Question 1

The source of MOST of a protostar's heat is

Accepted Answer

A)   gravitational energy released as the protostar expands.
B)   nuclear reactions in its core converting helium to carbon and oxygen.
C)   gravitational energy released as the star contracts.
D)   nuclear reactions in its core converting hydrogen into helium.E) C) and D)
F) A) and D)

Question 2

Giant molecular clouds, which are major sites of star formation, can be up to about

Accepted Answer

A)   10 times the size of the solar system and contain 2 to 3 solar masses of material.
B)   10 pc across and contain a few thousand solar masses of material.
C)   100 pc across and contain 2 million solar masses of material.
D)   1000 pc across and contain 100 million solar masses of material.E) B) and D)
F) A) and B)

Question 3

Where are protostars MOST likely to form?

Accepted Answer

A)   emission nebulae
B)   reflection nebulae
C)   dark nebulae
D)   planetary nebulaeE) B) and C)
F) A) and D)

Question 4

On the Hertzsprung-Russell diagram, in which direction does the position occupied by a star move after helium burning begins in the star's core?

Accepted Answer

A)   toward the upper right
B)   toward the lower left
C)   toward the lower right
D)   toward the upper leftE) A) and B)
F) A) and C)

Question 5

The Jeans instability describes the

Accepted Answer

A)   explosion of a star at the end of its life, the supernova phenomenon.
B)   overcoming of gas pressure by self-gravity in a cold and dense interstellar cloud, to form a star.
C)   conditions under which sufficient numbers of neutrinos can trigger the collapse of an interstellar cloud.
D)   expansion of a gas cloud after gravitational contraction because of buildup of great heat within the cloud from gravitational potential energy.E) B) and C)
F) C) and D)

Question 6

A low-mass red dwarf (0.08 M < M < 0.4 M)  experiences which of these fusion reactions as the last fusion reaction in its core?

Accepted Answer

A)   hydrogen fusion
B)   helium fusion
C)   carbon fusion
D)   silicon fusionE) B) and C)
F) All of the above

Question 7

How long will the Sun have spent as a main-sequence star when it finally begins to evolve toward the red giant phase?

Accepted Answer

A)   1 billion years
B)   1 million years
C)   1011 years
D)   1010 yearsE) A) and D)
F) All of the above

Question 8

The components of a binary star, particularly if they are close, can influence each other in various ways. Which of these is NOT a likely effect of one star on its companion?

Accepted Answer

A)   One star can be enshrouded in an accretion disk.
B)   The gravitational force of one star will make its companion move in an orbit rather than remaining stationary.
C)   Intense radiation from a hot star can produce nuclear reactions on the surface of a cooler companion and initiate a nova explosion.
D)   Mass can be transferred from one star to its companion.E) None of the above
F) A) and D)

Question 9

What is the relationship between a giant molecular cloud and an H II region?

Accepted Answer

A)   They are two names for the same entity.
B)   In giant molecular clouds, H II regions surround ultraviolet-emitting stars (types O and
B)  , which have ionized the hydrogen around them.
C)   In H II regions, giant molecular clouds are concentrations of other molecules like CO and H2O.
D)   Giant molecular clouds evolve into H II regions as the molecules other than hydrogen are used up in star formation.E) None of the above
F) A) and B)

Question 10

A protostar of about 1 solar mass is gradually contracting and becoming hotter. This activity causes its position in the Hertzsprung-Russell diagram to shift slowly

Accepted Answer

A)   upward and toward the right.
B)   downward and toward the left.
C)   upward and toward the left.
D)   downward and toward the right.E) A) and C)
F) A) and B)

Question 11

How large was the Sun when it first formed as a protostar?

Accepted Answer

A)   about 5 times its present diameter
B)   about 15 times its present diameter
C)   about 50 times its present diameter
D)   same diameter it has nowE) All of the above
F) None of the above

Question 12

Most interstellar matter is too cold to be observed optically. In which part of the electromagnetic spectrum might its radiation be detected?

Accepted Answer

A)   gamma ray
B)   ultraviolet
C)   infrared
D)   X-rayE) A) and C)
F) All of the above

Question 13

After hydrogen burning ends in a star's core, its position on the Hertzsprung-Russell diagram moves toward the

Accepted Answer

A)   lower right.
B)   upper left.
C)   lower left.
D)   upper right.E) B) and D)
F) A) and B)

Question 14

When any star on the main sequence was a protostar, it was

Accepted Answer

A)   larger and hotter.
B)   hotter but not necessarily larger.
C)   cooler but not necessarily larger.
D)   larger and cooler.E) B) and C)
F) A) and B)

Question 15

During helium burning in a star's later life, the chemical element produced by the combination of helium nuclei is

Accepted Answer

A)   the light isotope of helium, 3He.
B)   heavy hydrogen, 2H.
C)   beryllium, 8Be.
D)   carbon, 12C.E) A) and D)
F) B) and C)

Question 16

Nuclear fusion reactions that convert helium into carbon and oxygen in the central core of a star occur during which phase of a star's life?

Accepted Answer

A)   in the red giant stage, before the helium flash
B)   during and immediately after the (first)  red giant or supergiant stage
C)   during the protostar stage
D)   after the main-sequence phase, before the star becomes a red giantE) B) and C)
F) C) and D)

Question 17

What happens to the helium-rich core of a star after the core runs out of hydrogen?

Accepted Answer

A)   The core cools down and contracts.
B)   The core expands and cools down.
C)   The core heats up and expands.
D)   The core contracts and heats up.E) B) and C)
F) A) and B)

Question 18

How many brown dwarfs and sub-brown dwarfs are estimated to exist in the Milky Way Galaxy?

Accepted Answer

A)   about 450
B)   at least a million, but less than 100 million
C)   at least 100 billion
D)   at least 100 million, but less than 100 billionE) B) and D)
F) A) and D)

Question 19

Approximately what fraction of its main-sequence lifetime has the Sun completed at the present time?

Accepted Answer

A)   about 1/4
B)   less than 10 percent
C)   about 1/2
D)   about 3/4E) A) and D)
F) None of the above

Question 20

When does a star achieve hydrostatic equilibrium?

Accepted Answer

A)   when it becomes a main-sequence star
B)   when it leaves the main sequence
C)   when it becomes a protostar
D)   This question has different answers for stars of different masses. Massive stars never achieve equilibrium; small stars achieve hydrostatic equilibrium as soon as they reach the main sequence.E) A) and C)
F) None of the above

Exam 13: The Lives of Stars From Birth Through Middle Age

What is the relationship between a giant molecular cloud and an H II region?

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After hydrogen burning ends in a star's core, its position on the Hertzsprung-Russell diagram moves toward the

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During helium burning in a star's later life, the chemical element produced by the combination of helium nuclei is

Correct AnswerverifiedShow Answer

What happens to the helium-rich core of a star after the core runs out of hydrogen?

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On the Hertzsprung-Russell diagram, in which direction does the position occupied by a star move after helium burning begins in the star's core?

Correct AnswerverifiedShow Answer

Where are protostars MOST likely to form?

Correct AnswerverifiedShow Answer

The Jeans instability describes the

Correct AnswerverifiedShow Answer

How large was the Sun when it first formed as a protostar?

Correct AnswerverifiedShow Answer

The components of a binary star, particularly if they are close, can influence each other in various ways. Which of these is NOT a likely effect of one star on its companion?

Correct AnswerverifiedShow Answer

How long will the Sun have spent as a main-sequence star when it finally begins to evolve toward the red giant phase?

Correct AnswerverifiedShow Answer

Approximately what fraction of its main-sequence lifetime has the Sun completed at the present time?

Correct AnswerverifiedShow Answer

A protostar of about 1 solar mass is gradually contracting and becoming hotter. This activity causes its position in the Hertzsprung-Russell diagram to shift slowly

Correct AnswerverifiedShow Answer

Nuclear fusion reactions that convert helium into carbon and oxygen in the central core of a star occur during which phase of a star's life?

Correct AnswerverifiedShow Answer

Most interstellar matter is too cold to be observed optically. In which part of the electromagnetic spectrum might its radiation be detected?

Correct AnswerverifiedShow Answer

The source of MOST of a protostar's heat is

Correct AnswerverifiedShow Answer

Giant molecular clouds, which are major sites of star formation, can be up to about

Correct AnswerverifiedShow Answer

When does a star achieve hydrostatic equilibrium?

Correct AnswerverifiedShow Answer

A low-mass red dwarf (0.08 M < M < 0.4 M) experiences which of these fusion reactions as the last fusion reaction in its core?

Correct AnswerverifiedShow Answer

How many brown dwarfs and sub-brown dwarfs are estimated to exist in the Milky Way Galaxy?

Correct AnswerverifiedShow Answer

When any star on the main sequence was a protostar, it was

Correct AnswerverifiedShow Answer

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