Deck 9: The Formation and Structure of Stars

Question 1

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

Answer 1

molecular absorption lines in the spectra of cool stars

Question 2

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.

Answer 2

fainter and cooler than it really is.

Question 3

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.

Answer 3

produces energy in the core of the sun in the form of gamma-rays, positrons, and neutrinos.

Question 4

____ 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 5

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 6

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

Question 7

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 8

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 9

The lower edge of the main-sequence band is known as ____ and represents the location in the H-R 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 10

____ 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 11

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 12

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 13

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 14

____ 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 15

____ 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. 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 16

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 17

The diagram below is an H-R diagram. The line indicates the location of the main sequence. Which of the five labeled 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
E) 5

Question 18

The H-R diagram main-sequence of stars has a limit at the lower-mass or energy-output 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 19

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 20

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

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

Question 21

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

A) the outward flow of energy
B) the opacity of the gas
C) the temperature of the gas
D) the density of the gas
E) c and d

Question 22

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 23

____ 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 24

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 25

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 26

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 27

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 28

The extinction of starlight due to the interstellar medium

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

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*10¹¹ years
B) 1.8*10¹⁰ years
C) 1.8*10⁹ years
D) 1.8*10⁶ years
E) 1.8*10³ years

Question 30

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 31

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

A) 3.2*10⁷ years
B) 320 years
C) 3.2*10¹² years
D) 1*10⁹ years
E) 1*10¹¹ years

Question 32

Stars are born in

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

Question 33

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 34

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 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

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 37

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 38

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 39

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 40

The creation of ____ require(s) that a young hot star (T $\approx$ 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 41

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

Question 42

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

Question 43

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

Question 44

The central cores of which type of star is the lowest temperature?

A) protostars.
B) massive main-sequence stars.
C) low-mass main-sequence stars.

Question 45

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

Question 46

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

Question 47

In the collapse of a protostar ____________________ energy is converted to ____________________ energy.

Question 48

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

Question 49

Ionization from the ____ in the Orion Nebula makes it glow.

A) planets
B) coolest stars
C) hottest stars

Question 50

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

Question 51

Ionization from the ____________________ star in the Orion nebula makes it ____________________ brightly.

Question 52

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

Question 53

The youngest stars in the Orion Nebula and other star-forming regions also show

A) circumstellar disks.
B) jets.
C) both A and B

Question 54

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

Question 55

Some young star clusters contain large numbers of protostars.

Question 56

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 57

The hottest stars in the Orion Nebula ____________hydrogen gas and make it _____________.

A) ionize; glow
B) neutralize; glow
C) ionize; swallow the star

Question 58

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

Question 59

Most stars generate their energy by the thermonuclear fusion of ____________________ to form ____________________.

Question 60

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

Question 61

What is the difference between the proton-proton chain and the CNO cycle?

Question 62

Why do nuclear reactions in a star occur only near its center?

Question 63

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

Question 64

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

Question 65

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

Question 66

How does a gas cloud become hot enough to ignite nuclear reactions?

Question 67

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 68

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

Question 69

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

Question 70

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

Question 71

Hydrostatic equilibrium refers to the balance between weight and pressure.

Question 72

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

Question 73

What observations confirm the existence of protostars?

Question 74

The sun makes most of its energy by the proton-proton cycle.

Question 75

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

Question 76

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

Question 77

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 78

Some massive stars only live a few million years.

Question 79

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

Question 80

Main-sequence stars many times the mass of the sun produce their energy by the set of nuclear reactions called the CNO cycle.