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How does the diameter of the disk of Milky Way Galaxy compare to its thickness?

The diameter is about 100 times as great as the thickness

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

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

What do we mean by the interstellar medium?

the gas and dust that lies in between the stars in the Milky Way galaxy

What are the Magellanic Clouds?

two small galaxies that orbit the Milky Way Galaxy

How do disk stars orbit the center of the galaxy?

They all orbit in roughly the same plane and in the same direction

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 law) to the Sun's orbit around the center of the Galaxy

Elements heavier than hydrogen and helium constitute about _________ of the mass of the interstellar medium.


What do we mean by the star-gas-star cycle?

It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium

What are cosmic rays?

subatomic particles that travel close the speed of light

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

Which of the following molecules is the most abundant in molecular clouds?


Interstellar dust consists mostly of _________.

microscopic particles of carbon and silicon

Which of the following models best explains why our galaxy has spiral arms?

The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy.

What do we mean by a protogalactic cloud?

a cloud of hydrogen and helium that contracts to become a galaxy

Most stars in the Milky Way's halo are _________.

very old

What is an ionization nebula?

a colorful cloud of gas that glows because it is heated by light from nearby hot stars

What do halo stars do differently from disk stars?

They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.

Where does most star formation occur in the Milky Way Galaxy?

in the spiral arms

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.

What kind of object do we think lies in the center of the Milky Way Galaxy?

a 3 to 4 million solar mass black hole

If we could see our own galaxy from 2 million light-years away, it would appear _________.

as a flattened disk with a central bulge and spiral arms

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

About what percentage of the mass of a molecular cloud is in the form of dust?


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.

Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?

The 10% figure is too high because there are not enough heavy elements to make that much dust.

The most common form of gas in the disk of the Milky Way galaxy is _________.

atomic hydrogen gas

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.

Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.

have a greater abundance of heavy elements

Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?

the Chandra X-ray Observatory

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.

All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________.

globular clusters

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

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.

Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?

It is probably a halo star that is currently passing through the disk.

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.

Spiral arms appear bright because ________.

they contain more hot young stars than other parts of the disk

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.

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.

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 3 to 4 million solar mass object in a region no larger than our Solar System.

Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?

It is by far the brightest source of visible light lying in the direction of the galactic center.

Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in your textbook), the estimated number of galaxies in our observable universe is about ______.

100 billions

Which of the following is not one of the three major categories of galaxies?

globular galaxies

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

What two observable properties of a Cepheid variable are directly related to one another?

the period between its peaks of brightness and its luminosity

What does Hubble's law tell us?

The more distant a galaxy, the faster it is moving away from us.

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.

When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?

The inside of the balloon does not represent any part of our universe.

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

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

Cosmological redshift is the result of ______.

the expansion of the universe

Current estimates place the age of the universe at about _________.

14 billion years

Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______.

Infrared light

Which of the following is an important starting assumption in models of galaxy formation?

Some regions in the universe start out denser than others.

Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______.

may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies

Collisions between galaxies typically unfold over a period of _________.

hundreds of millions of years

Why are collisions between galaxies more likely than collisions between stars within a galaxy?

Relative to their sizes, galaxies are closer together than stars.

Which of the following features is not a feature of central dominant galaxies?

They are often spiral galaxies.

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 distinguishing feature of a starburst galaxy is _________.

a rate of star formation that may be 100 or more times greater than that in the Milky Way

The unusually bright centers found in some galaxies are called _________.

active galactic nuclei

According to current understanding, what is a quasar?

an active galactic nucleus that is particularly bright

Which of the following phenomena is probably not related to the presence of a supermassive black hole?

the presence of globular clusters in the halos of galaxies

The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly ______.

1 billion solar masses

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

In a photo like the Hubble Deep Field (Figure 15.1 in your textbook), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?

the galaxies that are farthest away

The most basic difference between elliptical galaxies and spiral galaxies is that _________.

elliptical galaxies lack anything resembling the disk of a spiral galaxy

Hubble's galaxy classification diagram (the "tuning fork") ______.

relates galaxies according to their shapes, but not according to any evolutionary status

Using the technique of main-sequence fitting to determine the distance to a star cluster requires that _____.

we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster

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

Suppose that Hubble's constant were 20 kilometers per second per million light-years. How fast would we 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 2,000 km/s

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.

What is the best way to determine a galaxy's redshift?

Take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory.

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.

What does cosmological redshift do to light?

stretches its wavelength

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.

We can study how galaxies evolve because _________.

we are really smart astronomers
the farther away we look, the further back in time we see

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.

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 due to other interactions, we'd expect this galaxy to be _________.

a spiral galaxy

Which of the following phenomena are not thought to be results of collisions or other interactions between galaxies?

The fact that spiral galaxies have both disk and halo components

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 characteristic is not generally true of a starburst galaxy?

The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy's center.

A quasar's spectrum is hugely redshifted. What does this large redshift tells us about the quasar?

the distance to the quasar

What evidence tells us that quasars are the centers of distant galaxies?

Images and spectra show quasars to be embedded at the centers of distant galaxies.

Most active galactic nuclei are found at large distances from us, with relatively few nearby. 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.

All of the following observations are real. Which one does not support the theory that active galactic nuclei are powered by accretion disks around massive black holes?

The most luminous active galactic nuclei have huge redshifts.

According to the theory 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

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 somehow.

Which of the following best summarizes what we mean by dark matter?

matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light

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.

The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that _________.

dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way

What is a rotation curve?

a graph showing how orbital velocity depends on distance from the center for a spiral galaxy

What is the primary way in which we determine the mass distribution of a spiral galaxy?

We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.

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.

Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?

Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.

Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?

measuring the temperatures of stars in the halos of the galaxies

When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?

It bends or distorts the light coming from galaxies located behind it.

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.

What is the distinguishing characteristic of what we call ordinary matter (such as the matter that makes up stars, planets, and people)?

It consists of atoms or ions with nuclei made from protons and neutrons.

Some dark matter may be ordinary matter that orbits in the halo of the galaxy. Which of the following would not be considered a type of ordinary dark matter?

globular clusters in the halo of the galaxy

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 best sums up current scientific thinking about the nature of dark matter?

Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.

When we speak of the large-scale structure of the universe, we mean _________.

the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe

The critical density of the universe is the _______.

average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist

What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?

Observations of white dwarf supernovae

Which of the following best sums up current scientific thinking about the nature of dark energy?

Dark energy probably exists, but we have little (if any) idea what it is.

Why do we call dark matter "dark"?

It emits no radiation that we have been able to detect.

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.

Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?

The orbital speeds would fall off sharply with increasing distance from the galactic center.

The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?

Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.

It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?

Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.

How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?

We detect this gas with X-ray telescopes.

Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?

The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.

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

If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?

They travel at speeds close to the speed of light.

Is space expanding within clusters of galaxies?

No, because their gravity is strong enough to hold them together even while the universe as a whole expands.

Which of the following statements about large-scale structure is probably not true?

Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.

Based on current evidence, a supercluster is most likely to have formed in regions of space where _________.

the density of dark matter was slightly higher than average when the universe was very young

Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?

The actual density, even with dark matter included, is less than about a third of the critical density.

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.

Based on our current understanding of physics, we can understand the conditions that prevailed in the early universe as far back in time as about _________.

one ten-billionth of a second after the Big Bang

What happens when a particle of matter meets its corresponding antiparticle of antimatter?

The combined mass of the two particles is completely transformed into energy (photons).

What is the significance of the Planck time?

Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.

The four fundamental forces that operate in the universe today are _________.

strong force, weak force, electromagnetic force, gravity

A "GUT" (grand unified theory) refers to theories that _________.

unify the strong force with the electromagnetic and weak forces

What do we mean by inflation?

a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence

Which of the following statements correctly summarizes the events in the early universe according to the Big Bang theory?

The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe.

Which statement about the cosmic microwave background is not true?

It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.

The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?

(1) the theory predicts the existence of and the specific characteristics of the observed cosmic microwave background; (2) the theory correctly predicts the observed overall chemical composition of the universe.

Measuring the amount of deuterium in the universe allows us to set a limit on _________.

the density of ordinary (baryonic) matter the universe

The idea of dark matter arose to explain gravitational effects observed in galaxies and clusters of galaxies. However, studies of the early universe (especially of the cosmic microwave background and of chemical abundances) also tell us something about dark matter. What do they tell us?

They add further support to the idea that dark matter really exists and is made of non-ordinary (nonbaryonic) matter, such as WIMPs.

Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?

the fact that the temperature of the cosmic microwave background is almost the same everywhere

The idea of inflation makes one clear prediction that, until the discovery of an accelerating expansion, seemed to contradict the available observations. What is this prediction?

The universe should be geometrically "flat" (in the four dimensions of spacetime).

Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?

Why is the sky dark at night?

What is the temperature of the universe (as a whole) today?


Which of the following statements cannot be tested by science today?

Prior to the Planck time, our universe sprouted from another universe.

How do we determine the conditions that existed in the very early universe?

We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.

Why can't current theories describe what happened during the Planck era?

We do not yet have a theory that links quantum mechanics and general relativity.

Which of the following statements best explains what we mean when we say that the electroweak and strong forces "froze out" at 10-38 second after the Big Bang?

These two forces first became distinct at this time.

According to the Big Bang theory, how many forces - and which ones - operated in the universe during the GUT era?

2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces

Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.

the electromagnetic and weak forces into the electroweak force

What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?

The basic chemical composition of the universe had been determined.

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