Deck 12: Nursery of the Stars: the Interstellar Medium and Star Formation

Question 1

How do astronomers measure the properties of molecular clouds?

A) through emission lines of H₂
B) through absorption lines of H₂
C) through emission lines of tracer molecules like CO₂
D) through interactions with nearby clouds

Answer 1

through emission lines of tracer molecules like CO₂

Question 2

Which of the following lists the phases of the interstellar medium in order,from densest to least dense?

A) molecular clouds, H I clouds, H II regions, warm interstellar medium, coronal gas
B) H II regions, warm interstellar gas, coronal gas, H I clouds, molecular gas
C) molecular clouds, H II regions, coronal gas, warm interstellar gas, H I clouds
D) H I clouds, molecular clouds, warm interstellar medium, H II regions, H I clouds, coronal gas

Answer 2

molecular clouds, H I clouds, H II regions, warm interstellar medium, coronal gas

Question 3

Giant molecular clouds are typically:

A) spherical.
B) hierarchical, with structure on a wide range of length scales.
C) dominated by magnetic fields.
D) found in the galactic halo.

Answer 3

hierarchical, with structure on a wide range of length scales.

Question 4

Which of the following adjectives best describes the interstellar medium?

A) ordered
B) dense
C) smooth
D) turbulent

Question 5

Which of the following is an important effect of interstellar dust?

A) a platform for the formation of molecules
B) pressure equilibrium in the interstellar medium
C) a source of diagnostic emission lines
D) heating of the interstellar medium

Question 6

What is the wavelength of radiation produced by the system in the figure below?

A) 91 nm
B) 121 nm
C) 656 nm
D) 21 cm

Question 7

An interstellar gas cloud has a temperature of 100 K and a density of 50 particles per cubic centimeter.How could an astronomer most easily observe this cloud?

A) atomic hydrogen absorption lines
B) molecular absorption lines
C) atomic hydrogen emission lines
D) molecular emission lines

Question 8

A gas cloud is in pressure equilibrium with its surroundings,which have a temperature of 10⁴ K.If the gas cloud is 100 times denser than the surrounding medium,what is its temperature?

A) 10⁴ K
B) 1 K
C) 10 K
D) 100 K

Question 9

How do dust particles affect the formation of interstellar molecules?

A) Dust particles collide and break into smaller molecules.
B) Dust particles are the building blocks of molecules.
C) Dust particles trap other atoms, which can form molecules.
D) Radiation from dust particles catalyzes dust formation.

Question 10

How are the properties of the densest parts of the interstellar medium measured?

A) gravitational effects
B) molecular lines
C) Ha emission
D) hydrogen 21 centimeter absorption

Question 11

An interstellar gas cloud with a density of 100 particles per cubic centimeter and with a temperature of 100 K is in equilibrium with a medium with a density of 0.1 particles per cubic centimeter and 10⁶ K.What is the most likely explanation?

A) supernova blast waves
B) photoionization
C) turbulence
D) the ideal gas law

Question 12

Which part of the interstellar medium is primarily photoionized?

A) molecular clouds
B) H I clouds
C) warm interstellar medium
D) coronal gas

Question 13

What is the typical size of an interstellar dust grain?

A) 50 microns
B) 5 microns
C) 5 nm
D) 500 nm

Question 14

Which of the following is a key constituent of the interstellar medium?

A) dust only
B) gas only
C) gas and dust
D) Neither gas nor dust, it is a vacuum.

Question 15

Which of the following provides the best example of turbulence?

A) a molecular cloud with large whorls containing many smaller whorls
B) a jet launched at near the speed of light by a supermassive black hole
C) a large network of convection cells inside a star
D) an accretion disk rotating around a black hole

Question 16

Which of the following phases of the interstellar medium is coldest?

A) H I clouds
B) molecular clouds
C) H II regions
D) coronal gas

Question 17

Which of the following is most strongly affected by interstellar dust?

A) infrared light
B) visible light
C) microwaves
D) radio waves

Question 18

Which of the following molecules is the most abundant species in giant molecular clouds in the Milky Way?

A) H₂O
B) CO₂
C) O₂
D) H₂

Question 19

What is the best evidence that giant molecular clouds formed through collisions of large gas flows?

A) the Doppler shifts of segments of the clouds
B) the balance between gravity and pressure in the clouds
C) the long lifetimes of the clouds
D) the magnetic field structure within clouds

Question 20

What is the typical density of the interstellar medium?

A) 0 particles/cm³ (vacuum)
B) 1 particle/cm³
C) 10¹² particles/cm³
D) 10¹⁹ particles/cm³

Question 21

A protostar forming without magnetic fields will most likely lack:

A) a molecular outflow.
B) an accretion disk.
C) an envelope.
D) the conditions necessary for fusion.

Question 22

Massive stars on the main sequence generate energy through the:

A) proton-proton chain.
B) CNO cycle.
C) triple alpha reaction.
D) onion model.

Question 23

Under which of the following circumstances will a protostar lack an accretion disk?

A) The protostar has a jet.
B) The protostar's molecular cloud had no heavy elements.
C) The protostar's molecular cloud was not spinning.
D) The protostar has no binary companion.

Question 24

Which of the following changes would help a molecular cloud to resist gravitational collapse longer?

A) increasing the cloud's average velocity relative to the center of the Milky Way.
B) decreasing the cloud's temperature
C) decreasing the fraction of metals in the cloud
D) changing the cloud's composition

Question 25

A T Tauri star is a:

A) low-mass main sequence star.
B) red giant star with strong winds.
C) white dwarf that undergoes novae.
D) protostar that produces X-ray flares.

Question 26

A star with which of the following masses is most likely to form in a molecular cloud?

A) 0.5 MSun
B) 1 MSun
C) 10 MSun
D) 100 MSun

Question 27

Two molecular clouds have identical densities,but cloud A has a radius four times larger than cloud B.How much longer will it take cloud A to collapse than cloud B?

A) They collapse on the same timescale.
B) twice as long
C) 4 times as long
D) 16 times as long

Question 28

The tracks with arrows shown on the figure below illustrate what evolutionary phase of the stellar life cycle?

A) main sequence
B) protostar
C) red giant
D) horizontal branch

Question 29

Which of the following effects would extend the lifetime of a protostar (before it becomes a star),if everything else remained the same?

A) increase the opacity of the protostar material
B) increase the luminosity of the protostar
C) increase the rate of fusion inside the protostar
D) increase the temperature of the protostar

Question 30

What is the net result of the CNO cycle?

A) One carbon nucleus and four hydrogen nuclei are transformed into one oxygen nucleus, two neutrinos, and energy.
B) One carbon nucleus and one helium nucleus are transformed into one oxygen nucleus, two neutrinos, and energy.
C) Three helium nuclei are transformed into one carbon nucleus and energy.
D) Four hydrogen nuclei are transformed into one helium nucleus, two neutrinos, and energy.

Question 31

What physical principle is responsible for accretion disks forming around protostars?

A) hydrostatic equilibrium
B) conservation of angular momentum
C) blackbody radiation
D) Wien's law

Question 32

Which of the following helps support molecular clouds against gravitational collapse?

A) thermal pressure
B) turbulence
C) magnetic fields
D) all of the above

Question 33

A protostar becomes a star when:

A) its luminosity reaches a maximum.
B) its surface temperature reaches 5,000 K.
C) it loses all its gravitational energy.
D) nuclear fusion begins.

Question 34

Astronomers discover a large number of Herbig-Haro objects arranged along two perpendicular lines centered on an obscured area of the sky,as shown in the figure below.That area most likely contains a:

A) supermassive black hole.
B) pair of supermassive black holes.
C) protostar.
D) pair of protostars.

Question 35

As a low-mass protostar approaches the main sequence,its central temperature ________ and its luminosity ________.

A) decreases; decreases
B) increases; increases
C) remains steady; increases
D) increases; decreases

Question 36

A brown dwarf is a:

A) low-mass star.
B) failed star.
C) stellar remnant supported by electron degeneracy pressure.
D) stellar remnant supported by neutron degeneracy pressure.

Question 37

Which regions of a molecular cloud collapse under gravity first?

A) those with the smallest linear size
B) the densest
C) the least massive
D) the most massive

Question 38

A massive protostar is more luminous than a low-mass protostar because it:

A) undergoes a different set of fusion reactions.
B) has a lower temperature.
C) releases less of its thermal energy.
D) had more potential gravitational energy before it collapsed.

Question 39

In what wavelength range do protostars emit most of their energy?

A) optical
B) radio
C) infrared
D) ultraviolet

Question 40

An object with a mass 10 times larger than Jupiter's forms on its own from gravitational collapse in a molecular cloud.What kind of object is it?

A) T Tauri object
B) M dwarf
C) brown dwarf
D) planet

Question 41

The figure below shows the HR diagram for an astronomical object.What kind of object is it?

A) an elliptical galaxy
B) a spiral galaxy
C) a globular cluster
D) an open cluster

Question 42

A gas cloud with a temperature of 100 K is embedded in a medium of temperature 3 × 10⁶ K and density 0.1 particle per cubic centimeter.If the gas cloud is neither expanding nor contracting,what is its density?

Question 43

Where do high-mass stars form?

A) in massive clumps
B) in small, isolated clumps
C) in binary systems
D) in globular clusters

Question 44

How might an astronomer measure the initial mass function?

A) catalog the masses of all the stars in the Milky Way
B) catalog the masses of all nearby galaxies
C) catalog the masses of all the molecular clouds in the Milky Way
D) catalog the masses of all the stars in a young open cluster

Question 45

Why do massive stars use the CNO cycle to generate energy,rather than the proton-proton chain?

A) It is more temperature sensitive.
B) Massive stars have more carbon.
C) It produces more energy per helium nucleus.
D) It uses up the carbon too quickly in low-mass stars.

Question 46

What is the criterion for a collapsing cloud to become a protostar?

Question 47

Which of the following effects would be most likely to increase the lifetime of a giant molecular cloud?

A) accelerating the formation of high-mass stars
B) accelerating the formation of low-mass stars
C) suppressing of the formation of high-mass stars
D) suppressing the formation of low-mass stars

Question 48

How can astronomers observe coronal gas in the interstellar medium?

Question 49

What would happen to the Milky Way's interstellar medium if more supernovae occurred than usual?

Question 50

What does it mean for a giant molecular cloud to have a hierarchy of structures?

Question 51

What kind of equilibrium characterizes the relation between gas clouds in the interstellar medium?

Question 52

Which of the following is likely to occur in a cluster with high-mass star formation?

A) elephant trunks
B) a decrease in the frequency of planets
C) triggered star formation
D) all of the above

Question 53

What is a proplyd?

A) the smallest form of molecular cloud
B) a bright knot in a protostellar jet
C) the visible result of colliding winds
D) none of the above

Question 54

An open cluster is a:

A) globular cluster in the Milky Way's halo.
B) gravitationally bound system of thousands of galaxies.
C) group of young stars that will later disperse.
D) group of molecular clouds.

Question 55

What are two reasons that molecules are so abundant within giant molecular clouds?

Question 56

Which of the following nuclei can a compact astronomical object with a mass of 0.05 M_Sun create through fusion?

A) none
B) deuterium
C) helium
D) carbon

Question 57

What factor provides a giant molecular cloud's primary support against gravitational collapse?

Question 58

List the five phases of the interstellar medium in order from coolest to hottest.

Question 59

What kind of interstellar feature is shown by the "X" in the figure below?

Question 60

Over what timescale do stars in a typical open cluster form,relative to each other?

A) thousands of years
B) millions of years
C) billions of years
D) They form nearly instantaneously.

Question 61

Suppose a star forms with 10 times the mass of the Sun and without any elements heavier than helium.How will the central temperature of this star compare to a star with the same mass but the same composition as the Sun?

Question 62

Astronomers have not found any stars more massive than about 125 MSᵤn in the Milky Way.What stops more massive stars from forming?

Question 63

Why does the initial mass function of stars differ from the actual distribution of stars in the Milky Way at the present day?

Question 64

Why is there some variation in the birth times of stars in open clusters?

Question 65

Why is carbon important to the energy-generation process in a massive star?

Question 66

Name two sources for the energy radiated by brown dwarfs.

Question 67

According to most astronomers,what two factors are necessary to launch a protostellar jet?

Question 68

How do the density and central temperature change as it approaches the main sequence?

Question 69

Protostars are surrounded by accretion disks and envelopes.How is the gas in each of these components moving?

Question 70

A Herbig-Haro object is moving at 100 km/s and lies 10 parsecs from a protostar.What is a lower limit for the lifetime of the protostar? Express your answer in years.