Question 1

The coldest molecular clouds in our galaxy have temperatures of approximately 1000 K.

Accepted Answer

A) True 
 B)False  False

Question 2

When a 3 M=$\odot$=protostar forms, it starts out at the top of the Hayashi track with a luminosity of 4,000 L=$\odot$= and a temperature of 3600 K. What is its radius at this point (give the answer in units of R=$\odot$=), and how many times larger is it at this stage compared to its radius as a main-sequence star, which is about 2.5 R=$\odot$=? For reference, the Sun's temperature is 5800 K.

Accepted Answer

The Stefan-Boltzmann law says that   , and thus the protostar's radius is   During its formation, the star contracts by a factor of 164/2.5 =66.

Question 3

Why does the surface temperature of a low-mass protostar remain nearly constant as its core contracts?

Accepted Answer

It remains nearly constant because of the presence of H^- ions in its atmosphere, which determine the opacity of the stellar atmosphere and act as a thermal regulator.

Question 4

If you could watch stars forming out of a gas cloud, which stars would form first?

Accepted Answer

A)  low-mass stars 
B)  medium-mass stars 
C)  high-mass stars 
D)  stars with low temperatures 
E)  stars with more heavy elements 
A)  low-mass stars 
B)  medium-mass stars 
C)  high-mass stars 
D)  stars with low temperatures 
E)  stars with more heavy elements

Question 5

The H- atom is important in protostars because it acts as a powerful temperature regulator.

Accepted Answer

A) True 
 B)False

Question 6

What critical event transforms a protostar into a normal main-sequence star?

Accepted Answer

A)  Planets form in the accretion disk. 
B)  The star grows suddenly larger in radius. 
C)  Triple alpha reactions begin in the core. 
D)  Nuclear fusion begins in the core. 
E)  Convection begins throughout the star's interior. 
A)  Planets form in the accretion disk. 
B)  The star grows suddenly larger in radius. 
C)  Triple alpha reactions begin in the core. 
D)  Nuclear fusion begins in the core. 
E)  Convection begins throughout the star's interior.

Question 7

Given the low efficiency of the star formation process, the initial mass of a molecular cloud fragment that formed a 2 M $\odot$  star was probably close to:

Accepted Answer

A)  10 M $\odot$ 
B)  50 M $\odot$ 
C)  100 M $\odot$ 
D)  500 M $\odot$ 
E)  1,000 M $\odot$ 
A)  10 M $\odot$ 
B)  50 M $\odot$ 
C)  100 M $\odot$ 
D)  500 M $\odot$ 
E)  1,000 M $\odot$

Question 8

What is the energy source that powers brown dwarf stars?

Accepted Answer

Brown dwarfs shine because they are stil

Question 9

How are typical interstellar gas clouds different from the clouds that we see in the Earth's sky?

Accepted Answer

ISM gas clouds are mostly made

Question 10

Figure 1
-Use Figure 1 to complete the following statement. At the start of the evolution of a protostar, the radius of a 60 M $\odot$  protostar is roughly _________ a 1 M $\odot$  main-sequence star.

Accepted Answer

A)  10 times bigger than 
B)  100 times bigger than 
C)  10 times smaller than 
D)  100 time smaller than 
E)  the same as 
A)  10 times bigger than 
B)  100 times bigger than 
C)  10 times smaller than 
D)  100 time smaller than 
E)  the same as

Question 11

Why is it possible for self-gravity to dominate pressure in molecular clouds but not in most interstellar clouds?

Accepted Answer

Molecular clouds are denser (m

Question 12

Electronic transitions from the H2 molecule are easily seen at radio wavelengths.

Accepted Answer

A) True 
 B)False

Question 13

A surprising fact about a 1 M $\odot$  protostar is that, even though nuclear reactions have not yet started in their cores, they are _________ than the Sun.

Accepted Answer

A)  hotter 
B)  rotating faster 
C)  smaller 
D)  denser 
E)  more luminous 
A)  hotter 
B)  rotating faster 
C)  smaller 
D)  denser 
E)  more luminous

Question 14

Why can't very bright protostars be seen in visible light?

Accepted Answer

They are cool, so they emit mo

Question 15

Why does a protostar continue to collapse as it is forming?

Accepted Answer

It continually radiates away t

Question 16

Dust in the ISM appears dark in _________ wavelengths and bright in _________ wavelengths.

Accepted Answer

A)  visible; ultraviolet 
B)  infrared; radio 
C)  infrared; visible 
D)  radio; ultraviolet 
E)  visible; infrared 
A)  visible; ultraviolet 
B)  infrared; radio 
C)  infrared; visible 
D)  radio; ultraviolet 
E)  visible; infrared

Question 17

A typical molecular cloud has a temperature of approximately:

Accepted Answer

A)  0.3 K 
B)  10 K 
C)  80 K 
D)  300 K 
E)  1000 K 
A)  0.3 K 
B)  10 K 
C)  80 K 
D)  300 K 
E)  1000 K

Question 18

If you wanted to observe heavy elements in the interstellar medium, where would be the best place to look?

Accepted Answer

A)  dust grains 
B)  cold gas 
C)  hot gas 
D)  warm gas 
A)  dust grains 
B)  cold gas 
C)  hot gas 
D)  warm gas

Question 19

The average density of the interstellar medium is many times less dense than the best vacuum on Earth.

Accepted Answer

A) True 
 B)False

Question 20

Stars with a mass from 0.01 M $\odot$  to 0.08 M $\odot$  are very different from the Sun because they:

Accepted Answer

A)  do not have strong enough gravity to form planets 
B)  have much higher temperatures than the Sun 
C)  cannot successfully execute the proton-proton chain reactions 
D)  form much faster than the Sun did 
E)  do not exhibit sunspots 
A)  do not have strong enough gravity to form planets 
B)  have much higher temperatures than the Sun 
C)  cannot successfully execute the proton-proton chain reactions 
D)  form much faster than the Sun did 
E)  do not exhibit sunspots

Exam 15: Star Formation and the Interstellar Medium

The coldest molecular clouds in our galaxy have temperatures of approximately 1000 K.

Why does the surface temperature of a low-mass protostar remain nearly constant as its core contracts?

If you could watch stars forming out of a gas cloud, which stars would form first?

The H- atom is important in protostars because it acts as a powerful temperature regulator.

What critical event transforms a protostar into a normal main-sequence star?

Given the low efficiency of the star formation process, the initial mass of a molecular cloud fragment that formed a 2 M ⊙\odot⊙ star was probably close to:

What is the energy source that powers brown dwarf stars?

How are typical interstellar gas clouds different from the clouds that we see in the Earth's sky?

Figure 1 -Use Figure 1 to complete the following statement. At the start of the evolution of a protostar, the radius of a 60 M ⊙\odot⊙ protostar is roughly _________ a 1 M ⊙\odot⊙ main-sequence star.

Why is it possible for self-gravity to dominate pressure in molecular clouds but not in most interstellar clouds?

Electronic transitions from the H2 molecule are easily seen at radio wavelengths.

A surprising fact about a 1 M ⊙\odot⊙ protostar is that, even though nuclear reactions have not yet started in their cores, they are _________ than the Sun.

Why can't very bright protostars be seen in visible light?

Why does a protostar continue to collapse as it is forming?

Dust in the ISM appears dark in _________ wavelengths and bright in _________ wavelengths.

A typical molecular cloud has a temperature of approximately:

If you wanted to observe heavy elements in the interstellar medium, where would be the best place to look?

The average density of the interstellar medium is many times less dense than the best vacuum on Earth.

Stars with a mass from 0.01 M ⊙\odot⊙ to 0.08 M ⊙\odot⊙ are very different from the Sun because they:

Why Learn Astronomy

Patterns in the Skymotions of Earth

Motion of Astronomical Bodies

Gravity and Orbits

Light

The Tools of the Astronomer

The Birth and Evolution of Planetary Systems

The Terrestrial Planets and Earths Moon

Atmospheres of the Terrestrial Planets

Worlds of Gas and Liquid the Giant Planets

Planetary Adornmentsmoons and Rings

Dwarf Planets and Small Solar System Bodies

Taking the Measure of Stars

Our Starthe Sun

Evolution of Low-Mass Stars

Evolution of High-Mass Stars

Relativity and Black Holes

The Expanding Universe

Galaxies

The Milky Waya Normal Spiral Galaxy

Modern Cosmology

Large-Scale Structure in the Universe

Life

Filters

The H^- atom is important in protostars because it acts as a powerful temperature regulator.

Given the low efficiency of the star formation process, the initial mass of a molecular cloud fragment that formed a 2 M _$\odot$ star was probably close to:

Electronic transitions from the H₂ molecule are easily seen at radio wavelengths.

A surprising fact about a 1 M _$\odot$ protostar is that, even though nuclear reactions have not yet started in their cores, they are _________ than the Sun.

Dust in the ISM appears dark in _ wavelengths and bright in _ wavelengths.

Stars with a mass from 0.01 M _$\odot$ to 0.08 M _$\odot$ are very different from the Sun because they: