Deck 18: The Interstellar Medium: Gas and Dust Among the Stars

Question 1

Molecular clouds are much warmer than typical interstellar gas clouds.

Answer 1

False

Question 2

The temperature inside an H II region is colder than inside an H I region.

Answer 2

False

Question 3

Dark dust clouds radiate mainly in the radio portion of the electromagnetic spectrum.

Answer 3

True

Question 4

There is as much mass in the voids between the stars as in the stars themselves.

Question 5

Reflection nebulae usually appear red.

Question 6

Dark nebulae are opaque to all wavelengths of electromagnetic radiation.

Question 7

We often find dark nebulae associated with bright H II regions.

Question 8

The dark dust clouds can be explored better with infrared than with visible light.

Question 9

Interstellar matter is distributed very evenly through the galaxy.

Question 10

The 21 cm line of hydrogen is strongly absorbed by interstellar dust.

Question 11

H II regions are usually found isolated from other stars or types of nebulae.

Question 12

Hydrogen is the major gas in the interstellar medium.

Question 13

Most molecules found in the giant molecular clouds are based on silicon.

Question 14

The "Local Bubble" was probably created by a nearby supernova in the distant past.

Question 15

Emission nebulae appear red because they are associated with cool red giant stars.

Question 16

The H I regions are mapped by the radiation created when an electron flips its spin.

Question 17

The gas in the interstellar medium consists of mostly heavy elements.

Question 18

The compositions of dark clouds can be determined through absorption spectra.

Question 19

Emission nebulae are blue.

Question 20

A dark cloud can be detected if it happens to block the light emitted by more distant stars.

Question 21

Interstellar gas is composed of:

A)only hydrogen.
B)90% hydrogen, 9% helium by weight.
C)10% hydrogen, 90% helium by numbers of atoms.
D)some hydrogen, but mainly carbon dioxide.
E)ammonia, methane, and water vapor.

Question 22

Due to absorption of shorter wavelengths by interstellar dust clouds, distant stars appear:

A)bluer.
B)brighter.
C)redder.
D)larger.
E)to have a higher radial velocity.

Question 23

Molecular hydrogen can best be detected by ultraviolet observations.

Question 24

The overall dimming of starlight by interstellar matter is called:

A)extinction.
B)reddening.
C)absorption.
D)emission.
E)scattering.

Question 25

The relative density of dust to gas is least in which place?

A)the interstellar medium
B)dark nebulae
C)the atmosphere of Earth
D)molecular clouds
E)reflection nebulae

Question 26

Which statement is true about the interstellar medium?

A)Gas contains a lot of carbon atoms.
B)Dust blocks the longest electromagnetic wavelengths.
C)Gas obscures the light from distant stars.
D)We know more about the gas than the dust.
E)Dust is spread uniformly through the galaxy.

Question 27

Emission nebulae like M42 occur only near stars that emit large amounts of:

A)visible light.
B)microwaves.
C)ultraviolet radiation.
D)infrared heat.
E)X-rays and gamma rays.

Question 28

Most interstellar clouds are:

A)much bigger than our solar system.
B)about the size of Earth.
C)similar in size to clouds in Earth's atmosphere.
D)much smaller than the clouds in Earth's atmosphere.
E)about the size of a wavelength of light.

Question 29

The spectra of interstellar gas clouds show that they have the same basic composition as:

A)interstellar dust.
B)Earth's atmosphere.
C)stars.
D)asteroids.
E)the Martian polar caps.

Question 30

What is the primary visible color of an emission nebula?

A)black, because of dust
B)blue from the scattering of light off the tiny molecules
C)blue due to ionized helium atoms
D)red due to ionized hydrogen atoms
E)red because of reddening by dust

Question 31

Some regions along the plane of the Milky Way appear dark because:

A)there are no stars in these areas.
B)stars in that region are hidden by interstellar gas.
C)stars in that region are hidden by dark dust particles.
D)many brown dwarfs in those areas absorb light which they turn into heat.
E)many black holes absorb all light from those directions.

Question 32

Which is the least dense?

A)interstellar gas
B)interstellar dust
C)the atmosphere of Earth
D)the atmosphere of Jupiter
E)the atmosphere of the Sun

Question 33

The ________ of light passing through thin dust clouds lets us map the Galaxy's magneticfield.

A)diffraction
B)granulation
C)ionization
D)polarization
E)absorption

Question 34

The density of interstellar dust is very low, yet it still blocks starlight because:

A)it is so cold it absorbs higher energy photons.
B)there is 100 times more opaque gas than dust present in the ISM.
C)the dust particles are about the same size as the light waves they absorb.
D)the dust particles are irregular in shape.
E)ice particles reflect all light back toward their stars, not toward us.

Question 35

Which of the following describes the shape of dust particles, based on polarization of light?

A)diamonds
B)cubes
C)spheres
D)disks
E)rodlike

Question 36

In the Milky Way Galaxy, gas and dust are found:

A)in molecular clouds.
B)everywhere.
C)in neutral hydrogen clouds.
D)in dark nebulae.
E)in emission nebulae.

Question 37

What effect do even thin clouds of dust have on light passing through them?

A)The light that passes through them is blueshifted due to the cloud's approach.
B)It dims and reddens the light of all more distant stars.
C)Even a little can completely block all light, such as the Horsehead Nebula.
D)Its motion causes the light of stars beyond to twinkle.
E)Its motion causes all light to be redshifted as it passes through these clouds.

Question 38

What feature of interstellar dust is inferred by the polarization of starlight?

A)its shape
B)its composition
C)its location in space
D)its presence in molecular clouds
E)its presence in dark nebulae

Question 39

The polarization of light passing though the dust grains shows that:

A)the dust grains are elongated in shape.
B)the dust grains are aligned by a weak interstellar magnetic field.
C)the dust grains are chiefly made of iron.
D)Both A and B are correct.
E)All of the above are correct.

Question 40

Molecular clouds can be detected by radio observations of molecular hydrogen.

Question 41

Interstellar dust clouds are best observed at what wavelength?

A)radio and infrared
B)visible only
C)UV and infrared
D)radio and X-ray
E)visible and UV

Question 42

Which of these is NOT a consequence of dust in the interstellar medium?

A)reddening of stars' light that passes through the dust
B)blue reflection nebulae around the Pleiades
C)red light from the emission nebulae
D)the dark nebulae that block 90% of the Milky Way from us visually
E)terrestrial planets like our own

Question 43

Unlike the ultraviolet photons originally emitted by the embedded stars of an emissionnebula, the photons emitted by the recombination of electrons with atoms:

A)do not escape the nebula.
B)escape the nebula.
C)are re-absorbed by neutral hydrogen atoms.
D)bounce around inside the nebula.
E)are scattered by the dust particles.

Question 44

Spectra of stars often show narrow absorption lines from interstellar matter. What type ofinterstellar matter produces these?

A)dust
B)neutral gas
C)ionized gas
D)molecules
E)all of the above

Question 45

In which wavelengths have the births of new stars been best mapped recently?

A)gamma rays
B)extreme ultraviolet
C)infrared
D)microwave
E)radio

Question 46

What two things are needed to create an emission nebula?

A)interstellar gas and dust
B)hydrogen fusion and helium ionization
C)cool stars and much interstellar dust
D)hot stars and interstellar gas, particularly hydrogen
E)hydrogen gas and carbon dust

Question 47

Why are dark dust clouds largely misnamed?

A)The cloud is an illusion, for the dust is evenly distributed around the Galaxy.
B)Dust clouds do radiate energy, but not as much light as the stars do.
C)It is ice, not dust, which make them look dark.
D)They contain much more gas than dust.
E)All of the above are correct.

Question 48

The average temperature of the typical dark dust cloud is about:

A)0 K.
B)2.73 K.
C)100 K.
D)3,000 K.
E)6,000 K.

Question 49

The "Local Bubble" was probably created by:

A)the strong solar winds created by the newborn Sun.
B)the Crab Nebula supernova of 1054 AD.
C)the magnetic fields of the Milky Way in our spiral arm.
D)a nearby supernova perhaps 300,000 years ago, brighter than the full Moon.
E)the interaction between the solar wind and the Oort Cloud.

Question 50

Which statement about the dark nebulae is true?

A)Hydrogen and helium are the chief absorbing and scattering agents.
B)They block the vast majority of radio waves from our Galaxy.
C)They can be penetrated only with longer wavelengths such as radio and infrared.
D)They comprise the majority of the mass of the Galaxy.
E)They can be penetrated only with shorter waves, such as UV and X-ray.

Question 51

Emission nebula take up about how much of the Milky Way Galaxy?

A)1%
B)5%
C)10%
D)99%
E)100%

Question 52

A large gas cloud in the interstellar medium that contains several type O and B stars wouldappear to us as:

A)a dark nebula.
B)an emission nebula.
C)bright blue.
D)a reflection nebula.
E)a dark patch against a bright background.

Question 53

Ionization of hydrogen in H II regions is most visible at

A)12,367 nm, the Paschen alpha line.
B)656.3 nm, Balmer alpha line for 3-2 electron transition.
C)21 cm in the radio region for the spin-flip of its electron.
D)121.3 nm, the Lyman alpha ultraviolet line for 2-1 electron transition.
E)473.2 nm, the blue-green emission line for 4-2 electron transition.

Question 54

What do "forbidden" lines reveal about interstellar space?

A)The density or pressure of this gas is much lower than can be produced in a laboratory.
B)The laws of physics are not the same in space as they are on Earth.
C)There are unknown elements in space that do not correspond to elements found on Earth.
D)There are molecules found in meteorites and interstellar dust that are not found on Earth.
E)All of the above are true.

Question 55

Astronomers use roman numerals to indicate the ionization state of a gas. Ionized hydrogenis indicated by which of the following symbols?

A)H II
B) $\mathrm { H } _ { 2 }$
C)H I
D) $\mathrm { H } _ { 1 } ^ { 1 }$
E)H III

Question 56

A "fuzzy" dark or light patch in the sky is called a:

A)nebula.
B)cloud.
C)galaxy.
D)comet.
E)dust lane.

Question 57

The gas density in an emission nebula is typically about how many particles per cc?

A)dozen
B)hundred
C)thousand
D)hundred thousand
E)million

Question 58

A reflection nebula is caused by:

A)starlight reflected off of a surface.
B)sunlight reflected off of a surface.
C)sunlight scattered by dust particles.
D)starlight scattered by dust particles.
E)emission by ionized gas.

Question 59

Charles Messier mapped the night sky and identified many objects now known to beemission nebulae in his search for objects that might be confused with:

A)comets.
B)asteroids.
C)galaxies.
D)dark nebulae.
E)reflection nebulae.

Question 60

Interstellar absorption lines are narrow primarily because:

A)there is very little interstellar matter.
B)molecules always produce narrow absorption lines.
C)most of the matter is ionized, and atoms moving fast produce sharp lines.
D)the matter is at a low temperature, and atoms are almost still.
E)All of the above are correct.

Question 61

Dust particles and magnetic fields in the interstellar medium are revealed by ________ oflight.

Question 62

When an electron in H changes its spin from the same to the opposite direction as the proton,it:

A)emits an X-ray photon.
B)absorbs a radio wave photon.
C)emits a radio wave photon.
D)absorbs a visible light photon.
E)neither emits nor absorbs a photon.

Question 63

What information does 21 cm radiation provide about the gas clouds?

A)their motion
B)their distribution
C)their density
D)their temperature
E)all of these

Question 64

Why is 21-cm radiation so important to the study of interstellar matter and the Galaxy?

A)It helps locate the galactic core in Sagittarius.
B)Emitted by hydrogen, it passes through interstellar dust and lets us map the entire Galaxy.
C)Emitted by carbon monoxide, it passes through interstellar gas and lets us see places rich inorganic molecules around the Galaxy.
D)It is emitted by most stars, enabling astronomers to map the entire Galaxy.
E)It is emitted only in hot regions of star formation, so the pattern of the spiral arms of theGalaxy can be mapped.

Question 65

Molecular cloud complexes are:

A)molecular clouds that have a high percentage of more complex molecules.
B)many molecular clouds close to each other, spanning regions as large as 50 parsecs.
C)molecular clouds that show a more complex, web-like structure.
D)molecular clouds that radiate more ultraviolet than radio energy.
E)molecular clouds at much higher density, as high as 1022 molecules/m3.

Question 66

The most common molecule in a molecular cloud is:

A)methane, $\mathrm { CH } _ { 4 } .$
B)molecular hydrogen, $\mathrm { H } _ { 2 }$
C)carbon monoxide, CO.
D)water, $\mathrm { H } _ { 2 } \mathrm { O } .$
E)ammonia, $\mathrm { NH } _ { 3 } .$

Question 67

What are the very cold (about 20 K), dense clouds of gas thought to be the most massiveobjects in the Galaxy called?

A)emission nebulae
B)absorption clouds
C)dust clouds
D)molecular clouds
E)reflection nebulae

Question 68

Emission nebulae occur only near stars that emit large amounts of ________ electromagneticradiation.

Question 69

Neutral hydrogen is most obvious in the electromagnetic spectrum:

A)in yellow light of the Sun's continuum.
B)at 656.3 nm, hydrogen alpha red.
C)at 21 cm in the radio region.
D)at 121.3 nm, the Lyman alpha ultraviolet line.
E)Neutral hydrogen has yet to be detected in space.

Question 70

Complex molecules in the interstellar medium are found:

A)uniformly throughout the disk of the Galaxy.
B)scattered evenly throughout the universe, a product of the Big Bang itself.
C)only around the supergiant stars like Betelgeuse that make their heavy atoms.
D)primarily in the dense dust clouds.
E)on the surfaces of the coolest class K and M stars only.

Question 71

Unlike atoms, molecules can:

A)rotate.
B)vibrate.
C)spin.
D)both A and B
E)all of the above

Question 72

Complex molecules in space are found:

A)in the photospheres of red giant stars.
B)inside dense dust clouds.
C)in the coronas of stars like our Sun.
D)scattered evenly throughout interstellar space.
E)surrounding the more energetic young stars.

Question 73

The overall dimming of starlight passing through dust clouds is called ________.

Question 74

What color is an H I region?

A)red from ionized hydrogen's alpha transition, such as in the Orion Nebula
B)red from reddening due to dust
C)colorless; it emits in the radio region
D)blue from the scattering of light off the tiny atoms, such as around the Pleiades
E)blue from ionized helium's alpha transition, such as the Horsehead Nebula

Question 75

If the spectra reveal stars A and B are identical, but star A looks bluer in color, then star B'slight has been ________ by interstellar dust.

Question 76

The "Local Bubble" is:

A)created by our Sun's solar wind, and extends well past Pluto's orbit.
B)created by the bipolar jets of the young Sun, perpendicular to the ecliptic.
C)about 100 pcs. across and transparent to extreme ultraviolet radiation.
D)the region around the Milky Way where we can trace out its magnetic field.
E)the cluster of galaxies our Milky Way and M-31 in Andromeda belong to.

Question 77

Interstellar reddening of starlight occurs because the interstellar dust preferentially ________the blue light.

Question 78

Which of the following has NOT yet been observed in space?

A)formaldehyde
B)methyl alcohol
C)DNA
D)ammonia
E)hydrogen cyanide

Question 79

An emission nebula appears primarily ________ in color, due to ionized hydrogen.

Question 80

Molecular hydrogen is not very useful for mapping molecular clouds because:

A)molecular hydrogen does not occur in most molecular clouds.
B)molecular hydrogen puts out radiation similar to most stars.
C)molecular hydrogen and atomic hydrogen cannot be distinguished in telescopes.
D)radiation from carbon monoxide overwhelms radiation from molecular hydrogen.
E)molecular hydrogen is essentially invisible in the radio portion of the spectrum.