Astronomy Exam #4

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According to modern science, approximately how old is the Sun?

4.6 billion years

The Sun will exhaust its nuclear fuel in about ______.

5 billion years.

Which of the following correctly describes how the process of gravitational contraction can make a star hot?

When a star contracts in size, gravitational potential energy is converted to thermal energy.

What two physical processes balance each other to create the condition known as gravitational equilibrium in stars?

The gravitational force and outward pressure.

The source of energy that keeps the Sun shining today is ______.

nuclear fusion.

What is the Sun made of (by mass)?

70% hydrogen, 28% helium, 2% other elements

From center outward, which of the following lists the "layers" of the Sun in the correct order?

core, radiation zone, convection zone, photosphere, chromosphere, corona

What are the appropriate units for the Sun's luminosity?

watts

The Sun's surface, as we see it with our eyes, is called the

photosphere

What is the solar wind?

It is the name we give to the gas (or plasma) particles flowing outward from the surface of the Sun into the solar system.

The fundamental nuclear reaction occurring in the core of the Sun is:

Nuclear fusion of hydrogen to helium.

What is the proton-proton chain?

It is the specific set of nuclear reactions through which the Sun fuses hydrogen into helium.

Why are neutrinos so difficult to detect?

hey have a tendency to pass through just about any material without any interactions.

To estimate the central temperature of the Sun, scientists ______.

use computer models to predict interior conditions.

Which statement best describes the solar neutrino problem?

Early experiments designed to detect solar neutrinos found them, but in fewer numbers than had been expected.

The light radiated from the Sun's surface reaches Earth in about 8 minutes, but the energy of that light was released by fusion in the solar core about ______.

one million years ago.

What happens to energy in the Sun's convection zone?

Energy is transported outward by the rising of hot plasma and sinking of cooler plasma.

What do sunspots, solar prominences, and solar flares all have in common?

The are all strongly influenced by magnetic fields on the Sun.

Which of the following is NOT a characteristic of the 11-year sunspot cycle?

The sunspot cycle is very steady, so that each 11-year cycle is nearly identical to every other 11-year cycle.

How is the sunspot cycle directly relevant to us here on Earth?

Solar flares and other activity associated with the sunspot cycle can disrupt radio communications and knock out sensitive electronic equipment.

In the late 1800s, Kelvin and Helmholtz suggested that the Sun stayed hot due to gravitational contraction. What was the major drawback to this idea?

It predicted that the Sun could shine for about 25 million years, but geologists had already found that Earth is much older than this.

When is/was gravitational contraction an important energy generation mechanism for the Sun?

It was important when the Sun was forming from a shrinking interstellar cloud of gas.

What do we mean when we say that the Sun is in gravitational equilibrium?

There is a balance within the Sun between the outward push of pressure and the inward pull of gravity.

ich of the following is the best answer to the question "Why does the Sun shine?"

As the Sun was forming, gravitational contraction increased the Sun's temperature until the core become hot enough for nuclear fusion, which ever since has generated the heat that makes the Sun shine.

How does the Sun's mass compare to Earth's mass?

The Sun's mass is about 300,000 times the mass of the Earth.

The Sun's average surface (photosphere) temperature is about ______.

5,800 K.

Why do sunspots appear dark in pictures of the Sun?

They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere.

The Sun's surface seethes and churns with a bubbling pattern. Why?

We are seeing hot gas rising and cool gas falling due to the convection that occurs beneath the surface.

Which of the following correctly compares the Sun's energy generation process to the energy generation process in human-built nuclear power plants?

The Sun generates energy by fusing small nuclei into larger ones, while our power plants generate energy by the fission (splitting) of large nuclei.

Which of the following best explains why nuclear fusion requires bringing nuclei extremely close together?

Nuclei normally repel because they are all positively charged, and can be made to stick only when brought close enough for the strong force to take hold.

The overall result of the proton-proton chain is:

4 H ( 1 He + energy.

ery second, the Sun converts 600 million tons of hydrogen into 596 million tons of helium. The remaining 4 million tons ______.

are converted to an amount of energy equal to 4 million tons times the speed of light squared.

Suppose that, for some unknown reason, the core of the Sun suddenly became hotter and the rate of nuclear fusion thereby increased. What would happen next?

The core would expand, reducing the pressure and temperature, and the rate of fusion would decrease until it returned to its original level.

Which of the following best explains why the Sun's luminosity gradually rises over billions of years?

Fusion steadily decreases the number of independent particles in the core (since 4 hydrogen nuclei become just 1 helium nucleus), which allows gravity to compress and heat the core. This core heating increases the rate of fusion and hence increases the Sun's luminosity.

Why does the Sun emit neutrinos?

Fusion in the Sun's core creates neutrinos as a byproduct.

Which of the following best describes the current status of our understanding of the solar neutrino problem?

Experimental evidence suggests that solar neutrinos can change from electron neutrinos to other types of neutrinos during their journey to Earth. If confirmed, the solar neutrino problem appears to be solved.

How can we best observe the Sun's chromosphere and corona?

The chromosphere is best observed with ultraviolet telescopes and the corona is best observed with X-ray telescopes.

The intricate patterns visible in an X-ray image of the Sun generally show ______.

extremely hot plasma flowing along magnetic field lines.

How can we measure the strength of magnetic fields on the Sun?

Magnetic fields can cause some spectral lines to split, so we can measure the Sun's magnetic field strength by carefully studying lines in the Sun's spectrum.

Satellites in low-Earth orbits are more likely to crash to Earth during the solar maximum periods of the sunspot cycle because ______.

Earth's upper atmosphere tends to expand during solar maximum, exerting drag on satellites in low orbits.

What is the approximate chemical composition (by mass) with which all stars are born?

three quarters hydrogen, one quarter helium, no more than 2% heavier elements

The total amount of power (in watts, for example) that a star radiates into space is called its ______.

luminosity.

Assuming that we can always measure the apparent brightness of a star, what does the luminosity-distance formula allow us to do?

It allows us to calculate a star's luminosity if we know its distance, or calculate its distance if we know its luminosity.

If star A has an apparent magnitude of 3 and star B has an apparent magnitude of 5, which star is brighter in our sky?

Star A

Ten parsecs is about ______.

32.6 light-years.

If star A is closer to us than star B, then Star A's parallax angle is ______.

larger than that of Star B.

From hottest to coolest, the order of the spectral types of stars is:

OBAFGKM

Our Sun is a star of spectral type _______.

G

Astronomers can measure a star's mass in only certain cases. Which case below might allow astronomers to measure a star's mass?

The star is a member of a binary star system.

Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?

spectroscopic binary

The axes on a Hertzsprung-Russell (H-R) diagram represent ______.

luminosity and surface temperature

What can we infer, at least roughly, from a star's luminosity class?

Its radius.

On an H--R diagram, stellar radii ______.

increase diagonally from the lower left to the upper right.

On an H--R diagram, stellar masses ______.

can be determined from a star's position in the diagram only for main sequence stars, and decrease from upper left to lower right.

How is the lifetime of a star related to its mass?

More massive stars live considerably shorter lives than less massive stars.

Each choice below lists a spectral type and luminosity class for a star. Which one is a red supergiant?

Spectral type M2, luminosity class I

What is the common trait of all main sequence stars?

They generate energy through hydrogen fusion in their core.

Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?

Earth would be inside the supergiant.

What is a white dwarf?

It is the remains of a star, comprised mostly of carbon, that no longer produces energy by nuclear fusion.

What is a Cepheid?

It is a type of variable star for which the period of variability can tell us its luminosity.

ich of the following statements comparing open and globular star clusters is NOT true?

Open and globular clusters each typically contain a few hundred stars.

What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?

It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.

All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the innate characteristics of a star?

Its mass and its stage of life.

Based on the definition of apparent brightness, which units are appropriate for its measurement?

watts per square meter

Suppose two stars are identical except that one is twice as far away from us as the other. Which statement is true

Both stars have the same luminosity, but the apparent brightness of the closer star is four times as great as that of the more distant star.

Suppose that a star has a parallax angle of 1/20 arcsecond. If so, it is ______.

20 parsecs away.

The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the definitions of absolute and apparent magnitude, we can conclude that ______.

Vega is nearer than 10 parsecs from Earth.

Which of the following statements about spectral types of stars is NOT generally true?

The spectral type of a star can be used to determine its distance.

Sirius is a spectral type A star whereas Rigel is a spectral type B star. What can we conclude?

Rigel has a higher surface temperature than Sirius.

Astronomers generally can measure a star's mass if only it is a member of a binary star system. What characteristics of the stars must we know to measure the masses of the stars in a binary system?

Their orbital period and average orbital distance.

Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours. The most likely explanation is that:

The star is a member of an eclipsing binary star system.

To answer this question, refer to the labels (a) through (e) on the following sketch of an H--R diagram. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents stars that are cool and dim?

D

To answer this question, refer to the labels (a) through (e) on the following sketch of an H--R diagram. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents stars of the largest radii?

E

To answer this question, refer to the labels (a) through (e) on the following sketch of an H--R diagram. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents the most common type of stars?

D

To answer this question, refer to the labels (a) through (e) on the following sketch of an H--R diagram. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents stars that are extremely bright and emit most of their radiation as ultraviolet light?

C

To answer this question, refer to the labels (a) through (e) on the following sketch of an H--R diagram. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents stars with the longest main-sequence lifetimes?

D

To answer this question, refer to the labels (a) through (e) on the following sketch of an H--R diagram. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents stars fusing hydrogen in their cores?

A

To answer this question, refer to the labels (a) through (e) on the following sketch of an H--R diagram. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents stars that have no nuclear fusion in their cores?

B

You observe a star in the disk of the Milky Way, and you want to plot the star on an H-R diagram. You will need to determine all of the following, except:

mass of the star

The approximate main-sequence lifetime of a star of spectral type O is ______.

3 million years

For a Cepheid variable star, the longer the time between peaks of brightness, the ______.

greater the luminosity of the star

How did astronomers discover the relationship between spectral type and mass for main sequence stars?

Astronomers discovered the relationship by measuring the masses of main sequence stars in binary systems, and assume that the same relationship holds for single stars as well.

The choices below each describe the appearance of an H--R diagram for a different star cluster. Which cluster is the youngest?

The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.

The choices below each describe the appearance of an H--R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?

The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

Gravity would cause main-sequence stars to collapse if it were not for the opposing ______.

thermal pressure

In the context of understanding stellar lives, by "high-mass" stars we mean:

stars with mass more than about 8 times the mass of our Sun.

The interstellar clouds called molecular clouds are:

the cool clouds in which stars form

What is a protostar?

A very young star (relative to its lifespan).

What can we learn about a star from a life track on an H-R diagram?

What surface temperature and luminosity the star will have at each stage of its life.

The vast majority of stars in a newly formed star cluster are ______.

less massive than the Sun.

When does a star become a main-sequence star?

A star becomes a main-sequence star when the rate of hydrogen fusion within the star's core becomes high enough to balance the rate at which the star radiates energy into space.

Which of the following statements about brown dwarfs is NOT true?

Brown dwarfs will eventually collapse to become white dwarfs.

Which of the following lists the stages of life for a low-mass star in the correct order?

protostar, main-sequence star, red giant, planetary nebula, white dwarf

When a main-sequence star exhausts its core hydrogen fuel supply:

the core shrinks while the rest of the star expands.

The main source of energy for a star as it grows in size to become a red giant is ______.

hydrogen fusion in a shell surrounding the central core.

The overall helium fusion reaction is:

three helium nuclei fuse to form one carbon nucleus.

What is a helium flash?

The sudden onset of helium fusion in the core of a low-mass star.

Which of the following statements about horizontal branch stars is NOT true?

Horizontal branch stars have inert (non-burning) carbon cores.

What is a planetary nebula?

Gas ejected from a low-mass star in the final stage of its life.

The ultimate fate of our Sun is to____.

become a white dwarf that will slowly cool with time.

What is the CNO cycle?

The CNO cycle is series of nuclear reactions with the final result being the fusion of four hydrogen nuclei into one helium nucleus.

Why is iron significant to understanding how a supernova occurs?

Iron cannot release energy either by fission or fusion, so a star with an iron core has no way to generate additional energy to counteract the crush of gravity.

ter a supernova explosion, the remains of the stellar core ______.

may be either a neutron star or a black hole.

Why is Supernova 1987A particularly important to astronomers?

It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.

Algol consist of a 3.7 Msun main-sequence star and a 0.8 Msun subgiant. Why does this seem surprising, at least at first?

The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.

Which two processes can generate energy to help a star maintain its internal thermal pressure?

nuclear fusion and gravitational contraction

Our Sun is considered to be a ______.

low-mass star

Close binary star systems are thought to form when _____.

a protostar is spinning so fast that it splits in two.

Angular momentum plays an important role in star formation. Which of the following characteristics of a protostellar system is probably NOT strongly affected by the star's angular momentum?

The onset of core hydrogen fusion.

Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?

A main-sequence star is hotter and dimmer than it was as a protostar.

der a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?

few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.

We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?

Brown dwarfs would outnumber all ordinary stars.

Where would a brown dwarf be located on an H-R diagram?

It would be located below and to the right of the lowest part of the main sequence.

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. During which stage is the star's energy supplied by primarily by gravitational contraction?

ii

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. During which stage does the star have an inert (non-burning) helium core?

iv

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. Which stage lasts the longest?

iii

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. During which stage does the star have an inert (non-burning) carbon core surrounded by shells of helium and hydrogen burning?

viii

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. What will happen to the star after stage viii?

Its outer layers will be ejected as a planetary nebula and its core will become a white dwarf.

What process distributes most of the carbon in the universe?

Winds from low-mass red giant stars.

Why can the fusion of carbon occur in intermediate- and high-mass stars but not in low-mass stars?

It is because the cores of low-mass stars never get hot enough for carbon fusion.

Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen--16, neon--20, and magnesium--24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?

At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.

Which of the following statements about various stages of core nuclear burning (hydrogen, helium, carbon, etc.) in a high-mass star is NOT true?

Each successive stage lasts for approximately the same amount of time

Which event marks the beginning of a supernova?

The sudden collapse of an iron core into a compact ball of neutrons.

Suppose that the star Betelgeuse (the upper left shoulder of Orion) were to supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?

Betelgeuse would remain a dot of light, but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime.

A spinning neutron star has been observed at the center of a ______.

supernova remnant.

You discover a binary star system in which one star is a 15 Msun main-sequence star and the other is a 10 Msun giant. How do we believe that a star system such as this might have come to exist?

The giant must once have been the more massive star, but is now less massive because it transferred some of its mass to its companion.

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