Chapter 20 and 21

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 star take longer. c. the most massive stars promptly explode as supernovae, destroying all remaining gas. d. the stars form at the same rate, regardless of their mass.

(check)

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.

A

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

A

The dust in the interstellar medium comes primarily from a. the stellar winds of main-sequence stars. b. the cooled material ejected from supernova explosions. c. cold cores of molecular clouds. d. all of the above

A

Use the figure shown 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

A

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

A

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

A

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

A

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

A (check)

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

B

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.

B

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.

B

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

B

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.

B

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.

B

The average density of the interstellar medium is a. much denser than the 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.

B

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.

B

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.

B (check)

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

C

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 occurs only in the cores of stars.

C

Stars with a mass from 0.01 M to 0.08 M 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.

C

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.

C

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

C

The coldest molecular clouds in our galaxy have temperatures of approximately a. 1000 K. b. 100 K. c. 10 K. d. 0 K.

C (check)

. 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

D

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.

D

Use the figure shown 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

D

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.

D

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.

D

Which of the following stars spend the longest time on their Hayashi tracks? a. 100 M stars b. 10 M stars c. 1 M stars d. 0.1 M stars e. 0.08 M stars

D

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.

D (check)

What is the most likely explanation for the dark area in the figure shown? 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

D (check)

. 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

E

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

E

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

E

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

E

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

E

Use the figure shown 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

E

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.

E