Astronomy Chapter 18

Dark nebulae are opaque to all wavelengths of electromagnetic radiation. True False

False

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

False

Emission nebulae are blue. True False

False

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

False

Interstellar matter is distributed very evenly through the galaxy. True False

False

Molecular clouds are much warmer than typical interstellar gas clouds. True False

False

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

False

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

False

Reflection nebulae usually appear red. True False

False

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

False

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

False

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

False

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

True

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

True

Hydrogen is the major gas in the interstellar medium. True False

True

Molecular hydrogen can best be detected by ultraviolet observations. True False

True

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

True

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

True

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

True

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

True

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

True

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

True

Which of the following has NOT yet been observed in space? a) DNA b) hydrogen cyanide c) methyl alcohol d) ammonia e) formaldehyde

a) DNA

Why is 21-cm radiation so important to the study of interstellar matter and the Galaxy? a) Emitted by hydrogen, it passes through interstellar dust and lets us to map the entire Galaxy. b) It is emitted only in hot regions of star formation, so the pattern of the spiral arms of the Galaxy can be mapped. c) It helps locate the galactic core in Sagittarius. d) Emitted by carbon monoxide, it passes through interstellar gas and lets us to see places rich in organic molecules around the Galaxy. e) It is emitted by most stars, enabling astronomers to map the entire Galaxy.

a) Emitted by hydrogen, it passes through interstellar dust and lets us to map the entire Galaxy.

Why are reflection nebulae blue and emission nebulae red? a) Reflection nebulae scatter blue light, and emission nebulae emit red light. b) Reflection nebulae emit blue light, and emission nebulae scatter red light. c) Reflection nebulae scatter blue light, and emission nebulae scatter red light. d) Reflection nebulae emit blue light, and emission nebulae emit red light. e) Reflection nebulae are hot, and emission nebulae are cool.

a) Reflection nebulae scatter blue light, and emission nebulae emit red light.

What is 21-cm radiation? With what element is it associated? a) The 21-cm radio radiation is emitted by clouds of "cold" hydrogen gas, with no stars nearby. b) The 21-cm radio radiation is emitted by clouds of "hot" hydrogen gas surrounding a newly formed star or group of stars. c) The 21-cm radio radiation is emitted by clouds of "hot" hydrogen gas, with no stars nearby. d) The 21-cm radio radiation is emitted by clouds of "cold" hydrogen gas surrounding a newly formed star or group of stars.

a) The 21-cm radio radiation is emitted by clouds of "cold" hydrogen gas, with no stars nearby.

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) There are unknown elements in space that do not correspond to elements found on Earth. c) The laws of physics are not the same in space as they are 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.

a) The density or pressure of this gas is much lower than can be produced in a laboratory.

Neutral hydrogen is most obvious in the electromagnetic spectrum: a) at 21 cm in the radio region. b) in yellow light of the Sun's continuum. c) at 656.3 nm, hydrogen alpha red. d) at 121.3 nm, the Lyman alpha ultraviolet line. e) Neutral hydrogen has yet to be detected in space.

a) at 21 cm in the radio region.

Unlike the ultraviolet photons originally emitted by the embedded stars of an emission nebula, the photons emitted by the recombination of electrons with atoms: a) escape the nebula. b) do not escape the nebula. c) bounce around inside the nebula. d) are re-absorbed by neutral hydrogen atoms. e) are scattered by the dust particles.

a) escape the nebula.

Stars interact with emission nebulae by a) exiting their atoms enough to emit light. b) causing them to contract. c) heating them so they explode. d) illuminating them like an advertising billboard.

a) exiting their atoms enough to emit light.

The overall dimming of starlight by interstellar matter is called: a) extinction. b) absorption. c) emission. d) reddening. e) scattering.

a) extinction.

In which wavelengths have the births of new stars been best mapped recently? a) infrared b) radio c) gamma rays d) microwave e) extreme ultraviolet

a) infrared

What feature of interstellar dust is inferred by the polarization of starlight? a) its shape b) its presence in molecular clouds c) its presence in dark nebulae d) its composition e) its location in space

a) its shape

The largest interstellar clouds are a) molecular clouds. b) dark dust clouds. c) emission nebulae. d) globules.

a) molecular clouds.

What is the primary visible color of an emission nebula? a) red due to ionized hydrogen atoms b) red because of reddening by dust c) blue due to ionized helium atoms d) black, because of dust e) blue from the scattering of light off the tiny molecules

a) red due to ionized hydrogen atoms

Due to absorption of shorter wavelengths by interstellar dust clouds, distant stars appear: a) redder. b) bluer. c) larger. d) brighter. e) to have a higher radial velocity.

a) redder.

The Rho Ophiuchi cloud, shown in Figure18.14 in the textbook ("Dark Dust Cloud"), is dark because a) starlight from behind the cloud does not penetrate the cloud. b) the stars in this region are young and faint. c) the region is too cold to sustain stellar fusion. d) there are no stars in this region.

a) starlight from behind the cloud does not penetrate the cloud.

Interstellar gas is composed of: a) ammonia, methane, and water vapor. b) 90% hydrogen, 9% helium by weight. c) 10% hydrogen, 90% helium by numbers of atoms. only hydrogen. d)some hydrogen, but mainly carbon dioxide.

b) 90% hydrogen, 9% helium by weight.

Astronomers use roman numerals to indicate the ionization state of a gas. Ionized hydrogen is indicated by which of the following symbols? a) H11 b) H II c) H I d) H III e) H2

b) H II

Why are molecules like CO, HCN, NH3, H2O, and H2CO important for studying the interstellar medium? a) They are important components of dust particles. b) They allow us to locate and trace out the structure of cool, dark clouds. c) They are the major constituents of cold dark clouds. d) They mark the locations where stars have formed. e) They are major sources of the 21-cm radiation.

b) They allow us to locate and trace out the structure

Why are dark dust clouds largely misnamed? a) The cloud is an illusion, for the dust is evenly distributed around the Galaxy. b) They contain much more gas than dust. c) Dust clouds do radiate energy, but not as much light as the stars do. d) It is ice, not dust, which make them look dark. e) All of the above are correct.

b) They contain much more gas than dust.

What type of nebula contains stars of spectral types O and B? a) an absorption nebula b) an emission nebula c) a dark nebula d) a molecular cloud e) a reflection nebula

b) an emission nebula

If a proton and an electron within a hydrogen atom initially have parallel spins, then change to have antiparallel spins, the atom must a) absorb energy. b) emit energy. c) become hotter. d) become larger.

b) emit energy.

Of the following objects, the one that shines most like an emission nebula shines is a) star like the Sun. b) glowing fluorescent light tube. c) red hot ember from a campfire. d) regular incandescent light-bulb with a filament.

b) glowing fluorescent light tube.

The gas density in an emission nebula is typically about how many particles per cc? a) dozen b) hundred c) million d) hundred thousand e) thousand

b) hundred

What makes up 90 percent of interstellar gas? a) carbon monoxide b) hydrogen c) carbon dioxide d) helium

b) hydrogen

Most interstellar clouds are: a) about the size of a wavelength of light. b) much bigger than our solar system. c) much smaller than the clouds in Earth's atmosphere. d) about the size of Earth. e) similar in size to clouds in Earth's atmosphere.

b) much bigger than our solar system.

Spectra of stars often show narrow absorption lines from interstellar matter. What type of interstellar matter produces these? a) dust b) neutral gas c) ionized gas d) molecules e) All of the above.

b) neutral gas

The relative density of dust to gas is least in which place? a) dark nebulae b) the atmosphere of Earth c) molecular clouds d) reflection nebulae e) the interstellar medium

b) the atmosphere of Earth

The density of interstellar dust is very low, yet it still blocks starlight because: a) ice particles reflect all light back toward their stars, not toward us. b) the dust particles are about the same size as the light waves they absorb. c) it is so cold it absorbs higher energy photons. d) the dust particles are irregular in shape. e) there is 100 times more opaque gas than dust present in the ISM.

b) the dust particles are about the same size as the light waves they absorb.

Interstellar absorption lines are narrow primarily because: a) there is very little interstellar matter. b) the matter is at a low temperature, and atoms are almost still. c) most of the matter is ionized, and atoms moving fast produce sharp lines. d) molecules always produce narrow absorption lines. e) All of the above are correct.

b) the matter is at a low temperature, and atoms are almost still.

Emission nebulae like M42 occur only near stars that emit large amounts of: a) infrared heat. b) ultraviolet radiation. c) visible light. d) microwaves. e) X-rays and gamma rays.

b) ultraviolet radiation.

Interstellar dust clouds are best observed at what wavelength? a) visible only b) Radio and X-ray c) Radio and infrared d) Visible and UV e) UV and infrared

c) Radio and infrared

Why is it useful for studying the interstellar medium? a) With its short wavelength, the 21-cm radiation cannot pass through dust clouds without being scattered. Therefore, we can detect clouds of cold hydrogen gas throughout the Galaxy, and determine distance to them. b) With its short wavelength, the 21-cm radiation cannot pass through dust clouds without being scattered. Therefore, we can detect clouds of cold hydrogen gas throughout the Galaxy, and determine their temperature and density. c) With its long wavelength, the 21-cm radiation can easily pass through dust clouds without being scattered. Therefore we can detect clouds of cold hydrogen gas throughout the Galaxy, and determine their temperature and density. d) With its long wavelength, the 21-cm radiation can easily pass through dust clouds without being scattered. Therefore, we can detect clouds of cold hydrogen gas throughout the Galaxy, and determine distance to them.

c) With its long wavelength, the 21-cm radiation can easily pass through dust clouds without being scattered. Therefore we can detect clouds of cold hydrogen gas throughout the Galaxy, and determine their temperature and density.

The telescope best suited to observing dark dust clouds is a) a large visible-light telescope. b) an orbiting ultraviolet telescope. c) a radio telescope. d) an X-ray telescope.

c) a radio telescope.

The "Local Bubble" is: a) created by the bipolar jets of the young Sun, perpendicular to the ecliptic. b) the region around the Milky Way where we can trace out its magnetic field. c) about 100 pcs. across and transparent to extreme ultraviolet radiation. d) created by our Sun's solar wind, and extends well past Pluto's orbit. e) the cluster of galaxies our Milky Way and M-31 in Andromeda belong to.

c) about 100 pcs. across and transparent to extreme ultraviolet radiation.

A large gas cloud in the interstellar medium that contains several type O and B stars would appear to us as: a) a dark nebula. b) bright blue. c) an emission nebula. d) a reflection nebula. e) a dark patch against a bright background.

c) an emission nebula.

What color is an H I region? a) red from reddening due to dust b) red from ionized hydrogen's alpha transition, such as in the Orion Nebula c) colorless; it emits in the radio region d) blue from ionized helium's alpha transition, such as the Horsehead Nebula e) blue from the scattering of light off the tiny atoms, such as around the Pleiades

c) colorless; it emits in the radio region

Which constituent of interstellar matter obscures visible light? a) comets b) atoms c) dust d) molecules e) asteroids

c) dust

What observation has enabled astronomers to determine the approximate shape of interstellar dust particles? a) how they absorb light b) how they reflect light c) how they polarize light d) how they emit light

c) how they polarize light

What are the main constituents of interstellar gas? a) ammonia, methane, and water vapor b) carbon and helium c) hydrogen and helium d) hydrogen and carbon

c) hydrogen and helium

A "fuzzy" dark or light patch in the sky is called a: a) galaxy. b) cloud. c) nebula. d) dust lane. e) comet.

c) nebula.

The ________ of light passing through thin dust clouds lets us map the Galaxy's magnetic field. a) absorption b) ionization c) polarization granulation diffraction

c) polarization

Complex molecules in the interstellar medium are found: a) only around the supergiant stars like Betelgeuse that make their heavy atoms. b) scattered evenly throughout the universe, a product of the Big Bang itself. c) primarily in the dense dust clouds. d)uniformly throughout the disk of the Galaxy. e) on the surfaces of the coolest class K and M stars only.

c) primarily in the dense dust clouds.

Which of the following best describes the shape of interstellar dust particles? a) a cube b) a sphere c) rod-like d) flat rectangles e) a disk

c) rod-like

A reflection nebula is caused by: a) starlight reflected off of a surface. b) emission by ionized gas. c) starlight scattered by dust particles. d) sunlight reflected off of a surface. e) sunlight scattered by dust particles.

c) starlight scattered by dust particles.

The density of atoms in the interstellar medium is most similar to a) wildfire smoke. b) deep ocean water. c) the interior of a TV tube. d) dark rain clouds.

c) the interior of a TV tube.

Despite its low density, dust can completely block starlight because of which of the following? a) the scarcity of stars compared to the amount of dust in the interstellar medium b) its dark composition c) the large distances involved d) its large size

c) the large distances involved

Emission nebula take up about how much of the Milky Way galaxy? a) 5% b) 99% c) 100% d) 1% e) 10%

d) 1%

The polarization of light passing though the dust grains shows that: a) the dust grains are aligned by a weak interstellar magnetic field. b) the dust grains are elongated in shape. c) the dust grains are chiefly made of iron. d) Both A and B are correct. e) All of the above are correct.

d) Both A and B are correct. a) the dust grains are aligned by a weak interstellar magnetic field. b) the dust grains are elongated in shape.

Unlike atoms, molecules can: a) rotate. b) vibrate. c) spin. d) Both A and B. e) All of the above.

d) Both A and B. vibrate and rotate

Why is 21-cm radiation important? a) It is produced by every object in the universe. b) It enables us to study the deep interiors of stars. c) It enables us to image magnetic fields directly. d) It enables us to study dark clouds in the interstellar medium.

d) It enables us to study dark clouds in the interstellar medium.

Which of the following describes the distribution of interstellar matter in the galaxy? a) It is very evenly distributed throughout the galaxy. b) Most of it is concentrated at the center of the galaxy, with little in the disk. c) Most of it is just above or below the disk of the galaxy. d) It is patchy throughout most of the galaxy.

d) It is patchy throughout most of the galaxy.

Molecular clouds are routinely studied using spectral lines from all but which of the following? a) Carbon monoxide. b) Water. c) Formaldehyde. d) Molecular hydrogen

d) Molecular hydrogen

The Local Bubble was probably created by: a) the strong solar winds created by the new-born Sun. b) the magnetic fields of the Milky Way in our spiral arm. c) the interaction between the solar wind and the Oort Cloud. d) a nearby supernova perhaps 300,000 years ago, brighter than the full Moon. e) the Crab Nebula supernova of 1054 AD.

d) a nearby supernova perhaps 300,000 years ago, brighter than the full Moon.

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 visible light photon. c) absorbs a radio wave photon. d) emits a radio wave photon. e) neither emits nor absorbs a photon.

d) emits a radio wave photon.

Complex molecules in space are found: a) scattered evenly throughout interstellar space. b) in the coronas of stars like our Sun. c) in the photospheres of red giant stars. d) inside dense dust clouds. e) surrounding the more energetic young stars.

d) inside dense dust clouds.

Which is the least dense? a) interstellar gas b) the atmosphere of Earth c) the atmosphere of Jupiter d) interstellar dust e) the atmosphere of the Sun

d) interstellar dust

Molecular cloud complexes are: a) molecular clouds at much higher density, as high as 1022 molecules/m3. b) molecular clouds that have a high percentage of more complex molecules. c) molecular clouds that radiate more ultraviolet than radio energy. d) many molecular clouds close to each other, spanning regions as large as 50 parsecs. e) molecular clouds that show a more complex, web-like structure.

d) many molecular clouds close to each other, spanning regions as large as 50 parsecs.

What are the very cold (about 20 K), dense clouds of gas thought to be the most massive objects in the Galaxy called? a) absorption clouds b) reflection nebulae c) emission nebulae d) molecular clouds e) dust clouds

d) molecular clouds

Some regions along the plane of the Milky Way appear dark because: a) stars in that region are hidden by interstellar gas. many brown dwarfs in those areas absorb light which b) they turn into heat. c) any black holes absorb all light from those directions. d) stars in that region are hidden by dark dust particles. e)there are no stars in these areas.

d) stars in that region are hidden by dark dust particles.

The spectra of interstellar gas clouds show that they have the same basic composition as: a) interstellar dust. b) Earth's atmosphere. c) the Martian polar caps. d) stars. e) asteroids.

d) stars.

A dark interstellar globule is about the same size as a) cloud in Earth's atmosphere. b) the entire planet Earth. c) star like the Sun. d) the Oort cloud.

d) the Oort cloud.

The chemical composition of the interstellar medium is basically similar to that of a) Venus. b) Mars. c) Earth. d) the Sun.

d) the Sun.

The average temperature of the typical dark dust cloud is about: a) 3,000 K. b) 6,000 K. c) 0 K. d) 2.73 K. e) 100 K.

e) 100 K.

Ionization of hydrogen in H II regions is most visible at: a) 473.2 nm, the blue-green emission line for 4-2 electron transition. b) 12,367 nm, the Paschen alpha line. 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) 656.3 nm, Balmer alpha line for 3-2 electron transition.

e) 656.3 nm, Balmer alpha line for 3-2 electron transition.

If an interstellar cloud lies between Earth and a hot star, we can detect its presence in the stellar spectrum of the star. Which of the following properties of the cloud CANNOT be determined from the stellar spectrum? a) elemental abundance b) velocity c) density d) temperature e) All of the above properties can be determined.

e) All of the above properties can be determined.

What effect do even thin clouds of dust have on light passing through them? a) Even a little can completely block all light, such as the Horsehead Nebula. b) The light that passes through them is blue shifted due to the cloud's approach. c) Its motion causes the light of stars beyond to twinkle. d) Its motion causes all light to be redshifted as it passes through these clouds. e) It dims and reddens the light of all more distant stars.

e) It dims and reddens the light of all more distant stars.

Which statement about the dark nebulae is true? a) They comprise the majority of the mass of the Galaxy. b) Hydrogen and helium are the chief absorbing and scattering agents. c) They can be penetrated only with shorter waves, such as UV and X-ray. d) They block the vast majority of radio waves from our Galaxy. e) They can be penetrated only with longer wavelengths such as radio and infrared.

e) They can be penetrated only with longer wavelengths such as radio and infrared.

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) Dust is spread uniformly through the galaxy. e) We know more about the gas than the dust.

e) We know more about the gas than the dust.

What information does 21 cm radiation provide about the gas clouds? a) their temperature b) their distribution c) their motion d) their density e) all of these

e) all of these

Charles Messier mapped the night sky and identified many objects now known to be emission nebulae in his search for objects that might be confused with: a) galaxies. b) asteroids. c) dark nebulae. d) reflection nebulae. e) comets.

e) comets.

In the Milky Way galaxy, gas and dust are found: a) in emission nebulae. b) in molecular clouds. c) in neutral hydrogen clouds. d) in dark nebulae. e) everywhere.

e) everywhere.

What two things are needed to create an emission nebulae? a) interstellar gas and dust b) cool stars and much interstellar dust c) hydrogen gas and carbon dust d) hydrogen fusion and helium ionization e) hot stars and interstellar gas, particularly hydrogen

e) hot stars and interstellar gas, particularly hydrogen

Molecular hydrogen is not very useful for mapping molecular clouds because: a) molecular hydrogen and atomic hydrogen cannot be distinguished in telescopes. b) radiation from carbon monoxide overwhelms radiation from molecular hydrogen. c) molecular hydrogen puts out radiation similar to most stars. d)molecular hydrogen does not occur in most molecular clouds. e) molecular hydrogen is essentially invisible in the radio portion of the spectrum.

e) molecular hydrogen is essentially invisible in the radio portion of the spectrum.

The most common molecule in a molecular cloud is: a) methane, CH4. b) water, H2O. c) ammonia, NH3. d) carbon monoxide, CO. e) molecular hydrogen, H2.

e) molecular hydrogen, H2.

If you took a large sample of interstellar material and compressed it to the density and temperature typical of Earth's atmosphere, how would it probably appear? a) green, due to the presence of doubly ionized atomic oxygen b) approximately the same as Earth's air red, due to the presence of large amounts of ionized c) hydrogen d) much clearer than Earth's atmosphere e) much more opaque than Earth's air

e) much more opaque than Earth's air

Which of these is NOT a consequence of dust in the interstellar medium? a) the dark nebulae that block 90% of the Milky Way from us visually b) blue reflection nebulae around the Pleiades c) terrestrial planets like our own d) reddening of stars' light that passes through the dust e) red light from the emission nebulae

e) red light from the emission nebulae

Which of the following describes the shape of dust particles, based on polarization of light? a) disks b) cubes c) spheres d) diamonds e) rodlike

e) rodlike