Deck 15: The Interstellar Medium and Star Formation

Question 1

Interstellar extinction refers to

A) planets in interstellar space absorbing starlight.
B) light being absorbed by dust in interstellar space.
C) asteroid impacts that result in mass extinctions of life.
D) the emission of red light by interstellar dust.
E) hot interstellar gas clouds that cause mass extinctions of life.

Answer 1

light being absorbed by dust in interstellar space.

Question 2

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

A) coldest.
B) least ionized.
C) hottest.
D) most localized, being found mostly around protostars.
E) most likely to be in a molecular state.

Answer 2

hottest.

Question 3

In the interstellar medium, molecules survive in regions with

A) very high temperatures.
B) type O and B stars.
C) lots of dust.
D) frequent supernovas.
E) a disk around a protostar.

Answer 3

lots of dust.

Question 4

The interstellar medium is divided into three different kinds of gas clouds.These are

A) cold gas at 100 K, warm gas at 8000 K, and hot gas at about 1 million K.
B) warm gas at 8000 K, hot gas at 1 million K, and superhot gas at 10 million K.
C) warm gas at 5000 K, warm-hot gas at 100,000 K, and hot gas at about 1 million K.
D) cold gas at 100 K, cool gas at 1000 K, and warm gas at 8000 K.
E) cold gas at 100 K, hot gas at 1 million K, and superhot gas at 10 million K.

Question 5

The figure below shows the spectrum of a star as it was emitted the star (right), and the spectrum after the light from this star has passed through a cloud of interstellar dust.After the star's light has passed through the dust, the star appears

A) twice as bright as before, but redder.
B) the same brightness, but redder than before.
C) half as bright, but bluer.
D) more than 10 times dimmer, and much redder.
E) non-existent; its light is completely blocked and only the emission from the dust is seen.

Question 6

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
E) the interiors of forming protostars.

Question 7

Molecular hydrogen atoms can be found

A) nowhere in the universe.
B) in dense clouds where they are shielded from stellar radiation.
C) only in the disk surrounding forming protostars.
D) in low-density clouds of hot gas surrounding hot stars.
E) only in the atmospheres of the giant planets, such as Jupiter.

Question 8

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 9

What is the most likely explanation for the dark area in the figure shown 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 supermassive black hole.
D) It is a region with thick dust blocking the starlight coming from behind it.
E) It is a dark star cluster.

Question 10

When looking at the space between stars, what are you most likely to see?

A) nothing; it is empty
B) spacecraft
C) gas
D) dark matter
E) planets

Question 11

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 12

Sitting in a hot tub with water at 100°F feels much hotter than standing outside in 100°F air on a hot 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 not burn up while floating in 10⁶ K intercloud gas.
D) the Solar System is immersed in a hot bubble of gas.
E) fusion occurs only in the cores of stars.

Question 13

Dust reddens starlight because it

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

Question 14

The average density of the interstellar medium is

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

Question 15

Which of the following is responsible for heating the bulk of the bubbles of 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 these sources.

Question 16

Interstellar clouds are

A) hydrogen gas condensed out of the interstellar medium, like water clouds in 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 Earth's atmosphere.
E) regions where hydrogen combines with oxygen to create water molecules.

Question 17

When radiation from an object passes through the interstellar medium,

A) the object appears refracted.
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 18

The average density of the interstellar medium is

A) much denser than Earth's atmosphere.
B) much less dense than the best vacuum on Earth.
C) about the same density as air on the peak of Mount Everest.
D) zero.
E) about as dense as Earth's atmosphere at sea level.

Question 19

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 20

The coldest molecular clouds in our galaxy have temperatures of approximately

A) 10,000 K.
B) 1000 K.
C) 100 K.
D) 10 K.
E) 0 K.

Question 21

Dust that glows with a peak emission wavelength of 40 $\mu$ m has a temperature of

A) 10 K.
B) 72 K.
C) 100 K.
D) 120 K.
E) 1000 K.

Question 22

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 23

One phenomenon that can slow star formation in a molecular cloud is

A) a lack of rotation.
B) the strength of its magnetic field.
C) an excess of H II atoms.
D) a large size.
E) nearby type O and B stars.

Question 24

The red emission (bright region at the top) in the figure shown 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 25

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 26

Electronic transitions from the H₂ molecule are easily seen at

A) X-ray wavelengths.
B) visible wavelengths.
C) radio wavelengths.
D) infrared wavelengths.
E) ultraviolet wavelengths.

Question 27

What primarily makes it difficult to observe the process of star formation?

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

Question 28

The entire process of star formation is an evolving balance between ________ and gravity.

A) rotation
B) surface temperature
C) pressure
D) radiation
E) distance

Question 29

If you wanted to study regions where star formation is currently happening, you could look for

A) bright X-ray sources.
B) type M stars.
C) red giant stars.
D) type G main-sequence stars.
E) O and B stars.

Question 30

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) an electron's magnetic poles flip and align with the magnetic poles of a proton in a hydrogen atom.
E) an electron combines with a proton to make a hydrogen atom.

Question 31

Stars forming in molecular clouds tend to form first in

A) the low-density periphery.
B) the high-density core.
C) random locations.
D) any location where the temperature is highest.
E) the location with the heaviest elements.

Question 32

We observe neutral hydrogen gas using

A) X-ray radiation from highly ionized atoms.
B) visible radiation at 656.3 nm from recombined hydrogen.
C) 21-cm emission.
D) microwave emissions.
E) ultraviolet radiation from helium and oxygen.

Question 33

The figure below shows 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

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 these

Question 36

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 these

Question 37

Of the following processes at work in molecular clouds, which is the one that will be responsible for the clouds' eventual collapse?

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

Question 38

Radio waves with 21-cm wavelengths are important because they

A) allow us to study the deep interiors of stars.
B) allow us to image magnetic fields directly.
C) allow us to study neutral hydrogen in the interstellar medium.
D) is produced by every object in the universe.
E) is the longest wavelength of light that can be naturally produced.

Question 39

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

A) 1 µm.
B) 30 µm.
C) 50 µm.
D) 100 µm.
E) 500 µm.

Question 40

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 41

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 these are true.

Question 42

Use the figure shown below 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 43

A protostar is

A) in hydrostatic equilibrium as it collapses.
B) not in hydrostatic equilibrium as it collapses.
C) heated to millions of degrees as it collapses.
D) flattened into a disk as it collapses.
E) powered by chemical reactions.

Question 44

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 45

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 shine primarily at infrared wavelengths.
E) they are never as luminous as the Sun.

Question 46

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 47

As a protostar is evolving, 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 48

Use the figure shown below to complete the following statement: At the start of the evolution of a protostar, the radius of a 60 M_SUN protostar is roughly ________ that of a 1 M_SUN main-sequence star.

A) 10 times bigger than
B) 1000 times bigger than
C) 10 times smaller than
D) 1000 times smaller than
E) the same as

Question 49

Objects with masses from 0.01 M_SUN to 0.08 M_SUN 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 50

A surprising fact about a 1 M_SUN 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 51

A protostar may evolve into either a main-sequence star or a

A) molecular cloud.
B) gas giant planet.
C) Herbig-Haro object.
D) brown dwarf.
E) white dwarf.

Question 52

If the surface of a star cools, the number of H ^- ions ________ and the atmosphere becomes more ________.

A) decreases; opaque
B) decreases; transparent
C) increases; transparent
D) increases; opaque
E) stays the same; transparent

Question 53

The H ^- atom is important in protostars because it acts as a

A) source of friction, stopping the cloud from collapsing too rapidly.
B) source of infrared radiation, causing the cloud to cool off rapidly.
C) temperature regulator.
D) source of buoyancy, allowing the atmosphere of the protostar to expand.
E) source of easily detectable radio-wave emission.

Question 54

Use the figure shown below 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 55

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

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

Question 56

The thermal energy of a contracting protostar comes from

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

Question 57

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 58

As a protostar evolves towards becoming a main-sequence star, its core temperature will

A) always increase.
B) always decrease.
C) first increase, then decrease.
D) first decrease, then increase.
E) always be the same value.

Question 59

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

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 60

Use the figure shown below to complete the following statement: A low-mass protostar remains roughly constant in ________ and decreases in ________ until it makes a turn towards the main sequence, as it follows its evolutionary track.

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

Question 61

The images in the figure below are both of the Milky Way.Why can more detail be seen in the infrared image than in the visible light image?

Question 62

When winds blow the gas away from a forming protostar, the protostar

A) expands rapidly to 100 times its original size.
B) is revealed as a main-sequence star.
C) becomes visible as a T Tauri star.
D) is unable to reach the main sequence.
E) becomes a brown dwarf.

Question 63

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

A) 100 M_SUN stars
B) 10 M_SUN stars
C) 1 M_SUN stars
D) 0.1 M_SUN stars
E) 0.08 M_SUN stars

Question 64

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_SUN of material (the amount needed to make a 1 M_SUN 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.

Question 65

When a molecular cloud fragments,

A) the least massive stars are the first to form, while the most massive stars take longer.
B) the most massive star are the first to form, while the least massive stars take longer.
C) the most massive stars promptly explode as supernovae, removing all remaining gas.
D) the stars form at the same rate, regardless of their mass.
E) interstellar dust is generated, but not stars.

Question 66

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

Question 67

Suppose we observe two molecular clouds containing dust.The dust in Cloud 1 peaks in emission at 50 mm, while the dust in Cloud 2 peaks in emission at 78 mm.How much warmer is the dust in Cloud 1 compared to Cloud 2?

Question 68

Herbig-Haro objects are almost always found

A) in pairs on either side of a young protostar.
B) far away from molecular clouds where stars form.
C) close to planets that are forming around protostars.
D) deep inside molecular clouds.
E) using X-ray telescopes.

Question 69

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

Question 70

Compare the volume of the Sun with the volume of interstellar space it dominates.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 71

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 these

Question 72

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

Question 73

Suppose the 21-cm photon of neutral hydrogen were instead emitted at 500 nm (i.e., a visible blue photon).Would it still be a useful probe of the Milky Way's structure? Why?

Question 74

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

A) type M main-sequence stars
B) type B main-sequence stars
C) Sun-like (type G) stars
D) low-mass red giants
E) type F main-sequence stars

Question 75

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 of different types would form at random times.

Question 76

Star formation is an inefficient process, with only a few percent of the initial cloud fragment ending up as stars.This means the initial mass of a molecular cloud fragment that formed a 2 M_SUN star was probably close to

A) 1 M_SUN.
B) 50 M_SUN.
C) 100 M_SUN.
D) 5,000 M_SUN.
E) 1,000,000 M_SUN.

Question 77

Bipolar outflows create bright bow shocks because

A) the gas "piles up" when it encounters ultraviolet light.
B) the material collides with particles in the interstellar medium.
C) the star's magnetic fields trap the outflow material there.
D) the gas cools enough to allow H ^- ions to build up.
E) the outflows encounter newly forming planets.

Question 78

Why do molecules readily exist in Earth's atmosphere but not in most of interstellar space?

Question 79

What is the difference between interstellar extinction and interstellar reddening?

Question 80

How long does it typically take for a protostar to form a 1 M_SUN star?

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