Deck 15: Star Formation and the Interstellar Medium

Question 1

Dust reddens starlight because

A) it re-emits the light it absorbs at red wavelengths.
B) it emits mostly in the infrared due to its cold temperature.
C) it is made mostly of hydrogen,which produces the red Halpha emission line.
D) it preferentially affects light at optical and shorter wavelengths.

Answer 1

it preferentially affects light at optical and shorter wavelengths.

Question 2

A protostar is usually in hydrostatic equilibrium during its collapse.

Answer 2

True

Question 3

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

Answer 3

False

Question 4

When winds blow the gas away from a forming protostar it becomes visible as a T Tauri star.

Question 5

Dust appears dark in __________ wavelengths and bright in __________ wavelengths.

A) optical;ultraviolet
B) infrared;radio
C) radio;ultraviolet
D) optical;infrared

Question 6

Stars forming in molecular clouds tend to form first in the low-density periphery.

Question 7

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

A) 1 $\mu$ m.
B) 30 $\mu$ m.
C) 50 $\mu$ m.
D) 100 $\mu$ m.

Question 8

The coldest molecular clouds in our galaxy have temperatures of approximately 1,000 K.

Question 9

Herbig-Haro objects are almost always found in pairs on either side of a young protostar.

Question 10

When radiation from an object passes through the interstellar medium,

A) the object appears dimmer.
B) the object appears bluer.
C) the object appears redder and dimmer.
D) the object's apparent velocity changes.

Question 11

When a molecular cloud fragments and stars form,the least massive stars are the first to form while the massive stars take longer to form.

Question 12

Neutral hydrogen atoms are found only inside dense clouds where they are shielded from stellar radiation.

Question 13

The H^- atom is important in protostars because it acts as a powerful temperature regulator.

Question 14

The average density of the interstellar medium is many times less dense than the best vacuum on Earth.

Question 15

The average density of the interstellar medium is

A) .
B) .
C) .
D) .

Question 16

The interstellar medium is divided up into three different kinds of gas clouds: cold gas at 10 K,warm gas at 8,000 K,and hot gas at about 1 million K.

Question 17

Star formation is a quiet and tranquil process.

Question 18

The dust in the interstellar medium comes primarily from stellar winds of main-sequence stars.

Question 19

The only effect of interstellar dust is that it makes objects appear redder than they really are.

Question 20

We observe molecular hydrogen gas using 21-cm emission.

Question 21

"Weather" in the interstellar medium is produced

A) only by supernovae.
B) by supernovae and strong winds from luminous stars.
C) by supernovae,strong winds from luminous stars,and fast-moving stars.
D) by supernovae,strong winds from luminous stars,fast-moving stars,and exploding planets.

Question 22

If you wanted to study regions where star formation is currently happening you could use

A) Halpha emission to look for O and B stars.
B) a radio telescope to search for molecular cloud cores.
C) infrared emission to identify T Tauri stars.
D) All of the above

Question 23

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) Gravity
D) Turbulence

Question 24

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

A) less luminous;hotter
B) larger;cooler
C) more luminous;cooler
D) smaller;hotter

Question 25

A typical molecular cloud has a temperature of approximately

A) 0.3 K.
B) 10 K.
C) 300 K.
D) 1,000 K.

Question 26

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.

Question 27

Molecular clouds are best observed at __________ wavelengths.

A) ultraviolet
B) optical
C) infrared
D) radio

Question 28

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.

Question 29

Molecular cloud cores are places where you might find

A) protostars.
B) Herbig-Haro objects.
C) carbon monoxide.
D) All of the above

Question 30

Warm ionized gas in the interstellar medium appears __________ when imaged in the optical region of the electromagnetic spectrum.

A) red
B) yellow
C) blue
D) dark

Question 31

An H II region signals the presence of

A) newly formed stars.
B) young stars.
C) O- and B-type stars.
D) All of the above

Question 32

What primarily makes it difficult to observe protostars?

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.

Question 33

An accretion disk forms around a collapsing protostar because infalling material must conserve

A) energy.
B) centrifugal force.
C) velocity.
D) angular momentum.

Question 34

Which of the following is responsible for heating the bulk of the very hot intercloud gas?

A) High-energy radiation from stars
B) Supernovae
C) Young O and B stars
D) The heating is an even mix of all three sources.

Question 35

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 in the 106 K intercloud gas.
D) the Solar System is immersed in a hot bubble of gas.

Question 36

A surprising fact about a 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) denser
D) more luminous

Question 37

If the temperature inside a protostar were to suddenly increase,the protostar would

A) expand.
B) contract.
C) remain the same size.
D) explode.

Question 38

We detect neutral gas in the interstellar medium by looking for radiation at 21 cm that arises when

A) an electron moves from the to 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) the spin of an electron flips and aligns with the spin of a proton in a hydrogen atom.

Question 39

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.

Question 40

In the interstellar medium,molecules survive only in regions with

A) low temperatures.
B) high densities.
C) lots of dust.
D) All of the above

Question 41

The H^- ion is very important in protostars because it

A) acts as a temperature regulator.
B) undergoes fusion and produces energy.
C) reacts with oxygen to produce water.
D) reduces angular momentum.

Question 42

Why do many stars form from a single molecular cloud?

Question 43

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

A) stars
B) stars
C) stars
D) stars

Question 44

What happens as a protostar contracts?

A) Its density rises.
B) Its temperature rises.
C) Its pressure rises.
D) All of the above are true.

Question 45

At what wavelength are H II regions most clearly visible,and why do H II regions mark the regions where new stars are currently being formed?

Question 46

Brown dwarfs are considered failed stars because

A) fusion never begins in their cores.
B) they never reach masses larger than 50 Jupiter masses.
C) convection never plays a role in their energy transport.
D) they primarily shine at infrared wavelengths.

Question 47

The source of energy for a contracting protostar comes from

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

Question 48

Given the low efficiency of the star formation process,the initial mass of a molecular cloud fragment that formed a star was probably close to

A) .
B) .
C) .
D) .

Question 49

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) We do not yet know which types of stars are most common in our galaxy.

Question 50

In the densest molecular clouds,the average density is approximately .If a cube of molecular cloud gas with this density contained of material (the amount needed to make a star),what would be the length of a side of the cube in units of AU? For reference,the mass of the Sun is ,the mass of a hydrogen atom is , .

Question 51

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

A)
B)
C) 3,000 years
D) 300 years

Question 52

Why is it possible for self-gravity to dominate pressure in molecular clouds but not in most interstellar clouds?

Question 53

Why do stars form most often within molecular clouds?

Question 54

A __________ is a failed star that shines primarily because of energy derived from its gravitational collapse.

A) black hole
B) brown dwarf
C) protostar
D) T Tauri star

Question 55

Why does the surface temperature of a protostar remain nearly constant as its core contracts?

Question 56

Why can we see dust in the interstellar medium better at far-infrared wavelengths than we can at optical wavelengths?

Question 57

Compare the volume of the Sun with the volume of interstellar space it occupies.Is the occupied percentage large or small? (Note: the radius of the ;the nearest star is 4.6 light-years away;

Question 58

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.

Question 59

The Hayashi track of a low-mass protostar in the H-R diagram is a path of approximately constant

A) mass.
B) luminosity.
C) temperature.
D) radius.

Question 60

If a protostar starts out on the Hayashi track with a temperature of 3,300 K and a luminosity of ,what is its approximate radius?

A) .
B) .
C) .
D) .

Question 61

What is the energy source that powers brown dwarf stars?

Question 62

How are Herbig-Haro objects related to T Tauri stars?

Question 63

Why do some theories of star formation hypothesize that massive stars that form in a molecular cloud may influence the number and mass of the lower mass stars that form in the cloud?

Question 64

When a protostar forms,it starts out at the top of the Hayashi track with a luminosity of and a temperature of 3,600 K.What is its radius at this point (give the answer in units of ),and how many times larger is it at this stage compared to its radius as a main-sequence star,which is about ? For reference,the Sun's temperature is 5,800 K.

Question 65

Why can't very bright protostars be seen in visible light?

Question 66

Why does a protostar continue to collapse as it is forming?

Question 67

Some molecular clouds have so much internal pressure that it exceeds their self-gravity.What keeps them from expanding and dissipating?