Deck 12: The Interstellar Medium: Star Formation in the Milky Way

Question 1

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

Answer 1

False

Question 2

Dark dust clouds are cooler than their surroundings.

Answer 2

True

Question 3

21- cm radiation is a result of an electron in hydrogen flipping its spin direction.

Answer 3

True

Question 4

The dust particles have exactly the same chemical composition as the gases of the interstellar medium.

Question 5

Conditions inside the darkest, densest nebulae are best studied with radio emission from molecules.

Question 6

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

Question 7

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

Question 8

Light from distant stars that must pass through dust arrives bluer than when it left its star.

Question 9

O and B type stars are usually found associated with emission nebulae.

Question 10

Emission nebulae get their red color from the red supergiants forming in them.

Question 11

We usually find dark molecular clouds beside bright emission nebulae.

Question 12

Emission nebulae are created by gas absorbing ultraviolet energy from the hot young stars within them, such as in the Orion Nebula.

Question 13

21- cm radio waves are emitted by all molecular hydrogen.

Question 14

Dark dust clouds are optically invisible, but give off radio energy.

Question 15

While ionized hydrogen glows bright red, neutral hydrogen is transparent visibly and best studied with 21- cm radio waves.

Question 16

The predominant color of emission nebulae is blue.

Question 17

Interstellar matter is distributed very evenly throughout the galaxy.

Question 18

Molecular clouds are much warmer than typical interstellar gas clouds.

Question 19

Dark nebulae are opaque to all wavelengths of electromagnetic radiation.

Question 20

Most molecules found in the interstellar medium are based on carbon.

Question 21

Interstellar gas is composed mainly of

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

Question 22

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

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

Question 23

Compared to an open cluster, a globular cluster will have more hot, blue stars.

Question 24

Young open clusters contain a lot of hot, young blue- white stars.

Question 25

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

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

Question 26

A star cluster with a lot of hot, blue stars must be relatively young.

Question 27

Globular clusters contain mostly blue stars.

Question 28

What is the primary visible color of an emission nebula?

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

Question 29

For bright stars to form, the protostars must condense in dense, dark nebulae.

Question 30

What happens to light passing through even thin clouds of dust?

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

Question 31

In stage 3 of star formation, the cloud has collapsed enough that its inner regions are opaque to their own radiation.

Question 32

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 33

Which statement is true about the interstellar medium?

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

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) the dust particles are irregular in shape.
C) there is 100 times more opaque gas than dust present in the interstellar medium.
D) ice particles reflect all light back toward their stars, not toward us.
E) the dust particles are about the same size as the light waves they absorb.

Question 35

Which statement about dark dust clouds is true?

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

Question 36

Stars evolve along the main sequence.

Question 37

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

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

Question 38

What two things are needed to create an emission nebula?

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

Question 39

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

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

Question 40

Globular clusters are typically much older than open clusters.

Question 41

Why are dark dust clouds largely misnamed?

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

Question 42

At what core temperature does hydrogen begin to fuse to helium?

A) 3,000 K
B) 5,800 K
C) 1 million K
D) 10 million K
E) 100 million K

Question 43

A fragment of a collapsing gas cloud that comes to equilibrium with a central temperature of 4 million K will become a

A) T- Tauri star.
B) stage 1 protostar.
C) black hole.
D) brown dwarf.
E) black dwarf.

Question 44

A cloud fragment too small to collapse into a main sequence star becomes a

A) T- Tauri object.
B) brown dwarf.
C) white dwarf.
D) pulsar.
E) planet of another star.

Question 45

How long does it take an M- type star to reach the main sequence, compared to a solar- type star?

A) longer than the age of the Galaxy
B) about twice as long
C) about the same, 30 million years
D) a tenth as long
E) about twenty times longer

Question 46

On an H- R diagram, a protostar would be

A) below and near the right side of the main sequence.
B) above and near the upper left of the main sequence.
C) on the main sequence at the extreme lower right.
D) below and to the left of the main sequence.
E) above and to the right of the main sequence.

Question 47

During the T- Tauri phase of a protostar, it

A) may develop very strong winds.
B) begins a period of reduced activity.
C) expands dramatically.
D) lies on the main sequence.
E) changes its spin direction.

Question 48

The single most important determinant of the temperature, density, radius, luminosity, and pace of evolution of a protostar is its

A) chemical composition.
B) mass.
C) magnetic field.
D) spin.
E) molecular composition.

Question 49

Neutral hydrogen atoms are best studied from their energy given off as

A) 121.3 nm as Lyman alpha emission in the UV.
B) red hydrogen alpha emission, at 656.3 nm.
C) 0.2 nm as X- rays.
D) 21- cm waves in the radio region.
E) Neutral hydrogen gives off no detectable radiation, since it is cold, not hot.

Question 50

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

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

Question 51

A newly formed protostar will radiate primarily at which wavelength?

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

Question 52

The most common molecule in a molecular cloud is

A) methane, CH4.
B) ammonia, NH3.
C) water, H2O.
D) molecular hydrogen, H2.
E) carbon monoxide, CO.

Question 53

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 54

Interstellar dust clouds are best observed at what wavelength?

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

Question 55

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

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

Question 56

When an electron in a hydrogen atom changes its spin from the same to the opposite direction as the proton, it

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

Question 57

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

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

Question 58

Which statement about the stages of starbirth is false?

A) By stage 7, the star has reached the main sequence.
B) Nuclear reactions begin in the core by stage 4.
C) At stage 1, only the cloud exists.
D) By stage 3, the star has formed a photosphere.
E) The T- Tauri wind is prevalent in stage 5.

Question 59

Complex molecules in the interstellar medium are found

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

Question 60

Approximately how many brown dwarfs are believed to exist in the Milky Way galaxy?

A) 100,000
B) one million
C) one billion
D) one hundred billion
E) ten trillion

Question 61

Which of these would typically be the brightest star in a young open cluster?

A) B1V
B) G2V
C) K3II
D) M3Ia
E) A2Ib

Question 62

Of all forms of electromagnetic energy, radiation gives us the least access to our entire Milky Way Galaxy.

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

Question 63

Which is characteristic of globular star clusters?

A) a mix of old and young stars, about 100,000 ly across
B) only brown dwarfs in a yellow ball 100 ly across
C) bright blue main sequence stars, and thousands of them
D) no remaining main sequence stars, but millions of white dwarfs
E) old age and hundreds of thousands of stars, only about 30 ly wide

Question 64

What are the characteristics of an open cluster of stars?

A) a few hundred, mostly main- sequence stars
B) old age and millions of members
C) mostly found above and below the galactic plane
D) All stars are much more massive than our Sun.
E) All stars are about the same age and luminosity.

Question 65

A region of ionized gas and dust, with imbedded hot young stars, is a(n)

A) dark nebula.
B) dust cloud.
C) nebular hole.
D) emission nebula.
E) absorption nebula.

Question 66

Neutral hydrogen clouds appear when viewed visually.

Question 67

Interstellar reddening and extinction are due to in the interstellar medium.

A) ice crystals
B) dust
C) helium gas
D) hydrogen gas
E) rogue planets

Question 68

While many elemental spectral lines are visible, almost all molecular lines lie in the portion of the spectrum, since they are at much lower energy.

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

Question 69

A reflection nebula appears blue because it scatters the _ light of the stars near it.

Question 70

Emission nebulae occur only near stars that emit large amounts of electromagnetic radiation.

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

Question 71

All globular clusters in our Milky Way are about how old?

A) a variety of ages, from newly born to twenty billions years old
B) around ten billion years old
C) one to three billion years old
D) less than a million years
E) ten to fifty million years old

Question 72

Why are star clusters ideal "laboratories" for stellar evolution?

A) Their stars are all about the same age, composition, and distance from us.
B) Like our Sun, they lie in the plane of the Milky Way.
C) The combined light of all the stars makes them easier to see.
D) Their stars are all about the same mass and temperature.
E) Their stars are all the same composition and stage in evolution.

Question 73

Which of these would typically be among the brightest stars in an ancient globular cluster?

A) M5
B) B3
C) A4
D) O3
E) K3

Question 74

A photosphere appears on a protostar near the end of stage

A) 1.
B) 2.
C) 3.
D) 6.
E) 7.

Question 75

Red sunset, red lunar eclipses, blue skies, and reflection nebulae are all the result of the
of visible light off tiny dust particles.

Question 76

Almost all of the complex molecules found in space are based on atoms.

A) silicon
B) carbon
C) oxygen
D) hydrogen
E) iron

Question 77

Neutral hydrogen can be detected because its electron occasionally its spin and creates the 21- cm emission line for radio observers.

Question 78

The gas in an HII region is hydrogen.

Question 79

Most stars in our part of the Galaxy are formed

A) in a singular event just after the Big Bang.
B) in globular clusters of millions of stars.
C) alone.
D) in associations of thousands of stars across a spiral arm of the Galaxy.
E) in open clusters of a few dozen.

Question 80

The interstellar gas in our portion of the Galaxy is about K in temperature.

A) 10
B) 100
C) 1,000
D) 10,000
E) 100,000