the following accurately describe some aspect of gravitational waves?
The existence of gravitational waves is predicted by Einstein's general theory of relativity.

The first direct detection of gravitational waves came in 2015.

Gravitational waves carry energy away from their sources of emission.

Gravitational waves are predicted to travel through space at the speed of light.
Consider a binary system of two neutron stars. How should the emission of gravitational waves affect this system?
It should cause the orbits of the two objects to decay with time.
With current technology, we expect to be able to detect (directly) gravitational waves from a binary system of two neutron stars or two black holes __________.
only from the instant when the two objects merge into one
LIGO detects gravitational waves because the lengths of its arms change as gravitational waves pass by. About how much are these lengths expected to change when LIGO detects gravitational waves from the merger of two neutron stars or two black holes?
by an amount smaller than the diameter of a proton
Given such small length changes (as noted in Part D), what can give scientists confidence that they have really detected a gravitational wave signal?
detecting the same changes at more than one location
The data in the figure show how the orbital period of the two neutron stars changes with time. Based on these data, what can we conclude?
shorter and the two neutron stars are moving closer together
Einstein's general theory of relativity predicts that two orbiting neutron stars should radiate gravitational waves. How does the fact that the orbital period is getting shorter support this prediction?
Gravitational waves must carry some energy away from the system, and this loss of energy must cause the orbits to get smaller over time.
Notice how closely the actual data points match the theoretical prediction (red curve). Which of the following statements are supported by this close match of prediction and data?
If gravitational waves really are responsible for the decay in this system, we should see similar decay in other systems with two neutron stars.

We can continue to test the predictions of general relativity by continuing to measure the orbital period of this system in the future.

These data provide strong evidence for the existence of gravitational waves as predicted by the general theory of relativity.
Why do we call dark matter "dark"?
It emits no or very little radiation of any wavelength.
Dark matter is purely hypothetical, because we have no way of detecting its presence.
False
Dark matter is inferred to exist because:
we can observe its gravitational influence on visible matter.
How do we know that there is much more mass in the halo of our galaxy than in the disk?
Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.
What evidence suggests that the Milky Way contains dark matter?
We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
How are rotation curves of spiral galaxies determined beyond radii where starlight can be detected?
through observations of the 21 cm line of atomic hydrogen
The distribution of the dark matter in a spiral galaxy is
approximately spherical and about ten times the size of the galaxy halo.
A large mass-to-light ratio for a galaxy indicates that
on average, each solar mass of matter in the galaxy emits less light than our Sun.
Which of the following is not evidence for dark matter?
the expansion of the universe
Which of the following is an example of baryonic matter?
You
What do we mean when we say that a particle is a weakly interacting particle?
It interacts only through the weak force and the force of gravity.
Why can't the dark matter in galaxies be made of neutrinos?
Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull.
the Bullet Cluster. What kind of cluster is it?
a galaxy cluster
Study the composite image of the Bullet Cluster, as well as the animations. What do the two large, red-colored regions in the composite image represent?
X-ray Emission from hot gas
There are also two large blue-colored regions in the composite image. These blue regions are labeled as "dark matter" based on __________.
observations of gravitational lensing by the cluster
From Parts A though C, you should understand the three major components shown in the composite Bullet Cluster image. Notice that the hot, X-ray-emitting gas (red) is not in the same place as most of the visible galaxies and most of the matter (blue). How do we think the hot gas came to be in a different place from the galaxies and most of the matter?
The Bullet cluster is actually two separate galaxy clusters that have collided, stripping out hot gas in the process.
Careful measurements show that the hot, X-ray-emitting gas in the Bullet Cluster contains about 7 times as much total mass as all the stars in the cluster's galaxies combined. Therefore, the fact that the most of the gravity is in the blue regions that surrounds the visible galaxies, rather than in the regions with the hot gas, indicates that __________.
there is even more matter surrounding the galaxies than there is in the hot gas
As noted in the Introduction, some scientists have proposed that dark matter does not really exist. According to this view, all matter is ordinary (baryonic), but at large distances from matter, gravity does not precisely obey either Newton's or Einstein's theories of gravity. Is this alternative view of gravity consistent with what we observe in the Bullet Cluster? Why or why not?
No. If all matter was ordinary, then the blue region representing the location of most of the matter would line up with the red region representing the hot gas.
Classify the given types of matter as either ordinary (baryonic) matter that contains protons and neutrons or as exotic (nonbaryonic) matter that consists of subatomic particles different from those that build atoms.
Ordinary (baryonic) matter:

matter in our bodies,
Matter in stars,
matter in brown dwarfs,
dark matter consisting of Jupiter-size objects in galactic halos

Exotic (nonbaryonic) matter:

matter that probably makes up the majority of dark matter.
dark matter consisting of weakly interacting subatomic particles.
What is a quasar?
the extremely bright center of a distant galaxy, thought to be powered by a supermassive black hole
Which of the following is evidence for supermassive black holes in active galaxies?
very high speed orbital motions around galactic nuclei

rapid changes in the luminosity of the galaxy nucleus

quasars emit approximately equal power at all wavelengths from infrared to gamma rays

the discovery of powerful jets coming from a compact core
The most active galactic nuclei are usually found at large distances from us; relatively few nearby galaxies have active galactic nuclei. What does this imply?
Active galactic nuclei tend to become less active as they age.
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.
How are the masses of supermassive black holes related to the masses of the bulges of their surrounding galaxies and what does this suggest about the role of supermassive black holes in galaxy evolution.
Masses of supermassive black holes are proportional to the masses of the bulge components. The role of supermassive black holes in galaxy evolution is not clear, but somehow these black holes seem to grow along with the bulge component of their host.
Which statement describes the trend among the galaxies shown on this graph?
More distant galaxies tend to have a larger infrared color ratio, but there are some exceptions.
You discover a new cluster of galaxies, and the brightest galaxy in this new cluster has an infrared color ratio of 0.65. Based on the data in the graph, what is the approximate distance to this new cluster?
1 Billion light years
A follow-up study showed that the galaxies that are exceptions to the general trend on the original graph have unusually brighter centers; that is, these galaxies contain active galactic nuclei. Based on this follow-up study, could you improve your confidence in your distance estimate for the new cluster discussed in Part B, and, if so, how?
Yes, by photographing the new cluster with better angular resolution to see whether its brightest galaxy has an unusually bright center.
Consider both the original study (with data shown on the graph) and the follow-up study showing that the exceptions to the general trend are galaxies with unusually bright centers (as described in Part C). Which of the following conclusions are supported by this pair of studies?
Galaxies with unusually bright nuclei are more common at greater distances.

Galaxies with unusually bright nuclei were more common when the universe was young than they are today.

On average, more distant galaxies have larger infrared color ratios.
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.
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.
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.
Dr. X believes that the Hubble constant is H0 = 55 km/s/Mpc. Dr. Y believes it is H0 = 80 km/s/Mpc. Which statement below automatically follows?
Dr. X believes that the universe is older than Dr. Y believes.
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?
32,500 km/s
Hubble's "constant" is constant in
Space
Based on current estimates of the value of Hubble's constant, how old is the universe?
between 12 and 16 billion years old
Why can't we see past the cosmological horizon?
Beyond the cosmological horizon, we are looking back to a time before the universe had formed.
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.
If we say that a galaxy has a look back time of 1 billion years, we mean that _________.
its light traveled through space for 1 billion years to reach us
Which of these galaxies is most likely to be oldest?
a galaxy in the Local Group
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.
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
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