Deck 10: Structure and Formation of Stars

Question 1

Convection is important in stars because it

A) increases the temperature of the star.
B) mixes the gases of the star.
C) transports energy outward in the star.
D) carries the neutrinos to the surface of the star where they can escape.
E) b and c

Answer 1

b and c

Question 2

Protostars are difficult to observe because

A) the protostar stage is very short.
B) they are surrounded by cocoons of gas and dust.
C) they radiate mainly in the infrared.
D) all of the above
E) they are all so far away that the light hasn t reached us yet.

Answer 2

all of the above

Question 3

What causes the outward gas pressure that balances the inward pull of gravity in a main sequence star?

A) The rapid outward flow of gas.
B) The rapid inward flow of the gas.
C) The high temperature of the gas.
D) The high density of the gas.
E) c and d

Answer 3

c and d

Question 4

Which of the following is NOT evidence of the existence of an interstellar medium?

A) extinction of light from distant stars
B) very narrow calcium absorption lines in the spectra of O and B stars
C) reddening of more distant stars
D) dark cloud silhouetted against background stars
E) molecular absorption lines in the spectra of cool stars

Question 5

____ are small luminous nebulae excited by nearby young stars.

A) T Tauri stars
B) Herbig-Haro objects
C) O associations
D) Bok Globule
E) Giant molecular clouds

Question 6

Opacity is

A) the balance between the pressure and force of gravity inside a star.
B) the force that binds protons and neutrons together to form a nucleus.
C) the force that binds an electron to the nucleus in an atom.
D) a measure of the resistance to the flow of radiation (photons) through a gas.
E) the temperature and density at which a gas will undergo thermonuclear fusion.

Question 7

The average star spends ____ of its lifetime on the main sequence.

A) 1%
B) 2%
C) 10%
D) 20%
E) 90%

Question 8

The carbon-nitrogen-oxygen cycle

A) operates at a slightly lower temperature than the proton-proton chain.
B) is most efficient in star less massive than the sun.
C) occurs when carbon and oxygen combine to form nitrogen, which produces energy.
D) produces the energy responsible for bipolar flows.
E) combines four hydrogen nuclei to form one helium nucleus, which produces energy.

Question 9

In the proton-proton chain

A) no neutrinos are produced.
B) energy is released because a hydrogen nucleus has a greater mass than a helium nucleus.
C) no photons are produced.
D) carbon serves as a catalyst for the nuclear reaction.
E) energy is produced in the form of gamma rays and the velocity of the created nuclei.

Question 10

The lower edge of the Main Sequence band is known as ____ and represents the location in the HR-diagram at which stars begin their lives as main sequence stars.

A) the Coulomb barrier
B) hydrostatic equilibrium
C) the birth line
D) the zero-age main sequence
E) the evolutionary track

Question 11

Emission nebulae are also called ____ because they are composed of ionized hydrogen.

A) HI regions
B) HII regions
C) Bok globules
D) HeI regions
E) reflection nebulae

Question 12

The free-fall contraction of a molecular cloud

A) can be initiated by shock waves from supernovae.
B) can be initiated by nearby spectral type G stars.
C) can be initiated by the rotation of the cloud.
D) causes the cloud to become transparent to ultraviolet radiation.
E) causes the particles in the cloud to decrease the speed with which they move.

Question 13

____ are star-like objects that contain less than 0.08 solar masses an will never raise their core temperatures high enough that the proton-proton chain can begin. Other minor fusion reactions do occur in these objects. They fall in a gap between the low-mass M dwarf stars and the massive planets in which nuclear fusion never occurs.

A) Brown dwarfs
B) Herbig-Haro objects
C) Bok globules
D) T-Tauri star
E) Main sequence stars

Question 14

The diagram below is an HR diagram. The line indicates the location of the main sequence. Which of the five labeled locations on the HR diagram indicates a luminosity and temperature similar to that of a T Tauri star?

A) 1
B) 2
C) 3
D) 4
E) 5

Question 15

Interstellar gas clouds may collapse to form stars if they

A) have very high temperatures.
B) encounter a shock wave.
C) rotate rapidly.
D) are located near main sequence spectral type K and M stars.
E) all of the above.

Question 16

____ occurs when most of the material collapsing to form a protostar has fallen into a disk around the star and a strong wind from the warm protostar ejects material from its poles.

A) An emission nebula
B) Hydrostatic equilibrium
C) The proton-proton chain
D) The thermonuclear fusion of hydrogen
E) A bipolar outflow

Question 17

____ is the thermonuclear fusion of hydrogen to form helium operating in the cores of massive stars on the main sequence.

A) The CNO cycle
B) The proton-proton chain
C) Hydrostatic equilibrium
D) The neutrino process
E) none of the above

Question 18

The visual pink glow of the Great Nebula in Orion

A) is a Herbig-Haro object.
B) is a reflection nebula of dust.
C) is an emission nebula of thin gas.
D) contains only young low mass stars.
E) is believed to be about 5 billion years old.

Question 19

The ____ is the nuclear fusion of three helium nuclei to form a carbon nucleus.

A) proton-proton chain
B) Carbon-Nitrogen-Oxygen cycle (CNO cycle)
C) triple-alpha process
D) Coulomb barrier
E) infrared cirrus

Question 20

The nuclear reactions in a star s core remain under control so long as

A) luminosity depends on mass.
B) pressure depends on temperature.
C) density depends on mass.
D) weight depends on temperature.
E) temperature depends on mass.

Question 21

Absorption lines due to the interstellar medium indicate that some components of the interstellar medium are cold and of a very low density because

A) the lines are blue shifted.
B) the lines are red shifted.
C) the lines are extremely broad.
D) the lines are extremely narrow.
E) the lines are much darker than the stellar lines.

Question 22

The main sequence has a limit at the lower end because

A) low mass stars form from the interstellar medium very rarely.
B) low mass objects are composed primarily of solids, not gases.
C) pressure does not depend on temperature in degenerate matter.
D) the lower limit represents when the radius of the star would be zero.
E) there is a minimum temperature for hydrogen nuclear fusion (the definition of a star).

Question 23

There is a main sequence mass-luminosity relation because

A) helium fusion produces carbon.
B) stars expand when they become giants.
C) more massive stars support their larger weight by making more energy.
D) the helium flash occurs in degenerate matter.
E) all stars on the main sequence have about the same radius.

Question 24

Absorption lines due to interstellar gas

A) are wider than the lines from stars because the gas is hotter than most stars.
B) are more narrow than the lines from stars because the gas has a lower pressure than stars.
C) indicate that the interstellar medium contains dust.
D) indicate that the interstellar medium is expanding away from the sun.
E) none of the above

Question 25

The H-R diagram of a young star cluster shows

A) that high mass stars have not yet reached the main sequence yet.
B) that low mass stars have not yet reached the main sequence yet.
C) mainly giant stars.
D) no stars since none have reached the main sequence yet.

Question 26

The creation of ____ require(s) that a young hot star (T 25,000 K) be relatively nearby.

A) Emission nebulae
B) HI regions
C) Molecular clouds
D) The cool gas of the interstellar medium
E) 21 cm radiation

Question 27

A jet has an angular length of 10 seconds of arc and flows from a star at a distance of 2,000 pc. How far does this jet extend away from the star?

A) 20,000 pc
B) 20,000 AU
C) 20, 000 ly
D) 20,000 miles
E) 20,000 km

Question 28

If the dust around a protostar radiates most strongly at 30 microns or 30,000 nm, what is the approximate temperature of that dust? Hint: T in K =3,000,000/wavelength in nm.

A) 100,000 K
B) 10,000 K
C) 1000 K
D) 100 K
E) 10 K

Question 29

What is the lifetime on the main sequence of a 2 solar mass star compared to the approximately 10 billion year life of our Sun? Pick the best answer.

A) 1.8 10 11 years
B) 1.8 10 10 years
C) 1.8 10 9 years
D) 1.8 10 6 years
E) 1.8 10 3 years

Question 30

The extinction of starlight due to the interstellar medium
I. is the greatest in the ultraviolet.
II. is the greatest in the infrared.
III. is caused by ionized hydrogen.
IV. is caused by dust particles.

A) I,III
B) II III
C) I, IV
D) II IV
E) only IV

Question 31

What is the lifetime of a 10 solar mass star on the main sequence?

A) 3.2 10 7 years
B) 320 years
C) 3.2 10 12 years
D) 1 10 9 years
E) 1 10 11 years

Question 32

____ are small dark nebulae about 1 light-year in diameter that contain 10 to 1,000 solar masses.

A) HI regions
B) HII regions
C) Emission nebulae
D) Bok globules
E) Reflection nebulae

Question 33

What must occur for an object to be considered a main sequence star?

A) Hydrostatic equilibrium
B) Nuclear fusion reaction in the core
C) Protostar life begins
D) Both a and b

Question 34

Molecular clouds can be observed

A) using infrared telescopes to detect ionized gas in the clouds.
B) using x-ray telescope to observe the x-ray radiation from the molecules in the cloud.
C) using radio telescopes to observe the CO emission from the clouds.
D) by looking for blue wispy regions near star clusters.
E) by looking for the 21-cm radiation from hydrogen.

Question 35

While on the main sequence a star s primary energy source comes from

A) nuclear fusion.
B) nuclear fission.
C) gravitational potential energy.
D) magnetic fields.

Question 36

As a star begin to form the initial energy source is from

A) nuclear fusion.
B) nuclear fission.
C) gravitational potential energy.
D) magnetic fields.

Question 37

Due to the dust in the interstellar medium, a distant star will appear to an observer on Earth to be

A) brighter and cooler than it really is.
B) brighter and hotter than it really is.
C) fainter and cooler than it really is.
D) fainter and hotter than it really is.
E) unchanged in brightness or apparent color.

Question 38

Stars are born in

A) reflection nebulae.
B) dense molecular clouds.
C) HII regions.
D) the intercloud medium.
E) the local bubble.

Question 39

Stars with masses less than ____ produce most of their energy via the proton-proton chain.

A) 0.01 solar masses
B) 0.1 solar masses
C) 1.1 solar masses
D) 11 solar masses
E) 110 solar masses

Question 40

The lowest mass object that can initiate thermonuclear fusion of hydrogen has a mass of about

A) 1 solar masses
B) 80 solar masses
C) 8 solar masses
D) 0.08 solar masses
E) 0.001 solar masses

Question 41

____________________ appear reddish in color due to the light emitted by the Balmer series of the hydrogen atom.

Question 42

Star clusters are important to the study the stellar evolution because stars in a given cluster have the same

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

Question 43

The proton-proton chain needs high temperature because

A) of the ground state energy of the hydrogen atom.
B) of the presence of helium atoms.
C) the protons must overcome the Coulomb barrier.
D) of the need for low density.
E) the neutrinos carry more energy away than the reaction produces.

Question 44

The capture of too few solar neutrinos by Davis in the solar neutrino experiment

A) has been disproved by the results of later experiments.
B) can be explained if the sun is not undergoing thermonuclear fusion of hydrogen in its core.
C) indicates that the sun s core is much hotter than expected.
D) indicates that the sun s core is convective.
E) none of the above

Question 45

____________________ is the resistance of a gas to the flow of radiation.

Question 46

What characteristic of a star primarily determines its location on the main sequence?

A) age
B) distance from the galactic center
C) mass
D) space motion
E) radius

Question 47

Which of the following is suggested as the best explanation of the missing solar neutrinos?

A) The sun is fusing helium but not hydrogen.
B) Nuclear reactions do not produce neutrinos as fast as theory predicts.
C) The sun may contain matter we haven t yet identified.
D) Neutrinos may oscillate between three different flavors.
E) None of the above

Question 48

The condition of ____________________ means that the force due to gravity pushing down on a layer is exactly equal to the pressure pushing outward on that layer.

Question 49

The proton-proton chain

A) combines two hydrogen nuclei to produce a single helium nucleus and energy.
B) splits a helium nucleus to produce 4 hydrogen nuclei and energy.
C) is the mechanism that increases the temperature between the photosphere and corona of the sun.
D) is the interactions between protons in Earth s atmosphere that produces auroras.
E) produces energy in the core of the sun in the form of gamma-rays, positrons, and neutrinos.

Question 50

Helioseismology shows that deeper layers of gas in the sun rotate at different speeds. When different parts of an object rotate at different rates it is called

A) the dynamo effect.
B) lagging.
C) reconnection.
D) differential rotation.
E) the Zeeman effect.

Question 51

Energy transport by ____________________ is important when photons cannot readily travel through a gas.

Question 52

Why do nuclear fusion reactions only take place in the interior of a star?

A) The magnetic fields are strongest there.
B) The temperature is the hottest in the center.
C) The density of material is very high in the center.
D) Both (b) and (c)
E) FALSE; Nuclear fusion reactions can take place in the core and corona where temperatures are high enough.

Question 53

Stars with masses greater than 1.1 solar masses use the ____________________ to convert hydrogen into helium and produce energy

Question 54

The pressure of an interstellar gas cloud depends on the cloud s ____________________ and ____________________.

Question 55

What force(s) are responsible for the collapse of an interstellar cloud?

A) nuclear
B) gravity
C) electric
D) magnetic
E) both c and d

Question 56

Why do higher mass stars live shorter lives on the main sequence than lower mass stars?

A) Higher mass stars burn through their nuclear fuel faster.
B) Lower mass stars don t get their energy from that same nuclear fusion source as higher mass stars.
C) Higher mass stars have less hydrogen fuel to burn.
D) Lower mass stars spend a longer time evolving to the main-sequence.
E) All of the above are false.

Question 57

A star on the ____________________ generates enough energy from nuclear fusion to halt gravitational collapse.

Question 58

In the proton-proton chain

A) no neutrinos are produced.
B) energy is released because a helium nucleus has a greater mass than a hydrogen nucleus.
C) no photons are produced.
D) carbon serves as a catalyst for the nuclear reaction.
E) energy is produced in the form of gamma rays and the velocity of the created nuclei.

Question 59

A(n) ____________________ is a cool, dense cloud of dust and gas that is undergoing gravitational contraction.

Question 60

The sun creates its energy by the process of

A) nuclear fission.
B) nuclear fusion.
C) convection.
D) conduction.
E) radiation.

Question 61

HII regions are found near stars cooler than 25,000 K because large amounts of ultraviolet photons would totally destroy the hydrogen atoms in the gas.

Question 62

If energy is carried by the bulk motion of matter, that is called ____________________.

Question 63

Nuclear fusion in stars is controlled by the dependence of density on mass.

Question 64

Pre-main sequence stars are above and to the right of the main sequence.

Question 65

The Orion region contains young main sequence stars and an emission nebula, but the original molecular cloud they formed out of has been dispersed.

Question 66

The thermal motions of the atoms in a gas cloud can make it collapse to form a protostar.

Question 67

The sun has a core in which energy travels outward primarily by radiation.

Question 68

T Tauri stars are believed to be young, high mass main sequence stars.

Question 69

The sun makes most of its energy by the CNO cycle.

Question 70

The dust in the interstellar medium can make distant stars look redder than they really are.

Question 71

A(n) ____________________ is a subatomic particle produced in nuclear fusion that can travel through the sun and escape to space without interacting with any particles in the sun.

Question 72

Energy flows by radiation or convection inside stars but almost never by conduction.

Question 73

Ninety percent of all stars fuse helium to form carbon and lie on the main sequence.

Question 74

Bok globules are small, dark fragments of the interstellar medium.

Question 75

Some massive stars only live a few million years.

Question 76

The pressure of a gas depends on the temperature and density of the gas.

Question 77

Some young star clusters contain large numbers of T Tauri stars.

Question 78

Hydrostatic equilibrium refers to the balance between weight and pressure.

Question 79

High mass stars evolve more slowly to the main sequence than lower mass stars.

Question 80

Molecular clouds are mapped using CO instead of hydrogen because CO is much more abundant than hydrogen in molecular clouds.