Deck 11: Inferring Patterns in Star Life Cycles

Question 1

The stellar remnant left at the end of a moderately low-mass star like our Sun is a brown dwarf.

Answer 1

False

Question 2

Heavy metals from the expulsion of elements in a supernova explosion can be recycled into new stars,which makes them metal-rich.

Answer 2

True

Question 3

Emission nebulae are glowing,ionized clouds of gas,powered by ultraviolet light that they absorb from nearby hot stars.

Answer 3

True

Question 4

Of the three phases of matter-gas,liquid,and solid-solids are by far the simplest to understand.

Question 5

When an average-sized star reaches the end of its main sequence life,it becomes a T Tauri star.

Question 6

A dark nebula is a large cloud of dust grains that is so opaque that it blocks any visible light coming from the stars behind it.

Question 7

A planetary nebula may take millions of years to fade from view from Earth's perspective.

Question 8

It can take hundreds of billions of years for a red dwarf to convert all of its hydrogen completely to helium.

Question 9

The stellar remnant that will be left after the Sun ends its main-sequence life is a white dwarf.

Question 10

A nebula that emits its own light with the characteristic emission line spectrum similar to that of a hot,thin gas is called a reflection nebula.

Question 11

Our Sun is currently undergoing the triple-alpha process for turning helium into carbon in the core.

Question 12

Stars between 0.4 and 4 solar masses go through two distinct red-giant stages.

Question 13

When stars enter their red-giant phase,their plot on the H-R Diagram moves down and to the left of the graph.

Question 14

Protostars with emission lines as well as absorption lines in their spectra and whose luminosity can change irregularly on time scales of a few days are called T Tauri stars.

Question 15

Population I stars are members of a second stellar generation,whereas Population II stars belong to an older first generation.

Question 16

Astronomers have to piece together the evolutionary history of stars by studying different stars at different stages in their life cycles.

Question 17

Since the Sun started its sustained thermonuclear reactions in the core,its diameter has tripled in size.

Question 18

The one characteristic that determines a main-sequence star's lifetime is mass.

Question 19

Stars join the main-sequence group when they begin hydrogen fusion in their cores.They leave the main-sequence group and become giant stars when their core hydrogen is depleted.

Question 20

A white dwarf is approximately the same size as Earth.

Question 21

The distinct blue color of the nebulosity around stars in young clusters,such as the Pleiades,is caused by

A) light emitted by very hot gas which has been heated by collisions in the interstellar gas.
B) preferential scattering of blue starlight by small dust grains in the interstellar material.
C) atoms of gas emitting light by fluorescence,having been excited by ultraviolet radiation from hot stars.
D) halos caused by refraction of starlight in ice crystals in the nebula,similar to halos seen occasionally in Earth's atmosphere.

Question 22

What is the most abundant element in the universe?

A) Helium
B) Hydrogen
C) Carbon
D) Oxygen

Question 23

What is the second most abundant element in the universe (after hydrogen)?

A) Iron
B) Carbon
C) Nitrogen
D) Helium

Question 24

A particular giant molecular cloud has a mass of 400,000 solar masses.What is the mass of helium in this cloud?

A) 4000 solar masses
B) 300,000 solar masses
C) 392,000 solar masses
D) 100,000 solar masses

Question 25

Certain stages of stellar evolution,such as birth of a protostar and post-main-sequence red-giant evolution,come about because of an imbalance between gravity and

A) high-energy neutrino pressure.
B) radiation intensity.
C) the centrifugal force from rotation.
D) internal gas pressure.

Question 26

How is gas distributed in interstellar space?

A) Uniformly distributed through space
B) In clumps concentrated in interstellar clouds
C) Concentrated in thin walls throughout the universe
D) Concentrated around existing stars because of the stars' gravitational pull

Question 27

Of the various phases of matter,the one that is easiest to model and understand is

A) solid.
B) liquid.
C) gas.
D) plasma.

Question 28

The space between stars is now known to contain

A) large quantities of dust that absorb light but no gas,either atomic or molecular.
B) gas,made up of atoms,molecules,and dust particles.
C) a perfect vacuum.
D) variable amounts of gas but no dust,because dust forms only in planetary systems near stars.

Question 29

Of the following components,which are responsible for the main spectral emission lines found in an emission nebula?

A) Neutral helium,carbon,oxygen,and iron atoms
B) Ionized helium atoms and electrons
C) Protons,positrons,helium nuclei,and neutrinos
D) Hydrogen atoms,protons,and electrons

Question 30

The most likely places in which stars and planetary systems are forming in the universe are

A) the rarified outer space between galaxies.
B) regions of hot gas in the spiral arms of galaxies.
C) gas and dust nebulae.
D) the centers of galaxies.

Question 31

What fraction of the measured mass of the universe is hydrogen (as represented by "standard cosmic abundances" in the interstellar medium)?

A) 25%
B) 98%
C) 1%
D) 74%

Question 32

New stars are formed

A) in huge,cool dust and gas clouds.
B) from free space,out of pure energy.
C) within supernova remnants.
D) by condensation of gas near black holes in the centers of galaxies.

Question 33

What fraction of the measured mass of the universe is helium (as represented by "standard cosmic abundances" in the interstellar medium)?

A) 25%
B) 1%
C) 8%
D) 74%

Question 34

What causes the characteristic blue color of a reflection nebula?

A) Scattering of starlight from dust grains within the nebula
B) Thermal (blackbody)radiation from dust grains heated to high temperatures by stellar UV
C) Electrons dropping from n = 3 to n = 2 in hydrogen atoms
D) Electrons dropping from n = 2 to n = 1 in hydrogen atoms

Question 35

The distinctive color of a reflection nebula is

A) blue,caused by the preferential scattering of starlight by very small dust grains.
B) starlight of all colors from cool stars,predominantly in the red part of the spectrum,reflected by ice crystals of water,ammonia,and methane.
C) a mixture of several specific colors coming from fluorescence of atoms excited by ultraviolet radiation emitted by hot stars.
D) red,coming from the emission of light from hydrogen gas.

Question 36

A particular giant molecular cloud has a mass of 400,000 solar masses.What is the mass of hydrogen in this cloud?

A) 100,000 solar masses
B) 4000 solar masses
C) 392,000 solar masses
D) 300,000 solar masses

Question 37

A reflection nebula is made visible by

A) thermal energy emitted as a continuous spectrum by very hot gas,much like that emitted by a hot body on Earth.
B) blue light,preferentially scattered by tiny dust grains.
C) emission lines from hydrogen,which itself has been ionized by UV light from embedded stars.
D) light from embedded stars,reflected over a wide range of wavelengths toward Earth by crystals of water.

Question 38

When protostars are FIRST formed from the interstellar medium,they are usually

A) detected easily because their light ionizes the surrounding interstellar gas,forming H II regions.
B) very bright in ultraviolet light due to numerous flares (like solar flares but hotter and brighter).
C) hidden from sight by dust clouds that emit infrared radiation.
D) detected by emission lines in their visible spectra,emitted by gas being blown off their surfaces into space.

Question 39

Which of the following would NOT be found in higher-than-average densities in an interstellar nebula?

A) Electrons
B) Neutrinos
C) Dust
D) Protons

Question 40

Protostars are

A) stars made almost entirely out of protons.
B) objects with masses less than about 0.08 solar masses,which do not have enough mass to become true stars.
C) old stars,contracting after using up all of their available hydrogen fuel.
D) very young objects,still contracting before becoming true stars.

Question 41

Some protostars,when they evolve onto the main sequence,are fully convective and use convection to transport material all the way from the core to the surface.Which stars show this characteristic?

A) Only the smallest,with masses less than about 0.4 solar masses
B) Intermediate,with masses from about 0.4 solar masses up to about 4 solar masses
C) The largest,with masses above about 4 solar masses
D) All stars,regardless of solar mass

Question 42

What is the most important factor that determines the point in the Hertzsprung-Russell diagram where a protostar reaches the main sequence?

A) The initial luminosity of the protostar
B) The size of the interstellar cloud from which the protostar formed
C) The initial temperature of the protostar
D) The mass of the protostar

Question 43

What is a protostar?

A) A contracting sphere of gas produced by the collapse of an interstellar cloud with,as yet,no nuclear reactions occurring in its interior
B) A shell of gas left behind from the explosion of a star as a supernova
C) A small,cold,interstellar cloud before it collapses to become a star
D) A star near the end of its life,just before it explodes as a supernova

Question 44

Which wavelength region is most useful for investigating dense cores inside giant molecular clouds?

A) Infrared
B) X-rays
C) Ultraviolet
D) Optical (visual)

Question 45

What occurrence defines the end of the protostar phase of a star's life and the start of the main-sequence phase?

A) Convection begins in its interior.
B) The star stops accreting mass from the interstellar cloud.
C) The star begins to expand to become a red supergiant.
D) The nuclear reactions in its core allow it to gain hydrostatic equilibrium.

Question 46

The smallest mass that a main-sequence star can have is about 0.08 solar mass.The reason for this is that

A) the temperature in the core of a contracting protostar of less than 0.08 solar masses does not get high enough for nuclear reactions to start.
B) protostars cannot form with masses less than 0.08 solar mass.
C) thermonuclear reactions begin so suddenly in stars of less than 0.08 solar mass that the star is disrupted by an explosion.
D) protostars of less than 0.08 solar masses are not massive enough to contract.

Question 47

At what stage of its life does a star pass through the protostar phase?

A) When it is expanding in size as a red giant or supergiant
B) Wfter all nuclear reactions have ended in its core
C) Before nuclear reactions begin in its core
D) While it is converting hydrogen to helium in its core

Question 48

How do massive protostars (4 solar masses)evolve on the H-R diagram?

A) Temperature decreases at approximately constant luminosity.
B) Temperature increases at approximately constant luminosity.
C) Luminosity decreases at approximately constant temperature.
D) Luminosity increases at approximately constant temperature

Question 49

A T Tauri star is at what stage of its stellar evolution?

A) A well-established main-sequence star
B) Just before red-giant phase-when variability begins
C) At the end of its life-decaying away and cooling
D) Protostar-before main-sequence phase

Question 50

The main energy-generation mechanism in main-sequence stars is

A) core hydrogen fusion.
B) shell hydrogen fusion.
C) core helium fusion.
D) Kelvin-Helmholtz contraction.

Question 51

What places a limit on the lifetime of a star?

A) Loss of the mass,and therefore of nuclear fuel,of the star into space by stellar winds
B) Amount of available nuclear fuel it contains
C) Collisions between stars in a galaxy that are sufficiently frequent for all stars to eventually be destroyed in this way
D) Buildup of spin as it evolves and contracts,causing it to eventually spin apart

Question 52

In which phase of a star's life are thermonuclear reactions converting hydrogen into helium in the core of a star?

A) The main-sequence phase
B) The horizontal branch phase
C) As the star moves up the red-giant branch for the first time
D) The protostar phase

Question 53

If a protostar were able to contract (get smaller)without any change to its surface temperature,what would happen to its luminosity?

A) It would decrease,due to the smaller surface area of the protostar.
B) It would remain the same because the temperature does not change.
C) It is not possible to predict the change in luminosity because other factors are involved.
D) It would increase due to the compression of the gas.

Question 54

A star first reaches the main sequence when it

A) starts to collapse.
B) becomes a protostar.
C) becomes a T Tauri star.
D) attains hydrostatic equilibrium.

Question 55

What event occurs at the end of the protostar stage of a star's life?

A) Nuclear reactions begin in its core,converting hydrogen into helium,generating energy and increasing internal pressure.
B) It explodes,forming a supernova remnant.
C) It begins a long period of contraction,in which gravitational energy is converted into heat.
D) Gas is spun off from its equator,forming planets.

Question 56

Prior to reaching the main sequence,a protostar's energy comes mostly from

A) gravitation.
B) nuclear fusion.
C) nuclear fission.
D) natural radioactivity.

Question 57

The source of a protostar's heat is

A) gravitational energy,released as the star contracts.
B) gravitational energy,released as the protostar expands.
C) nuclear reactions converting helium to carbon and oxygen in its core.
D) nuclear reactions converting hydrogen into helium in its core.

Question 58

What physical process is taking place inside a star that is in the main-sequence phase of its life?

A) Hydrogen is being converted to helium in its core.
B) Hydrogen is being converted to helium in a shell around a core where no nuclear reactions are occurring.
C) The gas in the star is contracting gravitationally without nuclear reactions taking place.
D) Helium is being converted to carbon in its core.

Question 59

A star's evolutionary track is

A) its movement when plotted on a Hertzsprung-Russell diagram,as it evolves in luminosity and temperature.
B) its motion through the dark,dense cloud from which it was formed,marked by a visible channel swept free of dust and gas.
C) the line across the Hertzsprung-Russell diagram denoting stars identified as main-sequence stars.
D) its movement when plotted on a map of the galaxy as it takes part in the overall galactic rotation.

Question 60

At what stage of its evolutionary life is the Sun?

A) Pre-main-sequence-variable star
B) Main-sequence-middle age
C) Post-main-sequence-red-giant (cool)phase
D) Just before supernova stage (perhaps 5 years)-late evolutionary stage

Question 61

A helium flash occurs in the cores of all stars

A) that contain helium.
B) of more than 2 solar masses.
C) of fewer than 2 solar masses.
D) that have become red giants.

Question 62

What is a red giant?

A) A large,red star burning hydrogen into helium in its core
B) A protostar in the "upper right" part of the Hertzsprung-Russell diagram
C) A large emission nebula
D) A star burning hydrogen into helium in a shell around the core

Question 63

The event that marks the end of the main-sequence lifetime of a star is the

A) formation of a nova.
B) formation of a planetary nebula.
C) end of hydrogen fusion in the core.
D) helium flash.

Question 64

Degeneracy occurs when

A) magnetic fields inhibit the motion of charged particles in sunspots.
B) solar wind particles ionize atoms in Earth's upper atmosphere.
C) thermonuclear reactions halt the contraction of a protostar.
D) electrons inside a star resist being pushed closer together than a certain limit.

Question 65

The study of stars in clusters has helped astronomers to understand

A) the reason for differences in surface temperatures of stars.
B) the mechanism of mass loss in stars.
C) the action of nuclear fusion in stars.
D) stellar evolution-the development of stars with time.

Question 66

The end of the life of a red dwarf star is predicted to be a sphere of almost pure helium,but we have not detected any of these.What do we believe is the reason?

A) These helium stars are very dim and consequently hard to detect.
B) There are very few red dwarfs,so their end products are expected to be rare.
C) The evolution rate for red dwarfs is so slow that none has yet evolved to its end stage.
D) Helium is a relatively light material,so these helium spheres are expected to dissipate in a short time.

Question 67

What happens to the helium-rich core of a star after the core runs out of hydrogen?

A) It heats and expands.
B) It contracts and heats.
C) It cools and contracts.
D) It expands and cools.

Question 68

What are the main general features that make clusters of stars useful to astronomers?

A) The stars are at the same distance from Earth,were formed at approximately the same time,and were made from the same chemical mix.
B) The stars are all at the same distance from Earth,have the same surface temperature,and joined the cluster at various times.
C) The stars all have the same apparent magnitude,the same surface temperatures,and the same sizes.
D) The stars all have the same intrinsic brightness but differ in size and surface temperature.

Question 69

How is the length of a star's lifetime related to the mass of the star?

A) Higher-mass stars run through their lives faster and have shorter lifetimes.
B) The lifetimes of stars are too long to measure,so it is not known how their lifetimes depend on mass.
C) Lower-mass stars run through their lives faster and have shorter lifetimes.
D) A star's lifetime does not depend on its mass.

Question 70

The hydrogen in the outer layers of the Sun cannot undergo fusion because the

A) hydrogen in the outer layers is fundamentally different from the hydrogen in the core.
B) hydrogen in the outer layers is pure,and hydrogen must be mixed with helium,as it is in the core,in order to undergo fusion.
C) temperature in the outer layers is too low to allow the hydrogen nuclei to come close enough together to undergo fusion.
D) hydrogen atoms in the surface layers are not ionized,as they are in the core,and ionization is one requirement for nuclear fusion.

Question 71

Why are the majority of stars in the sky in the main-sequence phase of their lives?

A) This is the longest-lasting phase in each star's life.
B) Most stars die at the end of the main-sequence phase.
C) This is the only phase that is common to all stars.
D) Most stars in the sky were created at about the same time,so they are all in the same phase of their lives.

Question 72

Important information can be obtained from the H-R diagram of a star cluster because all stars in the cluster have approximately the same

A) Doppler shift.
B) age.
C) spectral type.
D) luminosity class.

Question 73

Which of the following statements about the rate of stellar evolution is TRUE?

A) The more massive the original star,the faster the evolution.
B) Star mass has no bearing upon stellar evolution because all stars evolve at the same rate,controlled by nuclear fusion and core temperature.
C) The chemical makeup of the original nebula is the major factor in deciding the rate of evolution.
D) The more massive the original star,the slower the evolution because there is more material for thermonuclear burning.

Question 74

The evolution of a star is controlled mostly by its

A) initial mass.
B) location in the galaxy.
C) surface temperature.
D) chemical composition.

Question 75

The age of a cluster can be found by

A) observing its position in the sky with respect to the Sun.
B) measuring its speed of motion relative to the Sun.
C) carrying out a number count of the stars in the cluster.
D) determining the turnoff point on the main sequence of its H-R diagram.

Question 76

What "safety valve" operates in the gas of normal (nondegenerate)stars?

A) If the star gets too big,it will collapse into a black hole.
B) If the stellar gas is suddenly heated,it will expand and cool.
C) If the pressure gets too high,electrons will combine with protons to relieve the pressure.
D) If thermonuclear reactions proceed too quickly,the star will run out of fuel before anything drastic happens.

Question 77

When a star leaves the main sequence and expands toward the red-giant region,what is happening inside the star?

A) Hydrogen burning is taking place in a spherical shell just outside the core;the core itself is almost pure helium.
B) Hydrogen burning is taking place in a spherical shell just outside the core;the core has not yet started thermonuclear reactions and is still mostly hydrogen.
C) Helium burning is taking place in a spherical shell just outside the core;the core itself is almost pure carbon and oxygen.
D) Helium is being converted into carbon and oxygen in the core.

Question 78

All stars on the main sequence

A) are at a late stage of evolution after the red-giant stage.
B) are changing slowly in size by gravitational contraction.
C) generate energy by hydrogen fusion in their centers.
D) have approximately the same age to within a few million years.

Question 79

After a star becomes a red giant,hydrogen fusion

A) no longer occurs.
B) occurs in the core.
C) occurs in a shell around the core.
D) occurs only during the helium flash.

Question 80

Over which of the following stages of stellar evolution does the radius of a star remain approximately constant?

A) Birth and initial formation
B) Red-giant phase
C) Asymptotic giant branch phase
D) Main-sequence phase