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Astronomy Unit 4(Final Exam)

The circular bu relatively flat portion of the galazy is the ____
the first portion of the galaxy to form was the ____
A tightly packed group of a few hundred thousand very old stars is a ___
globular cluster
a ____ stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust
spiral arm
our milky way galxy is a ___
spiral galaxy
stars orbiting in the ____ bulge near the galazxy's center can have orbits highly inclined to the galactic plane
distance from the center of the milky way galaxy (closest to farthers.
1. the edge of the central bulge 2. our solar system 3. a cloud of gas and dust in the outskirts of the disk. 4. global cluster in the outskirst of the halo
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.
1. across the diameter of the galactic halo 2. across the diameter of the glactic disk 3. from the sun to the center of the galaxy 4. across the diameter of the central bulge 5. through the disk from top to bottom (The light-travel time converts directly to a distance in light-years,)
disk stars of the milky way
1.yellow 2. high-mass stars 3.youngest stars 4. the sun 5.stars that all orbit in nearly the same plane
halo stars
1. green 2. oldest stars 3. globular clusters 4. stars whose orbits can be inclined at any angle 5. stars with the smallest abundance of heavy elements
current age from oldest to youngest
1. red giant in globular cluster and red main -sequence star in globular cluster (formed at the same time)2. the sun 3. hot, blue, main-sequence star in disk
rank stars based on abundance of elements heavier than carbon from highest to lowest (assume both the blue and yellow disk stars are members of the same open cluster)
1. the sun and hot in open cluster in disk, blue main-sequence star in open cluster in the disk 2. red giant in globular cluster and red main-sequence star in the globular cluster

(A star's abundance of heavy elements (heavier than helium) depends primarily on when it was born. Very old stars lacked earlier generations of stars manufacturing heavy elements when they were born, and thus have very low levels. Globular cluster stars (very old stars) therefore have a very small abundance of heavy elements — typically less than 0.1 percent. Open clusters in the disk are generally quite young, and their stars therefore have more heavy elements — typically about 2 percent.)
key differences between the disk and halo
1. gas and dust are abundant in the disk but not the halo 2. stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits 3. disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red. 4. clusters of young stars are found only in the disk
Where are large dust clouds predominantly located in the galaxy M51?
within or on the edges of the spiral arms (As is generally true for spiral galaxies, large, star-forming dust clouds are located primarily within or on the edges of spiral arms.)
Where are the ionization nebulae predominantly located in the galaxy M51?
within or on the edges of the spiral arms of the galaxy (Like the dust clouds, ionization nebulae are located primarily within or on the edges of spiral arms. This is because spiral arms are regions of active star formation. Stars are born within dust clouds, and ionization nebulae are ionized by the intense light from massive, young stars.)
Why are the ionization nebulae so bright?
They are regions where gas is ionized by hot, young stars. (The gas in ionization nebulae is ionized by hot, young stars. That is why they are found in star-formation regions of the galaxy.)
diameter of the milky way
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
calculation of mass of a central object
M=(a)^3/(p)^2 (a=semimajor axis in au) (p=orbital period in years)
Which of these forms of radiation passes most easily through the disk of the Milky Way?
infared light
where are most of the milky way's globular clusters found?
in the halo
where would you least expect ot find an ionization nebula?
in the halo
where would the most likely place to find an ionization nebula?
in the spiral arm
we measure the mass of the black hole at the galactic center from:
the orbits of the stars in the galactic center
characteristics of spiral galaxies
1. contain abundant clouds of cool gas and dust 2. have significant, ongoing star-formation 3. have a flattened disk of stars 4. are rare in central regions of galaxy clusters 5. contain many bright, hot stars
characteristics of elliptical galaxies
1. are more reddish in color 2. contain primarily old, low-mass stars
our sun belongs to the ____ of the milky way galaxy
disk component
the globular cluster M13 belongs to the ____ of the milky way galaxy
spheroidal component
an _____contains hot, ionized gas but very little cool gas or dust
elliptical galaxy
the milky way is a _____
spiral galaxy
the type of galaxy known as an _____ was more common in the universe 10 billion years ago
irregular galaxy
scientists investigating ____ study how the distribution of galaxies changes with time
Which of these galaxies would you most likely find at the center of a large cluster of galaxies?
a large elliptical galaxy
what are the three main categories of galaxies?
irregular, elliptical, spiral
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
Which cosmic distance measurement techniques are considered standard candle techniques?
1. white dwarf supernovae (distant standards) 2. cepheids 3. main-sequence fitting (A standard candle technique requires knowing the luminosities of the objects whose distances we are trying to find.)
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. (Cepheids provide a key link in the cosmic distance chain. 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. White dwarf supernovae, in turn, allow us to calibrate Hubble's law. 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. )
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.
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% (Each step in the chain can be only as reliable as the prior steps. If the Earth-Sun distance were off by 10%, then all the other methods would also have to be off by the same 10% )
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
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 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
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. (The larger the value of Hubble's constant, the faster galaxies are being carried away from us with the expansion of the universe)
If all the stars on the main sequence of a star cluster are typically only one-hundredth as bright as their main-sequence counterparts in the Hyades Cluster, then that cluster's distance is
10 times as far as the Hyades's distance.
Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?
a white dwarf supernova
Why do virtually all the galaxies in the universe appear to be moving away from our own?
Because expansion causes all galaxies to move away from nearly all others.
What two observable properties of a Cepheid variable are directly related to one another?
the period between its peaks of brightness and its luminosity
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
What does Hubble's law tell us?
The more distant a galaxy, the faster it is moving away from us.
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.
Which of these items is a key assumption in our most successful models for galaxy formation?
Some regions of the universe were slightly denser than others.
Why should galaxy collisions have been more common in the past than they are today?
Galaxies were closer together in the past because the universe was smaller.
The luminosity of a quasar is generated in a region the size of
the solar system.
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 ______.
light emitted by hot gas in an accretion disk that swirls around the black hole
T/F: Spiral galaxies have more gas, dust, and young stars than elliptical galaxies.
T/F: A lenticular galaxy is another name for an elongated elliptical galaxy.
T/F:All spiral galaxies have both a disk and spheroidal component to their stellar populations.
Most active galactic nuclei are at large distances from us; relatively few nearby galaxies harbor active galactic nuclei. What does this imply?
Active galactic nuclei tend to become less active as they age.
Why do astronomers hypothesize that a massive black hole lies at the center of M 87?
A very small region at the center of M 87 releases an enormous amount of energy.
Assuming that the bright core of M 87 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. (The black hole itself does not emit any light. Rather, the bright light comes from hot gas surrounding the black hole. This gas becomes hot in the process of falling toward the black hole because a large fraction (up to 40%) of its gravitational potential energy is converted into thermal energy (through friction in the disk) as it falls inward. Most of this energy is ultimately radiated away as light, explaining why the central region is so bright. )
Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation?
We can learn the distance to the galaxy.
In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thus putting to rest the idea that it might have been a cloud within our own galaxy. How was he able to prove this?
By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation
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.
Does Hubble's law work well for galaxies in the Local Group? Why or why not?
No, because galaxies in the Local Group are gravitationally bound together.
Dark matter is inferred to exist because:
we can observe its gravitational influence on visible matter.
Which of the following best summarizes what we mean by dark energy?
It is a name given to whatever is causing the expansion of the universe to accelerate with time
An object the size of the Earth located in the halo of our galaxy would be an example of the form of dark matter known as ___
_____ are defined as subatomic particles that have more mass than neutrinos but do not interact with light
matter made from ordinary atoms represents what we call __
baryonic matter
a massive object can distort the light of more distant objects behind it through the phenomenon that we call ____
gravitational lensing
_____ of spiral galaxies provide strong evidence for the existence of dark matter
rotation curves
models show that ________ of the univers is better explained when we include the effects of dark matter along with the effects of luminous matter
large-scale structure
The flat part of the Milky Way's rotation curve tells us that stars in the outskirts of the galaxy:
orbit the galactic center just as fast as stars closer to the center.
What is a rotation curve?
a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
They respond to the weak force but not to the electromagnetic force, which means they cannot emit light
Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
Dark matter is the dominant form of mass in both clusters and in individual galaxies.
Based on current evidence, which of the following is considered a likely candidate for the majority of the dark matter in galaxies?
subatomic particles that we have not yet detected
Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?
There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
How does gravitational lensing tell us about the mass of a galaxy cluster?
Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.
What evidence tells us that at least some dark matter consists of objects made of ordinary matter that orbit in the halo of the galaxy?
We have detected gravitational lensing of distant objects that appears to be caused by unseen objects in the halo of our galaxy.
Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?
Dark energy does not exist and there is much more dark matter than we are aware of to date.
T/F:MACHOs (massive compact halo objects) have been searched for using the technique of gravitational lensing, but, to date, none have been found.
T/F: Dark matter in galaxies and clusters of galaxies is purely hypothetical because we have no way of detecting its presence.
How do we know the expansion of the universe is accelerating?
White dwarf supernova in distant galaxies are fainter than they would be if the expansion were not accelerating.
Which of the following best describes how galaxies are distributed on large scales in the universe?
Galaxies appear to be distributed in chains and sheets that surround great voids.
How do astronomers create three-dimensional maps of the universe?
By using a galaxy's position on the sky and its redshift to determine its distance along the line of sight.
A gravitational lens occurs when
a massive object bends light beams that are passing nearby.
Why does the Cepheid distance method fail us beyond about 20 Mpc?
Even with the HST, the most luminous Cepheids are too faint to be seen beyond.
The first attempt to map the Galaxy via star counts was done by:
William Herschel in the late eighteenth century.
T/F: Quasars may come from mergers of black holes in early galaxies.
Most of the new star formation in the Galaxy is found in the
spiral arms
T/F:It is harder to map the structure of the Milky Way in visible wavelengths than with radio and infrared waves.
What is true of the Local Group?
The Andromeda galaxy (M31), and the Milky Way are the two largest galaxies.
From the Sun, the distance to the Galactic Center is about
T/F: Among the ellipticals, E0 looks the most like huge globular star clusters.
What observations suggest the mass of the Galaxy goes much farther out than its visible disc?
the rotation curve of the outermost portions of the disc
Detailed measurements of the disk and central bulge region of our Galaxy suggest our Milky Way is a
barred spiral galaxy.
T/F: Unlike spirals, elliptical galaxies do not contain a flattened disk.
Which property is common to spiral galaxies?
ongoing star formation and emission nebula in the arms
Which of these would be made up of only population II stars?
elliptical gallaxies
T/F: Seyferts are irregular galaxies with very strong radio emission.
T/F: Population I stars came billions of years before Population II stars
T/F: The Virgo Cluster of galaxies is smaller than the Local Group.
In shape and component stars, elliptical galaxies are most like our own ___
Rotation curves for spiral galaxies show
most have dark halos
Based on galactic rotation curves and cluster dynamics, we think dark matter
comprises over 90% of the entire mass of the universe
What are Type I supernovae used for?
Standard candles for determining distances to other galaxies.
T/F: The additional energy given off by active galaxies is because of their higher stellar density.
The flattest of the ellipticals are class
Galactic disks appear blue because
O and B blue giants are much brighter than G, K, or M dwarfs.
According to Hubble's Law, the greater a galaxy's redshift, the
farther it is from us
T/F: When observing a quasar, we are probably looking at a stage of evolution in the very early universe.
T/F: In its early stages, the Milky Way was probably a quasar.
Which type of galaxy has a stellar disk, but without gas and dust?
Population II
1. They are made of almost nothing but hydrogen and helium. 2.Their orbits around the Galaxy resemble those of comets 3.The globular clusters are their most obvious groupings. 4. They formed first as the Galaxy formed.
The astronomer who originally classified galaxies into S, E, and Irr was
Edwin Hubble.
You observe a spiral galaxy with a large central bulge and tightly wrapped arms. It would be classified a
What is true of galaxies at the larger redshifts compared to ones nearby?
They appear smaller and more irregular.
T/F: A Population II star could have terrestrial planets orbiting it.
About how many galaxies are presently known in our Local Group?
The Tully-Fisher relation exists between the galaxy's luminosity and its
T/F: Radio galaxies emit only radio waves, and are not otherwise observable.