Stars 3

What is the approximate diameter of the disk of the Milky Way?

100,000 light-years

What does the interstellar medium consist of?

gas and dust

Approximately how long does it take stars in our region of the Milky Way to complete an orbit around the center of the Milky Way Galaxy?

200 million years

What produces the 21-cm line that we use to map out the Milky Way Galaxy?

atomic hydrogen

Why are stars no longer forming in the galactic halo?

The cold molecular gas required for star formation is in the galactic disk.

Which of the following represent the oldest stars of the Milky Way?

stars in globular clusters

Why can't we see the center of our galaxy with our eyes?

Interstellar dust and gas absorbs and scatters visible light before it can reach us from the center.

Which of the following provides the strongest evidence to support the claim that there is a black hole at the center of our galaxy?

The motions of the gas and stars indicate that the galactic center has a mass millions of times that of the Sun within a region less than 1 light-year across.

Which of the following provides the strongest evidence that halo stars are older, on average, than disk stars?

There are no blue halo stars

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

27,000 light-years

What is SgrA*?

a source of bright radio emission in the center of our galaxy

What do astronomers typically refer to as "heavy elements"?

elements besides hydrogen and helium

Which of the following best describes stars in the halo of our Galaxy?

old, red, and dim with low abundance heavy elements

Which of the following best describes the orbits of stars in the halo?

Halo stars have elliptical orbits, with random orientations.

Where are most elements heavier than hydrogen and helium made?

n the cores of stars, in supernovae, and during neutron star mergers

The galactic center lies in the direction of which constellation?

Sagittarius

Which of the following statements about the disk of the Milky Way is generally true?

Disk stars have a higher proportion of heavy elements, on average, than halo stars.

What are cosmic rays?

subatomic particles that travel close to the speed of light

How did Harlow Shapley conclude that the Sun was not in the center of the Milky Way Galaxy?

by mapping the distribution of globular clusters in the galaxy

What is the primary way in which stars with masses similar to the Sun ultimately return some of their material to the interstellar medium?

through planetary nebulae

What is the primary way in which high-mass stars recycle material into the interstellar medium?

through stellar winds and supernova explosions

Which of the following are typically found in the halo of our galaxy?

globular star clusters

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

in the spiral arms

Which statement is true about the relationship between Hubble's constant and the age of the universe?

The larger the value of Hubble's constant, the faster the universe is expanding and the younger the universe must be.

How do elliptical galaxies typically compare to spiral galaxies?

Elliptical galaxies are redder and rounder.

How does a lenticular galaxy differ from a spiral galaxy?

It has no spiral arms.

Which of the following expresses the major idea of Hubble's Law?

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

Which of the following types of galaxies is the most common type of galaxy found in large clusters?

ellipticals

Suppose you are observing a distant galaxy whose light has taken 12 billion years to reach us. What is the maximum age that this galaxy could have been when it emitted the light we are observing now? (Assume the age of the universe is about 14 billion years.)

about 2 billion years old

You observe a galaxy that is 100 million light-years away. Which of the following do you see?

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

White dwarf supernovae are good standard candles for distance measurements because

all white dwarf supernovae have about the same peak luminosity.

Nearly all galaxies contain Cepheid variable stars. Why, then, do we determine the most distant galaxy distances using white dwarf supernovae rather than Cepheids as standard candles?

White dwarf supernovae can be observed in galaxies that are too far away for us to see individual Cepheids.

In astronomy, a standard candle is an object with a known

luminosity.

The most massive galaxies in the universe are

ellipticals.

Why do elliptical galaxies appear yellow or red in color?

Their population is dominated by old, low-mass stars and old, red giant stars.

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

using Cepheid variables

Why do spiral galaxies appear blue in color?

Their light output is dominated by hot, massive blue stars.

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

ellipticals

What is the most accurate way to determine the distance to a relatively nearby star (within the Milky Way)?

stellar parallax

How did Edwin Hubble measure the distance to the Andromeda Galaxy?

He used Cepheid variables in the Andromeda Galaxy.

Why are Cepheid variables important?

Cepheid variables are stars that brighten and dim with a period that is directly related to the star's luminosity.

Based on current estimates of the value of Hubble's constant, approximately how old is the univers

14 billion years old

Which of the following types of galaxies are closest to being spherical in shape?

ellipticals

Why can't we see past the cosmological horizon?

It's impossible to look back to a time before the universe was born.

Which force binds together protons and neutrons in a nucleus?

strong force

How does the hypothesis of inflation explain the near-uniformity of the cosmic microwave background?

Prior to rapid inflation, all regions of space were close enough to bounce radiation back and forth and reach the same temperature.

Why do scientists think tiny quantum ripples should have been present in the very early universe?

Quantum mechanics tells us that the energy fields at any point in space be continually fluctuating as a result of the uncertainty principle.

Approximately how long after the Big Bang did the era of nucleosynthesis end, marking the time when the basic chemical composition of the universe was determined?

5 minutes

When we say that the electromagnetic and weak forces "freeze out" from the electroweak force at 10^-10 seconds after the Big Bang, what do we mean?

Prior to this time, the electromagnetic and weak forces could not be distinguished from each other, but they possessed separate identities following this time.

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

This radiation came from the heat of the universe itself.

The fact that the sky is dark at night proves that the universe cannot be

eternal and unchanging with an infinite number of stars.

If the electromagnetic force is stronger than gravity, then why is it overpowered by gravity on large scales?

Most objects are electrically neutral.

What is the main idea of the Big Bang theory?

All matter and energy in the universe was in an incredibly hot, dense state and space itself began expanding.

What happened to the quarks that existed freely during the particle era?

They combined to make protons, neutrons, and their antiparticles.

The cosmic microwave background is almost perfectly uniform in all directions, except for very small deviations in its temperature. What do scientists think these small deviations represent?

regions of slightly higher density that made it possible for galaxies to form

Which of the following hypothetical observations would demonstrate that the inflation model was incorrect?

The geometry of the universe is curved, not flat.

Which of the following lists the two key predictions of the Big Bang model that have been verified by observations?

(1) the existence of the cosmic microwave background and (2) the amount of helium relative to hydrogen in the universe

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.

What event made it possible for photons to begin to travel freely through the universe when the universe was about 380,000 years old?

The universe had expanded and cooled enough for electrons to combine with nuclei to form neutral atoms

What kinds of atomic nuclei were present when fusion ceased at the end of the era of nucleosynthesis?

hydrogen, helium, and trace amounts of deuterium and lithium

What direct evidence do we have that the weak and electromagnetic forces were once unified as a single electroweak force?

Particle accelerators on Earth can recreate the conditions of this era and have produced particles predicted by the electroweak theory.

In what sense is the cosmic microwave background evidence for the Big Bang?

because it was predicted to exist if the Big Bang really occurred

To date, physicists have investigated the behavior of matter and energy at temperatures as high as those that existed in the universe as far back as ________ after the Big Bang.

10-10 second

How do we know that there is much more mass in the halo of our galaxy than in the disk?

Stars and gas clouds 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.

Why isn't the space within our solar system or the Milky Way expanding according to Hubble's law?

The gravity exerted by the solar system and the Milky Way is strong enough to hold them together against the expansion of the universe.

Based on current understanding, what exactly is causing the expansion of the universe to accelerate?

We don't know, but we call it "dark energy."

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 measuring the sky positions and redshifts of many galaxies

Why were astronomers in the 1990s surprised to learn that the expansion rate of the universe is getting faster with time?

They expected gravity to be slowing the expansion rate with time.

Based on current evidence and models, visible, luminous matter (such as the stars within galaxies) amounts to approximately what percentage of the total mass and energy of the universe?

less than 1 percent

Which of the following provides important evidence for the existence of dark matter in clusters of galaxies?

the high temperature and distribution of gas between the galaxies in clusters

Galaxies sometimes have recession speeds that deviate somewhat from the exact speed predicted by Hubble's law. What do these so-called peculiar velocities reveal?

Question

velocities due to the gravitational tugs of nearby galaxies or clusters of galaxies

Why is dark matter called "dark matter"?

It does not interact strongly with light, but it does have gravity like other matter.

Which of the following is an example of ordinary (baryonic) matter?

you

In a mass-energy census of the universe, what are the approximate proportions of normal matter, dark matter, and dark energy?

27% dark matter, 68% dark energy, 5% normal matter

Given the observed average distance between galaxies today, which statement is true?

The faster the past expansion rate, the younger the age of the universe.

Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into the galaxy's disk?

WIMPS rarely interact with other particles and therefore cannot settle into galactic disks

Which of the following is not evidence for dark matter?

the expansion of the universe

Based on current evidence, which of the following is the most likely candidate to make up the majority of dark matter?

weakly interacting massive particles (WIMPs)

Based on current evidence and models, the total amount of matter (both luminous matter and dark matter) represents approximately what percentage of the total mass and energy of the universe?

about 30 percent

Which of the following best describes the distribution of the dark matter in a spiral galaxy?

approximately spherical and about 10 times the size of the galaxy halo

How are rotation curves of spiral galaxies determined for orbital distances beyond where starlight can be detected?

by observations of the 21 cm line of atomic hydrogen