Deck 16: Evolution of Low-Mass Stars

Question 1

A feature unique to main-sequence stars is that

A) hydrostatic equilibrium exists at all radii.
B) energy transport occurs via convection throughout much of their interiors.
C) carbon burning occurs in their cores.
D) they emit strong surface winds.
E) hydrogen burning occurs in their cores.

Answer 1

hydrogen burning occurs in their cores.

Question 2

As a main-sequence star burns its core supply of hydrogen, what happens?

A) Helium begins to fuse throughout the core.
B) Helium fuses in a shell surrounding the core.
C) Helium fusion takes place only at the very center of the core, where temperature and pressure are highest.
D) Helium builds up in the core.
E) Helium builds up everywhere in the star's interior.

Answer 2

Helium builds up in the core.

Question 3

A 10 M_SUN star will evolve through the same phases as a

A) 1 M_SUN star.
B) 5 M_SUN star.
C) 20 M_SUN star.
D) 0.5 M_SUN star.
E) 0.08 M_SUN star.

Answer 3

20 M_SUN star.

Question 4

The Sun will become a red giant star in about

A) 15 billion years.
B) 10 billion years.
C) 5 billion years.
D) 1 billion years.
E) 50 million years.

Question 5

The ________ a main-sequence star is, the more hydrogen it has to burn, and the ________ its main-sequence lifetime.

A) more massive; shorter
B) more massive; longer
C) less massive; shorter
D) less massive; longer
E) larger; longer

Question 6

Stars evolve primarily because

A) they convert all their mass to energy.
B) they lose all their mass into space.
C) their core temperatures decrease steadily as they evolve.
D) they use up the fuel in their cores.
E) they run out of rotational energy.

Question 7

If a main-sequence star's core temperature increased, fusion reaction rates would ________ because the protons would be moving ________.

A) decrease; slower
B) increase; faster
C) increase; slower
D) remain unchanged; the same speed as before
E) decrease; faster

Question 8

The percentage of hydrogen in the Sun's core today is roughly ________ of what it was originally.

A) half
B) one-third
C) one-quarter
D) 10 percent
E) less than 1 percent

Question 9

The low-end mass cutoff for stars occurs at approximately

A) 0.08 M_SUN.
B) 0.1 M_SUN.
C) 0.8 M_SUN.
D) 1 M_SUN.
E) 10 M_SUN.

Question 10

The largest low-mass stars have masses that are approximately

A) 1.5 M_SUN.
B) 1 M_SUN.
C) 3 M_SUN.
D) 5 M_SUN.
E) 10 M_SUN.

Question 11

How long will a 2 M_SUN star live as a main-sequence star?

A) 12 million years
B) 180 million years
C) 1.8 billion years
D) 12 billion years
E) 18 billion years

Question 12

The luminosity of a star depends on

A) its mass and age.
B) its mass.
C) its age.
D) its distance.
E) its mass, age, and distance.

Question 13

What factor is most important in determining a star's position on the main sequence and its subsequent evolution?

A) temperature
B) pressure
C) mass
D) radius
E) color

Question 14

If the Milky Way formed stars at approximately a constant rate over the last 14 billion years, what fraction of the M-type stars that ever formed in it can still be found as main-sequence stars today? (Note that M-type stars have a mass of approximately 0.5 M_SUN.)

A) 10 percent
B) 33 percent
C) 50 percent
D) 75 percent
E) 100 percent

Question 15

Using the data in the figure shown below, identify the spectral type of a star that has a main-sequence lifetime of about 500 billion years.

A) A5
B) F5
C) K0
D) G2
E) M5

Question 16

The main-sequence lifetime of a star is given by which equation?

A) t M/L
B) t L/M
C) t M ²/L
D) t L²/L
E) t M/L²

Question 17

Use the figure shown below and the relationship t $\infty$ M/L to estimate the main-sequence lifetime of a star with a mass equal to 10 times that of the Sun.(Note that the Sun's main-sequence lifetime is about 10¹⁰ years.)

A) 3 million years
B) 30 million years
C) 300 million years
D) 3 billion years
E) 30 billion years

Question 18

Which star spends the longest time as a main-sequence star?

A) 0.5 M_SUN
B) 1 M_SUN
C) 3 M_SUN
D) 6 M_SUN
E) 10 M_SUN

Question 19

As a low-mass main-sequence star runs out of fuel in its core, it grows more luminous. How is this possible?

A) It explodes.
B) It begins to fuse helium in the core.
C) The core expands as it runs out of fuel.
D) The core shrinks, bringing more hydrogen fuel into the fusing region.
E) Convection takes place throughout the interior, bringing more fuel to the core.

Question 20

When a star depletes its core supply of hydrogen, the core will

A) explode.
B) spin faster.
C) merge with the outer layers.
D) shrink.
E) immediately begin fusing helium into carbon.

Question 21

When a star is stably fusing helium in its core, it is a ________ star.

A) main-sequence
B) asymptotic giant branch
C) helium flash
D) horizontal branch
E) white dwarf

Question 22

When matter is packed so tightly together that quantum physics will not allow the electrons to get any closer, it is called

A) electron-degenerate.
B) neutron-degenerate
C) nucleon-degenerate.
D) quantum-limited.
E) exotic matter.

Question 23

During evolutionary phase A in the figure shown below, the star is ________.In evolutionary phase B, it is ________.

A) expanding; expanding
B) expanding; contracting
C) contracting; losing mass
D) contracting; contracting
E) gaining mass; contracting

Question 24

Helium burns in the core of a horizontal branch star via the ________ and produces ________.

A) triple-alpha reaction; carbon
B) proton-proton chain; lithium
C) triple-alpha reaction; oxygen
D) proton-proton chain; iron
E) proton-proton chain; calcium

Question 25

The following figure illustrates the

A) proton-proton chain.
B) process of electron-degeneracy.
C) beryllium decay process.
D) alpha decay chain.
E) triple-alpha process.

Question 26

As a red giant star evolves, hydrogen shell burning proceeds increasingly faster due to

A) rotational energy from the star's rapid rotation.
B) heat released from the core's contraction.
C) pressure from the contracting envelope.
D) release of energy stored in magnetic fields.
E) energy from the fusion of heavier elements.

Question 27

As a subgiant star becomes a red giant, its luminosity increases while its temperature remains approximately constant.What does this mean?

A) The radius is decreasing.
B) The radius is increasing.
C) The star is getting hotter.
D) The star is losing mass.
E) The star is rotating more slowly.

Question 28

A 1-M_SUN red giant star's energy comes from

A) hydrogen burning to helium in its core.
B) helium burning to carbon in its core.
C) hydrogen burning to helium in a shell surrounding its core.
D) helium burning to carbon in a shell surrounding its core.
E) hydrogen burning to carbon in a shell surrounding its core.

Question 29

A low-mass star that burns helium in its core and hydrogen in a shell surrounding the core is ________ than a similar star that burns hydrogen only ________.

A) more luminous; in its core
B) more luminous; in a shell around a degenerate core
C) less luminous; in a shell around a degenerate core
D) less luminous; in its core
E) The relative luminosities depend strongly on the mass of the star.

Question 30

A star's surface temperature during the horizontal branch phase is determined primarily by its

A) luminosity.
B) mass and chemical composition.
C) magnetic field strength.
D) rotation rate.
E) radius.

Question 31

When a star burns hydrogen in a shell, how does the energy released compare with when the star burned hydrogen in the core?

A) There will be an equal amount of energy produced.
B) There will be less energy produced.
C) There will be more energy produced.
D) There will be more energy produced only if the star's mass exceeds that of the Sun.
E) There will be less energy produced only if the star's mass is less than that of the Sun.

Question 32

If there were mixing processes in a main-sequence star with a radiative zone (there aren't) that churned up all the material in the interior, we would expect that the main-sequence lifetime would be ________ because ________.

A) shorter; the star would turn into a giant faster
B) shorter; the star would burn hydrogen faster and have a higher luminosity
C) longer; helium nuclei have a higher mass than hydrogen nuclei
D) shorter; the star would never turn into a red giant
E) longer; more hydrogen would be available to burn

Question 33

________ keeps the core of a red giant star from collapsing.

A) Pressure from protons
B) Pressure from neutrons
C) Radiation pressure from hydrogen fusion
D) Degenerate pressure from electrons
E) Angular momentum

Question 34

Place the evolutionary stages shown in the figure below in order from earliest to latest.

A) 1, 2, 3
B) 2, 3, 1
C) 3, 2, 1
D) 3, 1, 2
E) 2, 1, 3

Question 35

When helium fusion begins in the core of a red giant star, the situation quickly gets out of control because electron-degeneracy pressure does not respond to changes in

A) luminosity.
B) density.
C) gravity.
D) temperature.
E) magnetic field strength.

Question 36

Degenerate refers to a state of matter at

A) low density.
B) high density.
C) low luminosity.
D) high luminosity.
E) high temperature.

Question 37

When a spectral-type G2 star like the Sun leaves the main sequence, its

A) luminosity and surface temperature both stay the same.
B) luminosity and surface temperature both decrease.
C) luminosity increases and its surface temperature decreases.
D) luminosity and surface temperature both increase.
E) luminosity decreases and its surface temperature increases.

Question 38

The figure below depicts helium fusion in a late-stage star with certain elements labeled A, B, and C.What are these elements, in the order A, B, C?

A) hydrogen, helium, carbon
B) hydrogen, lithium, beryllium
C) helium, carbon, iron
D) helium, beryllium, carbon
E) hydrogen, hydrogen, carbon

Question 39

Using the figure shown below, identify the star with the smallest radius.

A) star A
B) star B
C) star C
D) star D
E) star E

Question 40

During which phase of the evolution of a low-mass star does it have two separate regions of nuclear burning occurring in its interior?

A) pre-main sequence
B) main sequence
C) red giant
D) horizontal branch
E) white dwarf

Question 41

When a star leaves the main sequence, which of the following is a possible outcome for a planet orbiting that star?

A) The planet may be engulfed by the expanding star.
B) The planet may move to a more distant orbit.
C) The planet may move to a less distant orbit.
D) The planet may be destroyed by the star's tidal forces.
E) All of these are possibilities.

Question 42

A low-mass main-sequence star's climb up the red giant branch is halted by

A) the end of hydrogen shell burning.
B) the beginning of helium fusion in the core.
C) electron-degeneracy pressure in the core.
D) instabilities in the star's expanding outer layers.
E) an explosion that destroys the star.

Question 43

A particular asymptotic giant branch star has approximately the same mass as the Sun but 100 times its radius.Compared with the Sun, what is the escape velocity from that star?

A) 0.01 times that of the Sun
B) 0.1 times that of the Sun
C) the same as that of the Sun
D) 10 times that of the Sun
E) 100 times that of the Sun

Question 44

What is a planetary nebula?

A) a planet surrounded by a glowing shell of gas
B) the disk of gas and dust surrounding a young star that will soon form a star system
C) the ejected envelope of a giant star surrounding the remnant of a star
D) a type of young, medium-mass star
E) leftover gas from a supernova explosion

Question 45

A star like the Sun will eventually become a(n) ________ star.

A) proton-degenerate brown dwarf
B) neutron-degenerate black hole
C) electron-degenerate white dwarf
D) electron-degenerate red dwarf
E) type M main-sequence

Question 46

Asymptotic giant branch stars have high mass-loss rates because they

A) are rotating quickly.
B) have weak magnetic fields.
C) have strong magnetic fields.
D) have low surface gravity.
E) have high surface temperatures.

Question 47

In a white dwarf, what is the source of pressure that halts its contraction as it cools?

A) thermal pressure of the extremely hot gas
B) electrons packed so closely that they become incompressible
C) neutrons that resist being pressed further together
D) carbon nuclei that repel each other strongly because they each contain six protons
E) rapid rotation

Question 48

Asymptotic giant branch stars have ________ luminosities, ________ radii, and ________ escape velocities.

A) large; large; large
B) large; small; large
C) large; large; small
D) small; large; small
E) small; small; large

Question 49

A star like the Sun will lose ________ of its mass before it evolves to become a white dwarf.

A) almost none
B) about half
C) about 90%
D) the vast majority (about 99%)
E) absolutely none

Question 50

What ionizes the gas in a planetary nebula and makes it visible?

A) X-ray photons emitted by a pulsar
B) ultraviolet photons emitted by a white dwarf
C) the shock wave from a supernova
D) hydrogen burning in the nebular gas
E) infrared photons from a nova explosion

Question 51

You observe a 0.8 M_SUN white dwarf in a binary orbit around a main-sequence star of mass 1.4 M_SUN.Which of the following is most likely the original mass of the star that became the white dwarf?

A) 0.5 M_SUN
B) 1 M_SUN
C) 0.8 M_SUN
D) 1.4 M_SUN
E) 3 M_SUN

Question 52

Even while the Sun is still on the main sequence, it may eventually threaten life on Earth due to its

A) increasing size.
B) increasing luminosity.
C) decreasing luminosity.
D) more energetic solar flares.
E) decreasing size.

Question 53

One star in a binary will almost always become a red giant before the other because

A) one star is always larger in radius than the other.
B) binaries always have one star twice as massive as the other.
C) small differences in main-sequence masses yield large differences in main-sequence lifetimes.
D) the more massive binary star always gets more mass from the less massive binary star when both are main-sequence stars.
E) one star always spins faster than the other.

Question 54

As a white dwarf star gradually cools, its radius stays approximately constant.What is happening to the white dwarf's luminosity?

A) It stays the same.
B) It increases.
C) It increases and then decreases periodically.
D) It decreases.
E) You can't tell from the information given.

Question 55

The Sun eventually will become a

A) nova.
B) neutron star.
C) black hole.
D) white dwarf.
E) type M main-sequence star.

Question 56

An asymptotic giant branch star loses mass to the interstellar medium because

A) it explodes violently.
B) it has low surface gravity.
C) collisions with particles in the interstellar medium strip away gas.
D) its rotation speed increases.
E) its helium core has less gravitational attraction than a hydrogen one.

Question 57

The gas in a planetary nebula is composed of

A) primarily hydrogen from the surrounding interstellar medium.
B) primarily hydrogen from the post-asymptotic giant branch star.
C) hydrogen and heavier elements like helium and carbon processed in the core of the post-asymptotic giant branch star.
D) primarily helium from the post-asymptotic giant branch star.
E) carbon and helium from the nuclear reactions that took place on the horizontal branch.

Question 58

The core of an asymptotic giant branch star is filled with

A) hydrogen gas.
B) electron-degenerate hydrogen.
C) helium gas.
D) electron-degenerate helium.
E) electron-degenerate carbon.

Question 59

What is the escape velocity from the surface of a 1 M_SUN AGB star that has a radius of 100 R_SUN?

A) 60 km/s
B) 120 km/s
C) 240 km/s
D) 620 km/s
E) 800 km/s

Question 60

What would you need to measure in a planetary nebula to determine how long ago its parent star died?

A) the mass of the white dwarf
B) the mass and radius of the white dwarf
C) the nebula's temperature and radius
D) the nebula's radius and expansion velocity
E) the composition of the gas in the nebula

Question 61

The figure below depicts the evolution in chemical composition of the Sun's interior at three different times during its life.How do the relative percentages of hydrogen and helium change at a fractional radius of 0.1 solar radii, from the time the Sun formed until it exhausts its core supply of hydrogen in 5 billion years?

Question 62

A 1-M_SUN star in a binary system could eventually create which chemical element and eject it into the interstellar medium?

A) carbon
B) helium
C) iron
D) all of these
E) none of these

Question 63

Explain the two different forms of pressure that support the core of a low-mass main-sequence star and the core of a low-mass red giant star.

Question 64

Why does the core of a main-sequence star have to be hotter to burn helium into carbon than hydrogen into helium?

Question 65

Calculate the main-sequence lifetimes of the following stars of different spectral types: B0 (18 M_SUN), B5 (6 M_SUN), A5 (2 M_SUN), F5 (1.3 M_SUN), and M0 (0.5 M_SUN).What trend do you notice in your results?

Question 66

A Type Ia supernova occurs when a white dwarf exceeds a mass of

A) 0.8 M_SUN.
B) 1.4 M_SUN.
C) 2.3 M_SUN.
D) 5.4 M_SUN.
E) 10 M_SUN.

Question 67

How can the core of a star be degenerate with respect to the electrons but nondegenerate with respect to the nuclei?

Question 68

Novae and Type Ia supernovae can occur in binary star systems because ________ can mean large differences in their ________.

A) large differences in the stars' temperatures; sizes
B) small differences in the stars' masses; main-sequence lifetimes
C) small differences in the stars' sizes; main-sequence lifetimes
D) small differences in the stars' separation; main-sequence lifetimes
E) large differences in the stars' masses; separation

Question 69

What are the three major mass ranges for stars?

Question 70

How many times longer does a 2 M_SUN main-sequence star live compared to a 10 M_SUN main-sequence star?

Question 71

Describe the structure of a red giant star just before the helium flash takes place.How does this compare with the structure of a horizontal branch star?

Question 72

Considering how long it took for life to arise on Earth, which of the stellar spectral types shown below would be the LEAST likely to have planets with life?

A) B0
B) G2
C) K0
D) M0
E) M5

Question 73

Consider a 1 M_SUN star's journey up the red giant branch.Its luminosity will change from 10 L_SUN to nearly 1,000 L_SUN.How will its temperature and radius change as the star ascends? (Recall that
L R²T⁴.)

Question 74

A nova is the result of which explosive situation?

A) mass transfer onto a white dwarf
B) helium burning in a degenerate stellar core
C) a white dwarf that exceeds the Chandrasekhar limit
D) the collision of members of a binary system
E) runaway nuclear reactions in the core

Question 75

Explain what the triple-alpha process is and when it takes place in evolving stars.

Question 76

If an 0.8 M_SUN white dwarf could accrete matter from a binary companion at a rate of 10^-9 M_SUN/yr, how long would it take before it exploded as a Type Ia supernova?

A) 600,000 years
B) 20 million years
C) 200 million years
D) 600 million years
E) 1 billion years

Question 77

What are two ways that Type Ia supernovae can be produced?

A) mass transfer and stellar mergers
B) helium flash and stellar mergers
C) mass transfer and helium flash
D) helium fusion and mass transfer
E) iron fusion and mass transfer

Question 78

The Roche lobe of a star is the

A) distance from the star where planets will be destroyed.
B) maximum distance between stars in a binary system.
C) limit to the star's radius before it is officially a red giant.
D) region around that star where its gravity is dominant.
E) maximum distance within which all planets will be tidally locked to the star.

Question 79

A Type Ia supernova can be as luminous as

A) 10 billion suns.
B) 1 billion suns.
C) 1 million suns.
D) 1,000 suns.
E) 10 suns.

Question 80

If a main-sequence star were gaining mass by being in an interacting binary system, what would happen to that star's luminosity and why?

A) The luminosity would increase because the star would become a nova.
B) The luminosity would increase because the star's central pressure would rise and the rate of nuclear reactions would increase.
C) The luminosity would decrease because the outgoing energy has to pass through more layers in the star.
D) The luminosity would decrease because high-mass stars are fainter.
E) The luminosity would decrease because the star would quickly turn into a white dwarf.