Deck 7: The Structure and Formation of Stars

Question 1

Relative to a sunlike star, how should the region where convection dominates be different within a cooler star?

A) Convection should dominate within a smaller fraction of the star's surface.
B) Convection should dominate within a larger fraction of the star's surface.
C) Convection should dominate within a smaller fraction of the star's core.
D) Convection should dominate within a larger fraction of the star's core.

Answer 1

Convection should dominate within a larger fraction of the star's surface.

Question 2

Which of the following is the best example of energy transport by conduction?

A) Your feet are warmed when you hold them in front of a fire.
B) Your feet are warmed when you wear socks.
C) Your feet get cold when you stand on a cold floor.
D) Your feet get cold when you hold them over a cool air vent.

Answer 2

Your feet get cold when you stand on a cold floor.

Question 3

What is opacity?

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 temperature and density at which a gas will undergo thermonuclear fusion
D) a measure of the resistance to the flow of radiation (photons) through a gas

Answer 3

a measure of the resistance to the flow of radiation (photons) through a gas

Question 4

What is the name of the process by which the Sun turns mass into energy?

A) nuclear fission
B) nuclear fusion
C) convection
D) radiation

Question 5

Why is convection important in stars?

A) because it mixes the star's gases and increases the temperature of the star
B) because it mixes the star's gases and transports energy outwards
C) because it carries energy toward the core of the star
D) because it carries the neutrinos to the surface of the star where they can escape

Question 6

What is produced in the proton-proton chain?

A) two hydrogen nuclei, a single helium nucleus, and energy in the form of visible light
B) four hydrogen nuclei and energy in the form of gamma rays
C) a helium nucleus and energy in the form of gamma rays
D) two hydrogen nuclei and energy in the form of visible light

Question 7

Why do nuclear fusion reactions only take place in the interior of a star (rather than at the surface)?

A) The magnetic fields are strongest there.
B) The temperature and density are highest in the centre.
C) The core is the only place where hydrogen is found.
D) The strong nuclear force is only active in the centres of stars.

Question 8

Why does the CNO cycle not occur in low-mass stars?

A) Low-mass stars do not have enough carbon, nitrogen, and oxygen.
B) Low-mass stars do not live long enough to reach this stage.
C) Low-mass stars do not have high enough temperatures in their cores.
D) Low-mass stars do not have enough convection to cycle elements.

Question 9

What does the strong force do?

A) It binds electrons to the nucleus in an atom.
B) It holds the Moon in orbit around the Earth.
C) It creates the magnetic field associated with sunspots.
D) It binds protons and neutrons together to form a nucleus.

Question 10

What does solving equations on a computer have to do with making a stellar model?

A) The equations apply the laws of stellar structure at locations within the star.
B) Equations can describe the H-R diagram and a star's location on it.
C) The mass-luminosity equation tells you how to find a star's luminosity given its mass.
D) Equations are used to model the nuclear reactions inside a star.

Question 11

While on the main sequence, what is a star's primary energy source?

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

Question 12

Which of the following relationships is the key to nuclear reactions in a star's core remaining under control?

A) Luminosity depends on mass.
B) Pressure depends on temperature.
C) Density depends on mass.
D) Weight depends on temperature.

Question 13

What concept explains why both fusion and fission release energy?

A) proton-proton chain energy
B) Coulomb barrier energy
C) strong force energy
D) nuclear binding energy

Question 14

How does the temperature inside a star determine how energy flows inside it?

A) The radiation rate depends on temperature.
B) The dependence of opacity on temperature makes convection happen.
C) The dependence of opacity on temperature makes conduction happen.
D) The temperature determines how much energy is produced at each layer.

Question 15

What happens in the proton-proton chain?

A) Two protons are fused to make a helium nucleus.
B) Three protons are fused to make a lithium nucleus.
C) A helium nucleus is split into four protons.
D) Four protons are fused to make a helium nucleus.

Question 16

Why does the proton-proton chain need high temperatures?

A) High temperatures increase the ground state energy of the hydrogen atom.
B) High temperatures increase the velocity of the protons so they can overcome the Coulomb barrier.
C) High temperatures lower the density of the gas.
D) High temperatures allow the neutrinos to carry more energy away than the reaction produces.

Question 17

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 gas
C) the high temperature and density of the gas
D) the low mass of helium nuclei

Question 18

What happens in the carbon-nitrogen-oxygen (CNO) cycle?

A) Carbon nuclei are split 3 ways to make helium nuclei.
B) Carbon and oxygen combine to form nitrogen, which produces energy.
C) Carbon and nitrogen combine to form oxygen and energy.
D) Four hydrogen nuclei combine to form one helium nucleus and energy.

Question 19

What is the most dominant type of energy transport in the cores of most stars?

A) conduction
B) radiation
C) convection
D) nuclear fusion

Question 20

What is the term for the process that fuses hydrogen into helium in the cores of massive main-sequence stars?

A) the CNO cycle
B) the proton-proton chain
C) hydrostatic equilibrium
D) the neutrino process

Question 21

Refer to the H-R diagram. Which point represents a star in which the CNO cycle is occurring?

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

Question 22

What would happen if the nuclear reactions in a star began to produce too much energy?

A) The star would shrink.
B) The star would expand.
C) The star would collapse.
D) Nothing would happen.

Question 23

How much of its lifetime does the average star spend on the main sequence?

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

Question 24

Which of the following is most similar in size to a brown dwarf?

A) the planet Jupiter
B) a red dwarf
C) a white dwarf
D) a Bok globule

Question 25

Why is there a main sequence mass-luminosity relation?

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

Question 26

The Sun has an expected main-sequence lifetime of about 10 billion years. About how long would a 2-solar-mass star stay on the main sequence?

A) 10 million years
B) 1 billion years
C) 10 billion years
D) 20 billion years

Question 27

On the H-R diagram, the line indicates the location of the main sequence. Which of the four labelled locations on the H-R diagram indicates a luminosity and temperature similar to that of a T Tauri star?

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

Question 28

What would happen if the nuclear reactions in a star began to produce too little energy?

A) The star would expand.
B) The star would explode.
C) The star would contract.
D) Nothing would happen.

Question 29

How did observations at the Sudbury Neutrino Observatory solve the solar neutrino problem?

A) They showed that the "missing neutrinos" had changed into a different type.
B) They showed that other experiments had miscounted the number of solar neutrinos.
C) They showed that models for the number of neutrinos produced by the Sun were wrong.
D) They showed that neutrinos were not escaping from the core of the Sun.

Question 30

Which of the following is considered to be the best explanation for 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 flavours.

Question 31

What happens to cores of main-sequence stars as they age?

A) They cool down because they run out of hydrogen fuel.
B) They cool down because they expand.
C) They heat up because they contract.
D) They heat up because helium is more tightly bound than hydrogen.

Question 32

Which of the following is closest to the time a 10-solar-mass star spends on the main sequence?

A) 10 million years
B) 1 billion years
C) 10 billion years
D) 100 billion years

Question 33

Which of the following are star-like objects that contain less than 0.08 solar masses and will never raise their core temperatures high enough that the proton-proton chain can begin?

A) brown dwarfs
B) Herbig-Haro objects
C) Bok globules
D) T Tauri stars

Question 34

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

A) age
B) distance from the galactic centre
C) mass
D) radius

Question 35

Stars with masses below a certain threshold produce most of their energy via the proton-proton chain. What is that threshold?

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

Question 36

Which of the following statements is true?

A) Low mass stars consume fuel rapidly.
B) Massive stars consume fuel rapidly.
C) Red giants are hotter than very massive stars.
D) White dwarfs are readily visible by naked eye.

Question 37

Why does the main sequence have a limit at the lower end?

A) Low mass stars form from the interstellar medium very rarely.
B) Low mass objects are composed primarily of solids, not gases.
C) The lower limit represents a star with zero radius.
D) A minimum temperature is required for hydrogen nuclear fusion to take place.

Question 38

What is the approximate mass of the lowest mass object that can initiate the nuclear fusion of hydrogen?

A) 0.08 solar mass
B) 1 solar mass
C) 8 solar masses
D) 80 solar masses

Question 39

The lower edge of the main-sequence band represents the location in the H-R diagram at which stars begin their lives as main-sequence stars. What is it called?

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

Question 40

Refer to the H-R diagram. Which point represents a star in which the proton-proton chain is occurring?

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

Question 41

What would the spectrum of an emission nebula contain?

A) the blackbody spectrum emitted by the hot stars that heat the gas
B) the blackbody spectrum emitted by dust grains in the nebula
C) bright bands at wavelengths emitted from transitions of hydrogen
D) dark bands at wavelengths emitted from transitions of hydrogen

Question 42

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 nuclear fusion like higher mass stars do.
C) Higher mass stars have less hydrogen fuel to burn.
D) Lower mass stars spend a longer time evolving to the main sequence.

Question 43

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

A) brighter and redder than it really is
B) brighter and bluer than it really is
C) fainter and redder than it really is
D) fainter and bluer than it really is

Question 44

What evidence can be used to show that space is not a vacuum?

A) ultraviolet radiation emitted by dust
B) narrow emission lines seen in stellar spectra
C) the presence of bright nebulae
D) the blue colour of stars seen near dark regions

Question 45

Why are emission nebulae associated with young stars?

A) Young stars have not moved far from the gaseous nebulae where they were born.
B) Only hot stars can ionize nearby gas; hot stars are high-mass, so must be young.
C) The dust that makes emission nebulae glow is destroyed by old stars.
D) Young stars produce the blue light that is scattered by emission nebulae.

Question 46

What causes the dimming of starlight in the interstellar medium?

A) dust particles
B) ionized hydrogen
C) neutral hydrogen
D) molecular gas

Question 47

Consider two stars of the same mass: star 1 has just moved on to the main sequence, and star 2 is about to leave it. How are these two stars different?

A) Star 2 has more helium in its core and a hotter surface.
B) Star 2 has more helium in its core and a cooler surface.
C) Star 1 is more luminous and has a hotter surface.
D) Star 1 is more luminous and has a cooler surface.

Question 48

In which part of the electromagnetic spectrum do interstellar dust grains emit the most light?

A) radio
B) infrared
C) visible
D) UV

Question 49

What causes reflection nebulae to appear blue while emission nebulae appear pink?

A) Reflection nebulae contain less hydrogen than emission nebulae.
B) Reflection nebulae are hotter than emission nebulae.
C) Reflection nebulae are less ionized than emission nebulae.
D) Long wavelengths of light are preferentially scattered by small dust grains.

Question 50

You observe several massive stars within a cool nebula. Which of the following is the most likely explanation?

A) The massive stars ejected some of their mass to form the nebula.
B) The massive stars formed within the nebula.
C) The massive stars are flying through the nebula.
D) The massive stars have reduced the temperature of the nebula's gas.

Question 51

Because they are composed of ionized hydrogen, emission nebulae have another name - what is that name?

A) HI regions
B) HII regions
C) Bok globules
D) HeI regions

Question 52

The formation of one of the following phenomena requires that a young hot star (T = 25,000 K) be relatively close by. Which one?

A) emission nebulae
B) HI regions
C) molecular clouds
D) 21-centimetre radiation

Question 53

How is it possible to observe dark nebulae?

A) They emit radiation only in the infrared.
B) Their gravity affects the motion of nearby stars.
C) They block light from more distant stars.
D) They are sometimes illuminated by nearby bright stars.

Question 54

What force is responsible for the collapse of an interstellar cloud?

A) gravity
B) nuclear
C) electric
D) magnetic

Question 55

Under what conditions might interstellar gas clouds collapse to form stars?

A) if they encounter a shock wave
B) if they have very high temperatures
C) if they rotate rapidly
D) if they are located near main-sequence spectral type K and M stars

Question 56

Where are stars born?

A) reflection nebulae
B) dense molecular clouds
C) HII regions
D) the intercloud medium

Question 57

Which of the following factors explain why infrared telescopes are used to observe protostars?

A) density and composition
B) density and the presence of dust
C) temperature and the presence of dust
D) temperature and luminosity

Question 58

Star clusters are important to the study of stellar evolution because all the stars in a given cluster have one thing in common. What is it?

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

Question 59

In which option below are the stellar types sorted from shortest to longest main-sequence lifetime?

A) O, A, K, M
B) A, B, F, G
C) K, F, B, O
D) B, A, M, G

Question 60

What can initiate the free-fall contraction of a molecular cloud?

A) shock waves from supernovae
B) nearby spectral type G stars
C) the rotation of the cloud
D) the cloud becoming transparent to ultraviolet radiation

Question 61

Upon observing a nebula, you discover that many stars are surrounded by disks of gas and dust. What can you conclude about these stars?

A) They are very young.
B) They are very old.
C) They are red giants.
D) They are white dwarfs.

Question 62

In which way are protostars different from main-sequence stars of the same mass?

A) Protostars are smaller than main-sequence stars.
B) Protostars are hotter than main-sequence stars.
C) Protostars have a different energy source from main-sequence stars.
D) Protostars have longer lifetimes than main-sequence stars.

Question 63

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

Question 64

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

Question 65

Why are protostars difficult to observe?

A) The protostar stage is very long.
B) They are all so far away that the light hasn't reached us yet.
C) Most of their radiation is in the form of X-rays.
D) They are surrounded by cocoons of gas and dust.

Question 66

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

A) Hydrostatic equilibrium ends.
B) Nuclear fusion reactions in the core begin.
C) Helium flash occurs in the core.
D) Gravitational contraction begins.

Question 67

What is the term for a group of 10 to 100 stars that formed at the same time but are so widely scattered in space that their mutual gravity cannot hold them together?

A) a globular cluster
B) an open cluster
C) an association
D) a spherical component

Question 68

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

Question 69

Which of the following are small dark nebulae about 1 light-year in diameter, with masses from 10 to 1,000 solar masses?

A) HII regions
B) emission nebulae
C) Bok globules
D) reflection nebulae

Question 70

What does the time a protostar takes to contract from a cool interstellar gas cloud to a main-sequence star mainly depend on?

A) luminosity
B) mass
C) magnetic field
D) location on the main sequence

Question 71

Why is the presence of massive stars in the Orion Nebula an indication that star formation is currently occurring?

A) The Orion Nebula contains large amounts of gas and dust.
B) Massive stars have short lives, so they must have formed recently.
C) Massive stars trigger star formation in nearby gas clouds.
D) The Orion Nebula contains large numbers of brown dwarfs and supernova remnants.

Question 72

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

Question 73

What occurs when some of the material collapsing to form a protostar is ejected from its poles?

A) A protoplanetary disk forms.
B) The star stops collapsing.
C) The T Tauri phenomenon occurs.
D) Jets extend into the interstellar medium.

Question 74

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

Question 75

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

Question 76

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

Question 77

As a star begins to form, what is its primary energy source?

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

Question 78

Which of the following are small luminous nebulae excited by the jets of nearby protostars?

A) T Tauri stars
B) Bok globules
C) O associations
D) Herbig-Haro objects

Question 79

What is the source of the pink glow within the Great Nebula in Orion?

A) a Herbig-Haro object
B) a reflection nebula made of dust
C) young low-mass stars
D) an emission nebula made of gas

Question 80

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.