Deck 15: Star Formation and the Interstellar Medium

Question 1

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

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

Answer 1

high-mass stars

Question 2

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

Answer 2

False

Question 3

A protostar is usually in hydrostatic equilibrium as its collapses.

Answer 3

True

Question 4

The lowest-density gas in the interstellar medium is also the coldest.

Question 5

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

Question 6

Star formation in a molecular cloud can be slowed by the strength of its magnetic field and turbulence caused by supernovae and stellar winds from massive stars.

Question 7

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

Question 8

The interstellar medium is divided up into three different kinds of gas clouds: cold gas at 10 K, warm gas at 8000 K, and hot gas at about 1 million K.

Question 9

The dust in the interstellar medium comes primarily from the stellar winds of main-sequence stars.

Question 10

Molecular hydrogen atoms are found only inside dense clouds where they are shielded from stellar radiation.

Question 11

When radiation from an object passes through the interstellar medium:

A) the object appears dimmer
B) the object appears bluer
C) the object appears bluer and dimmer
D) the object appears redder and dimmer
E) the object's apparent velocity changes

Question 12

Stars forming in molecular clouds tend to form first in the low-density periphery.

Question 13

When winds blow the gas away from a forming protostar it becomes visible as a T Tauri star.

Question 14

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

Question 15

We observe neutral hydrogen gas using 21-cm emission.

Question 16

The average density of the interstellar medium is:

A) 1 atom/cm³
B) 1,000 atom/cm³
C) 10⁴ atom/cm³
D) 10⁶ atom/cm³
E) 10¹² atom/cm³

Question 17

When a molecular cloud fragments and stars form, the least massive stars are the first to form while the most massive stars take longer to form.

Question 18

Herbig-Haro objects are almost always found in pairs on either side of a young protostar.

Question 19

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

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

Question 20

When looking at the space between stars, what might you see?

A) Nothing; it is empty.
B) spacecraft
C) gas
D) dark matter
E) none of the above

Question 21

Sitting in a 100°F hot tub feels much hotter than standing outside on a 100°F day. This analogy illustrates why:

A) interstellar dust is dark at optical wavelengths, but bright in the infrared
B) supernovae can heat their shells to such high temperatures
C) an astronaut would feel cold standing in the 10⁶ K intercloud gas
D) the Solar System is immersed in a hot bubble of gas
E) fusion only occurs in the cores of stars

Question 22

What is the most likely explanation for the dark area in the image below?

A) It is a region where there are no stars.
B) It is a region with lots of dark matter.
C) It is a super-massive black hole.
D) It is a region with thick dust blocking the starlight coming from behind.
E) It is a dark star cluster.

Question 23

Interstellar clouds are:

A) hydrogen gas, condensed out of the interstellar medium, like water clouds in the Earth's atmosphere
B) regions where hydrogen tends to be denser than the surrounding gas
C) regions where water condenses out of the interstellar medium
D) oxygen gas, condensed out of the interstellar medium, like water clouds in the Earth's atmosphere
E) regions where hydrogen combines with oxygen to create water molecules

Question 24

"Weather" in the interstellar medium is produced:

A) only by supernovae
B) by supernovae and strong winds from luminous stars
C) by supernovae, strong winds from luminous stars, and fast-moving stars
D) by supernovae, strong winds from luminous stars, fast-moving stars, and exploding planets
E) by supernovae, strong winds from luminous stars, fast-moving stars, exploding planets, and black holes

Question 25

The red emission in the photo below is due to:

A) carbon monoxide (CO)
B) warm, neutral hydrogen
C) molecular hydrogen (H₂)
D) ionized hydrogen (H II region)
E) dust

Question 26

Dust reddens starlight because:

A) it re-emits the light it absorbs at red wavelengths
B) it emits mostly in the infrared due to its cold temperature
C) it is made mostly of hydrogen, which produces the red H-alpha emission line
D) it preferentially affects light at visible and shorter wavelengths
E) it primarily moves away from Earth

Question 27

An H II region signals the presence of:

A) newly formed stars
B) young stars
C) ionized hydrogen gas
D) O- and B-type stars
E) all of the above

Question 28

The figures below show the spectrum of a star, along with five other spectra labeled A through E. Which one of the labeled spectra shows what the spectrum of that star would look like if it were viewed through a significant amount of interstellar dust?

A) A
B) B
C) C
D) D
E) E

Question 29

Dust that is heated to 30 K will emit a blackbody spectrum that peaks at:

A) 1 $\mu$ m
B) 30 $\mu$ m
C) 50 $\mu$ m
D) 100 $\mu$ m
E) 500 $\mu$ m

Question 30

Warm ionized gas in the interstellar medium appears _________ when imaged in the optical region of the electromagnetic spectrum.

A) red
B) yellow
C) white
D) blue
E) dark

Question 31

If you wanted to study regions where star formation is currently happening you could use:

A) H-alpha emission to look for O and B stars
B) 21 cm radiation to find neutral hydrogen clouds
C) radio emission from carbon monoxide (CO) to find molecular cloud cores
D) infrared emission to identify T Tauri stars
E) all of the above

Question 32

Which of the following is responsible for heating the bulk of the very hot intercloud gas?

A) high-energy radiation from stars
B) supernovae
C) young O and B stars
D) planetary nebulae
E) The heating is an even mix of all of the sources above.

Question 33

Below are three pictures of the disk of the Milky Way, taken in three different wavelength ranges. Put the three pictures in order from shortest to longest wavelength.

A) I, II, III
B) II, III, I
C) I, III, II
D) II, I, III
E) III, I, II

Question 34

For an object in hydrostatic equilibrium, if the temperature inside the object were to increase, the object would:

A) expand
B) contract
C) implode
D) remain the same size
E) explode

Question 35

In the interstellar medium, molecules survive only in regions with:

A) low temperatures
B) high densities
C) lots of dust
D) all of the above

Question 36

A typical molecular cloud has a temperature of approximately:

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

Question 37

Molecular cloud cores are places where you might find:

A) protostars
B) Herbig-Haro objects
C) molecular hydrogen (H₂)
D) carbon monoxide (CO)
E) all of the above

Question 38

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

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

Question 39

We detect neutral gas in the interstellar medium by looking for radiation at 21 cm that arises when:

A) an electron moves from the n = 1 to n = 2 state in a hydrogen atom
B) an electron is ionized from a hydrogen atom
C) carbon monoxide (CO) gas is excited by stellar radiation
D) the spin of an electron flips and aligns with the spin of a proton in a hydrogen atom
E) an electron combines with a proton to make a hydrogen atom

Question 40

Molecular clouds, which have temperatures of around 10 K, are best observed at _________ wavelengths.

A) X-ray
B) ultraviolet
C) optical
D) infrared
E) radio

Question 41

Of the following processes at work in molecular clouds, which is the one that inevitably dominates the clouds' evolution?

A) magnetic fields
B) conservation of angular momentum
C) pressure
D) gravity
E) turbulence

Question 42

As a protostar evolves, its temperature:

A) decreases because it is radiating
B) decreases because of gravitational contraction
C) decreases because of angular momentum
D) increases because of nuclear fusion
E) increases due to the kinetic energy of infalling material

Question 43

The entire process of star formation is really just an evolving balance between:

A) heat and rotation
B) core temperature and surface temperature
C) pressure and gravity
D) radiation and heat
E) luminosity and distance

Question 44

The Hayashi track of a low-mass protostar in the H-R diagram is a path of approximately constant:

A) density
B) luminosity
C) age
D) temperature
E) radius

Question 45

What happens as a protostar contracts?

A) Its density rises.
B) Its temperature rises.
C) Its radius decreases.
D) Its pressure rises.
E) All of the above are true.

Question 46

A young protostar is _________ than the Sun even though its surface temperature is _________.

A) less luminous; hotter
B) larger; cooler
C) smaller; the same
D) more luminous; cooler
E) smaller; hotter

Question 47

Because angular momentum must be conserved, as a gas cloud contracts due to gravity it will also:

A) spin slower
B) spin faster
C) increase in temperature
D) decrease in temperature
E) stay the same temperature

Question 48

A protostar's evolutionary "track" in the H-R diagram traces out:

A) only how the protostar's radius changes with time
B) how the protostar's luminosity, temperature, and radius change with time
C) only how the protostar's luminosity changes with time
D) only how the protostar's spectral type changes with time
E) the protostar's location in the molecular cloud

Question 49

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

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 50

Brown dwarfs are considered failed stars because:

A) they never reach masses larger than 50 Jupiter masses
B) hydrogen fusion never begins in their cores
C) convection never plays a role in their energy transport
D) they primarily shine at infrared wavelengths
E) they are never as luminous as the Sun

Question 51

Magnetic fields inside a molecular cloud act to:

A) inhibit gravitational collapse
B) fragment the cloud into numerous cores
C) modulate the temperature of the molecules
D) increase the formation of dust grains
E) increase the formation of protostars

Question 52

What primarily makes it difficult to observe protostars?

A) They occur in dusty regions.
B) They have low luminosities.
C) They do not shine at any wavelength until they become T Tauri stars.
D) The star formation process happens so quickly.
E) They are too small to be seen.

Question 53

A _________ is a failed star that shines primarily because of energy derived from its gravitational collapse rather than nuclear burning.

A) black hole
B) brown dwarf
C) Herbig-Haro object
D) protostar
E) T Tauri star

Question 54

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.

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

Question 55

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

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

Question 56

Figure 1
Use Figure 1 to complete the following statement. A high-mass protostar remains roughly constant in _________ and increases in _________ as it follows its evolutionary track.

A) temperature; luminosity
B) radius; temperature
C) luminosity; radius
D) luminosity; temperature
E) radius; luminosity

Question 57

Which of the following traits does NOT help slow or prevent the collapse of a gas cloud?

A) high mass
B) high temperature
C) turbulence
D) magnetic fields
E) angular momentum

Question 58

The H^- ion is very important in protostars because it:

A) reacts with oxygen to produce water
B) undergoes fusion and produces energy
C) helps make the protostars denser
D) acts as a temperature regulator
E) reduces angular momentum

Question 59

An accretion disk forms around a collapsing protostar because infalling material must conserve:

A) energy
B) centrifugal force
C) gravity
D) velocity
E) angular momentum

Question 60

The source of energy for a contracting protostar comes from:

A) thermonuclear energy
B) kinetic energy
C) chemical energy
D) gravitational potential energy
E) radiation energy

Question 61

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

Question 62

Figure 1
Use Figure 1 to complete the following statement. As a protostar contracts:

A) the luminosity decreases
B) the luminosity increases
C) the temperature increases
D) the temperature decreases
E) either the luminosity decreases or the temperature increases

Question 63

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

Question 64

How are each of the following types of ISM detected by astronomers: hot intercloud gas, H II regions, neutral hydrogen gas, and molecular clouds.

Question 65

Why can we see dust in the interstellar medium better at far-infrared wavelengths than we can at optical wavelengths?

Question 66

How are H II regions and the hot intercloud gas heated?

Question 67

Figure 1
Use Figure 1 to complete the following statement. A low-mass protostar remains roughly constant in _________ and decreases in _________ as it follows its evolutionary track.

A) temperature; luminosity
B) radius; temperature
C) luminosity; radius
D) luminosity; temperature
E) radius; luminosity

Question 68

The most common types of stars in our galaxy are:

A) high-mass stars
B) low-mass stars
C) an equal mix of high- and low-mass stars
D) low-mass stars near the Sun and high-mass stars far away
E) We do not yet know which types of stars are most common in our galaxy.

Question 69

Why do H II regions mark the regions where new stars are currently being formed?

Question 70

What is the interstellar medium made of? Give rough percentages of each.

Question 71

Some molecular clouds have so much internal pressure that it exceeds their self-gravity. What keeps them from expanding and dissipating?

Question 72

If a 1 M _$\odot$ protostar starts out on the Hayashi track with a temperature of 3300 K and a luminosity of 320 L _$\odot$ , what is its approximate radius?

A) 25 R _$\odot$
B) 55 R _$\odot$
C) 75 R _$\odot$
D) 105 R _$\odot$
E) 125 R _$\odot$

Question 73

Compare the volume of the Sun with the volume of interstellar space it occupies. Is the occupied percentage large or small? Consider the volume around the Sun to be a sphere whose radius is equal to the distance to the nearest star, which is equal to 5 light-years. (Note: the radius of the Sun is 7 * 10⁵ km, and 1 light-year = 9.5 * 10¹² km.)

Question 74

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:

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

Question 75

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.

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 76

Which of the following stars spend the longest time on their Hayashi tracks?

A) 100 M _$\odot$ stars
B) 10 M _$\odot$ stars
C) 1 M _$\odot$ stars
D) 0.1 M _$\odot$ stars
E) 0.08 M _$\odot$ stars

Question 77

How long does it typically take for a protostar to form a 1 M _$\odot$ star?

A) 3 * 10⁷ years
B) 3 *10⁵ years
C) 3,000 years
D) 300 years
E) 30 years

Question 78

At what wavelength are H II regions most clearly visible, and why do H II regions mark the regions where new stars are currently being formed?

Question 79

Where have astronomers observed the existence of planets?

A) in our Solar System
B) orbiting stars other than the Sun
C) orbiting stars in binary systems
D) traveling on their own through the Milky Way, not orbiting a star
E) all of the above

Question 80

In the densest molecular clouds, the average density is approximately 300 atoms/cm³. If a cube of molecular cloud gas with this density contained 100 M _$\odot$ of material (the amount needed to make a 1 M _$\odot$ star), what would be the length of a side of the cube in units of AU? For reference, the mass of the Sun is 2 * 10³⁰ kg, the mass of a hydrogen atom is 1.7 *10 ^{- 27} kg, and 1 AU = 1.5 * 10¹¹ m.