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I observe a galaxy that is 100 million light-years away: what do I see?

the light from the galaxy as it was 100 million years ago and it is redshifted

Which of the following types of galaxies are reddest in color?


Which of the following statements about galaxies is true?

Small galaxies outnumber large galaxies but large galaxies produce most of the light in the universe.

How does a lenticular galaxy differ from a normal spiral galaxy?

It has no spiral arms.

Which types of galaxies are most commonly found in large clusters?


How do observations of distant galaxies help us learn about galaxy evolution?

Observations at different distances show galaxies of different ages and therefore different stages of evolution.

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

What two quantities did Edwin Hubble plot against each other to discover the expansion of the Universe?

velocity and distance

What is a consequence of Hubble's Law?

the more distant a galaxy is from us, the faster it moves away from us

What is the most accurate way to determine the distance to a nearby galaxy?

using Cepheid variables

Compared with stars in the disk, orbits of stars in the halo...

are elliptical, with random orientation.

Applying the NewtonÕs version of KeplerÕs third law (or the orbital velocity law) to a star orbiting in the disk at a location 40,000 light-years from the center of the Milky Way Galaxy allows us to determine ________.

the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center

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.

Based on current estimates of the value of Hubble's constant, how old is the universe?

between 12 and 16 billion years old

What is the most accurate way to determine the distance to a very distant irregular galaxy?

using a white-dwarf supernova as a standard candle

Which of the following statements about the disk of the Milky Way is FALSE

Disk stars are all younger than 5 billion years.

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

have a greater abundance of heavy elements

What kinds of objects lie in the disk of our galaxy?

open clusters
O and B stars
old K and M stars
gas and dust

What comprises as the oldest members of the Milky Way?

globular clusters

What makes up the interstellar medium?

gas and dust

Spiral arms appear bright because...

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

Why do we believe 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 much larger than expected given the amount of visible matter.

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.

Approximately how far is the Sun from the center of the galaxy?

28,000 light-years

How should we expect the Milky Way's interstellar medium to be different in 80 billion years than it is today?

The total amount of gas will be much less than it is today.

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.

What is a shock wave?

a wave of pressure that moves faster than the speed of sound

What produces the 21-cm spectral line, seen in the radio portion of the electromagnetic spectrum, that we use to map out the Milky Way Galaxy?

atomic hydrogen

Where do most dust grains found in the interstellar medium form?

in the winds of red giant stars

Suppose you read somewhere that 10 percent of the matter in the Milky Way is in the form of dust grains. Should you be surprised? If so, why?

The 10 percent figure cannot be correct, because dust grains are solid but only about 2 percent of the matter in our galaxy is made of anything other than hydrogen and helium.

What are the Magellanic Clouds?

two small galaxies that orbit the Milky Way Galaxy

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

a several million solar-mass black hole

Approximately how long does it take the Sun to orbit the Milky Way Galaxy?

230 million years

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

in the spiral arms

What is a good analogy that best describes how the structure of the galaxyÕs spiral arms is maintained?

Like cars slowing in traffic to look at an accident, stars slow as they pass through the spiral arms.

Which constellation lies in the direction toward the galactic center?


Why do we call dark matter "dark"?

It emits no or very little radiation of any wavelength.

What is a quasar?

the extremely bright center of a distant galaxy, thought to be powered by a massive black hole

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

If there is no dark matter in the Milky Way Galaxy or other spiral galaxies, what is the best alternative explanation for the observations?

Our understanding of gravity is not correct for galaxy-size scales.

Which of the following methods used to determine the mass of a cluster does not depend on Newton's laws of gravity?

measuring the amount of distortion caused by a gravitational lens

Why wasn't the intracluster medium in galaxy clusters discovered until the 1960s?

The medium emits X rays, which are blocked by the Earth's atmosphere and require X-ray satellites in space in order to be observed.

Which of the following statements about rich clusters of galaxies (those with thousands of galaxies) is NOT true?

Galaxies in the central regions are predominantly spirals, while elliptical galaxies roam the outskirts.

What is NOT evidence for dark matter?

the expansion of the universe

Some MACHOs emit radiation. Why are they considered dark matter?

They are not as bright as a normal star and are not visible across great distances of space.

T or F. Approximately 90 percent of the mass of the Milky Way is located in the halo of the galaxy in the form of dark matter.


Which of the following gives the two main assumptions of theoretical models of galaxy evolution?

Hydrogen and helium filled all of space, and certain 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.

Approximately how many stars does a dwarf elliptical galaxy have?

less than a million

Approximately how many stars does a globular cluster have?

less than a million

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 distance measurements.

What is Hubble's law?

The recession velocity of a galaxy is directly proportional to its distance from us.

T or F. The larger the value of Hubble's constant, the more rapid the expansion of the universe and hence the younger the universe.


Why isn't space expanding within systems such as our solar system or the Milky Way?

Their gravity is strong enough to hold them together against the expansion of the universe.

What does the universe look like on very large scales?

Galaxies appear to be distributed in chains and sheets that surround great voids.

If all the "dark matter" in the Universe were to be, somehow, instantaneously removed, which of the following would NOT happen?

The Solar System would fly apart.

Recent measurements of the expansion rate of the universe reveal that the expansion rate of the universe is doing something astronomers did not expect. What is that?

The measurements indicate that the expansion is accelerating, rather than slowing under the influence of gravity.

What is meant by "dark energy"?

the agent causing the universal expansion to accelerate

Why can't current theories describe what happened during the Planck era, which ended 10-43 second after the Big Bang?

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

What do we mean by inflation?

a sudden expansion of the universe after the strong force froze out from the GUT force

What kinds of atomic nuclei formed during the era of nucleosynthesis, a period from approximately 0.001 to a few hundred seconds following the Big Bang?

hydrogen and helium and trace amounts of lithium, beryllium, and boron

Why did the era of nuclei end when the universe was about 300,000 years old?

The universe had expanded and cooled to a temperature of about 3,000 K, cool enough for stable, neutral atoms to form.

Evidence that the cosmic background radiation really is the remnant of a Big Bang comes from predicting characteristics of remnant radiation from the Big Bang and comparing these predictions with observations. Four of the five statements below are real. Which one is fictitious?

The cosmic background radiation is expected to contain spectral lines of hydrogen and helium, and it does.

Where do the photons in the cosmic background radiation originate?

the end of the era of nuclei

Why does the Big Bang theory predict that the cosmic background radiation should have a perfect thermal radiation spectrum?

The background radiation came from the heat of the universe, with a peak corresponding to the temperature of the universe.

Why do we expect the cosmic background radiation to be almost, but not quite, the same in all directions?

The overall structure of the universe is very uniform, but the universe must have contained some regions of higher density in order for galaxies to form

What are the two key observational facts that led to widespread acceptance of the Big Bang model?

the cosmic background radiation and the high helium content of the universe

T or F. The observed composition of ordinary matter in the universe—roughly 75 percent hydrogen and 25 percent helium—closely matches theoretical predictions based on the Big Bang model.


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.

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

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


According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?

During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.

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