Question 1

What kind of gas cloud is most likely to give birth to stars?

Accepted Answer

A) a hot,dense gas cloud 
B) a cold,dense gas cloud 
C) a cold,low-density gas cloud 
D) a hot,low-density gas cloud 
A) a hot,dense gas cloud 
B) a cold,dense gas cloud 
C) a cold,low-density gas cloud 
D) a hot,low-density gas cloud

Question 2

What is the range of timescales for star formation?

Accepted Answer

A) from 1 million years for the most massive stars up to 10 million years for the least massive stars 
B) from 1 million years for the most massive stars up to 100 million years for the least massive stars 
C) from 1 million years for the least massive stars up to 10 million years for the most massive stars 
D) from 1 million years for the least massive stars up to 100 million years for the most massive stars 
E) about 30 million years for all stars,whatever mass 
A) from 1 million years for the most massive stars up to 10 million years for the least massive stars 
B) from 1 million years for the most massive stars up to 100 million years for the least massive stars 
C) from 1 million years for the least massive stars up to 10 million years for the most massive stars 
D) from 1 million years for the least massive stars up to 100 million years for the most massive stars 
E) about 30 million years for all stars,whatever mass

Question 3

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

Accepted Answer

A) 1% 
B) 10% 
C) 50% 
D) 98% 
A) 1% 
B) 10% 
C) 50% 
D) 98%

Question 4

When does a protostar become a main-sequence star?

Accepted Answer

A) when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space 
B) when a piece of a molecular cloud first begins to contract into a star 
C) when it becomes luminous enough to emit thermal radiation 
D) at the instant that the first hydrogen fusion reactions occur in the protostar's core 
A) when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space 
B) when a piece of a molecular cloud first begins to contract into a star 
C) when it becomes luminous enough to emit thermal radiation 
D) at the instant that the first hydrogen fusion reactions occur in the protostar's core

Question 5

No stars have been found with masses greater than 300 times our Sun because

Accepted Answer

A) molecular clouds do not have enough material to form such massive stars. 
B) they would fragment into binary stars because of their rapid rotation. 
C) they would generate so much power that they would blow themselves apart. 
D) they shine exclusively at X-ray wavelengths and become difficult to detect. 
E) they are not bright enough to be seen nearby. 
A) molecular clouds do not have enough material to form such massive stars. 
B) they would fragment into binary stars because of their rapid rotation. 
C) they would generate so much power that they would blow themselves apart. 
D) they shine exclusively at X-ray wavelengths and become difficult to detect. 
E) they are not bright enough to be seen nearby.

Question 6

Close binary star systems are thought to form when

Accepted Answer

A) two interstellar gas clouds happen to contract so close together that there's no room for a disk or planets. 
B) the protostellar disk around a protostar has enough material to form a second star. 
C) gravity pulls two neighboring protostars quite close together,but angular momentum causes them to orbit each other rather than colliding. 
D) a protostar emits two jets,each of which turns into a star. 
A) two interstellar gas clouds happen to contract so close together that there's no room for a disk or planets. 
B) the protostellar disk around a protostar has enough material to form a second star. 
C) gravity pulls two neighboring protostars quite close together,but angular momentum causes them to orbit each other rather than colliding. 
D) a protostar emits two jets,each of which turns into a star.

Question 7

What are the mechanisms that restrict the mass range of stars to about 0.1 to 100 solar masses?

Accepted Answer

The low end is set by the resistance to

Question 8

For every star with a mass greater than 10 solar masses,about how many stars are there with masses less than a solar mass?

Accepted Answer

A) 1 
B) 3 
C) 10 
D) 30 
E) 200 
A) 1 
B) 3 
C) 10 
D) 30 
E) 200

Question 9

What happens to the visible light radiated by stars located within a dusty gas cloud?

Accepted Answer

A) It is blocked by dust and its energy is thereby lost. 
B) It is absorbed by dust,which heats the dust grains so that they emit the absorbed energy as infrared light. 
C) It is reflected by dust back to the star from whence it came. 
D) It passes through the cloud unaffected. 
A) It is blocked by dust and its energy is thereby lost. 
B) It is absorbed by dust,which heats the dust grains so that they emit the absorbed energy as infrared light. 
C) It is reflected by dust back to the star from whence it came. 
D) It passes through the cloud unaffected.

Question 10

What happens to the rotation of a molecular cloud as it collapses to form a star?

Accepted Answer

A) The rotation rate remains the same and results in stellar rotation. 
B) The rotation dissipates and any residual is left in small overall rotation of the star. 
C) The rotation rate increases and results in fast rotation of the star. 
D) The rotation rate increases and results in a flattened disk of material around a protostar. 
E) The rotation increases the speed of collapse and produces more massive stars. 
A) The rotation rate remains the same and results in stellar rotation. 
B) The rotation dissipates and any residual is left in small overall rotation of the star. 
C) The rotation rate increases and results in fast rotation of the star. 
D) The rotation rate increases and results in a flattened disk of material around a protostar. 
E) The rotation increases the speed of collapse and produces more massive stars.

Question 11

The surface of a protostar radiates energy while its core

Accepted Answer

A) shrinks and cools. 
B) shrinks and maintains a constant temperature. 
C) shrinks and heats. 
D) expands and cools. 
A) shrinks and cools. 
B) shrinks and maintains a constant temperature. 
C) shrinks and heats. 
D) expands and cools.

Question 12

Describe the four distinct stages in the life track of a solar-mass protostar on the H-R diagram and explain why the track is the shape it is.

Accepted Answer

1.The protostar forms from a collapsing

Question 13

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?

Accepted Answer

A) A main-sequence star is hotter and brighter than it was as a protostar. 
B) A main-sequence star is cooler and dimmer than it was as a protostar. 
C) A main-sequence star is cooler and brighter than it was as a protostar. 
D) A main-sequence star is hotter and dimmer than it was as a protostar. 
A) A main-sequence star is hotter and brighter than it was as a protostar. 
B) A main-sequence star is cooler and dimmer than it was as a protostar. 
C) A main-sequence star is cooler and brighter than it was as a protostar. 
D) A main-sequence star is hotter and dimmer than it was as a protostar.

Question 14

Where does a 1-solar-mass protostar appear on an H-R diagram?

Accepted Answer

A) to the right of the main sequence,and lower down than the Sun 
B) to the right of the main sequence,and higher up than the Sun 
C) to the left of the main sequence,and higher up than the Sun 
D) Nowhere-only stars that have fusion in their cores can be shown on H-R diagrams. 
A) to the right of the main sequence,and lower down than the Sun 
B) to the right of the main sequence,and higher up than the Sun 
C) to the left of the main sequence,and higher up than the Sun 
D) Nowhere-only stars that have fusion in their cores can be shown on H-R diagrams.

Question 15

Why does the rotation of a protostar slow down over time?

Accepted Answer

A) All rotating objects slow down over time. 
B) Magnetic fields can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away. 
C) The onset of fusion causes the rotation rate to slow dramatically. 
D) Magnetic fields of other stars interact with the magnetic fields of the protostars,slowing its rotation. 
A) All rotating objects slow down over time. 
B) Magnetic fields can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away. 
C) The onset of fusion causes the rotation rate to slow dramatically. 
D) Magnetic fields of other stars interact with the magnetic fields of the protostars,slowing its rotation.

Question 16

No stars have been found composed solely of Hydrogen and Helium (and no heavier elements).

Accepted Answer

A) True 
 B)False

Question 17

The typical density and temperature of molecular clouds are

Accepted Answer

A) 100 molecules per cubic centimeter,10-30 Kelvin. 
B) 300 molecules per cubic centimeter,10-30 Kelvin. 
C) 1000 molecules per cubic centimeter,10-30 Kelvin. 
D) 100 molecules per cubic centimeter,100-300 Kelvin. 
E) 300 molecules per cubic centimeter,100-300 Kelvin. 
A) 100 molecules per cubic centimeter,10-30 Kelvin. 
B) 300 molecules per cubic centimeter,10-30 Kelvin. 
C) 1000 molecules per cubic centimeter,10-30 Kelvin. 
D) 100 molecules per cubic centimeter,100-300 Kelvin. 
E) 300 molecules per cubic centimeter,100-300 Kelvin.

Question 18

What effect are magnetic fields thought to have on star formation in molecular clouds?

Accepted Answer

A) They can help resist gravity,so that more total mass is needed before the cloud can collapse to form stars. 
B) They accelerate the star formation process. 
C) They allow small stars to form in isolation within gas clouds. 
D) None-there are no magnetic fields in interstellar space. 
A) They can help resist gravity,so that more total mass is needed before the cloud can collapse to form stars. 
B) They accelerate the star formation process. 
C) They allow small stars to form in isolation within gas clouds. 
D) None-there are no magnetic fields in interstellar space.

Question 19

Which star spends the longest time in the protostellar phase of life?

Accepted Answer

A) a 1-solar-mass star 
B) a 2-solar-mass star 
C) a 3-solar-mass star 
D) a 4-solar-mass star 
E) a 5-solar-mass star 
A) a 1-solar-mass star 
B) a 2-solar-mass star 
C) a 3-solar-mass star 
D) a 4-solar-mass star 
E) a 5-solar-mass star

Question 20

The interstellar clouds called molecular clouds are

Accepted Answer

A) the clouds in which elements such as carbon,nitrogen,and oxygen are made. 
B) clouds that are made mostly of complex molecules such as carbon dioxide and sulfur dioxide. 
C) the hot clouds of gas expelled by dying stars. 
D) the cool clouds in which stars form. 
A) the clouds in which elements such as carbon,nitrogen,and oxygen are made. 
B) clouds that are made mostly of complex molecules such as carbon dioxide and sulfur dioxide. 
C) the hot clouds of gas expelled by dying stars. 
D) the cool clouds in which stars form.

Exam 16: Star Birth

What kind of gas cloud is most likely to give birth to stars?

What is the range of timescales for star formation?

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

When does a protostar become a main-sequence star?

No stars have been found with masses greater than 300 times our Sun because

Close binary star systems are thought to form when

What are the mechanisms that restrict the mass range of stars to about 0.1 to 100 solar masses?

For every star with a mass greater than 10 solar masses,about how many stars are there with masses less than a solar mass?

What happens to the visible light radiated by stars located within a dusty gas cloud?

What happens to the rotation of a molecular cloud as it collapses to form a star?

The surface of a protostar radiates energy while its core

Describe the four distinct stages in the life track of a solar-mass protostar on the H-R diagram and explain why the track is the shape it is.

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?

Where does a 1-solar-mass protostar appear on an H-R diagram?

Why does the rotation of a protostar slow down over time?

No stars have been found composed solely of Hydrogen and Helium (and no heavier elements).

The typical density and temperature of molecular clouds are

What effect are magnetic fields thought to have on star formation in molecular clouds?

Which star spends the longest time in the protostellar phase of life?

The interstellar clouds called molecular clouds are

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