Deck 12: Evolution of Low-Mass Stars

Question 1

Once the core of a low-mass main-sequence star runs out of hydrogen,fusion in the star stops until the core temperature is high enough for helium fusion to begin.

Answer 1

False

Question 2

A star like the Sun will eventually become an electron degenerate white dwarf star.

Answer 2

True

Question 3

A 10 M _$\bigodot$ star will evolve through the same phases as a 1 M _$\bigodot$ star.

Answer 3

False

Question 4

The Sun eventually could become a nova.

Question 5

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

A) Temperature
B) Pressure
C) Mass
D) Radius

Question 6

The Sun will become a red giant star in about 2 billion years.

Question 7

When a star burns hydrogen in a shell,it will never produce as much energy (per unit time)as when it burns hydrogen in the core because the core has a higher temperature.

Question 8

Pressure from degenerate electrons keeps the core of a red giant star from collapsing.

Question 9

The percent of hydrogen in the Sun's core today is roughly half of what it had originally.

Question 10

In stellar fusion the term "ash" refers to:

A) dusty product of fire from oxidation occurring within the core of the star
B) the result of nuclear fusion that collects in the core
C) the result of nuclear fusion that collects in the outer layers of a star
D) dusty product of fire from oxidation occurring in the outer layers of the star

Question 11

A low-mass star that burns helium in its core and hydrogen in a shell is more luminous than a similar star that burns only hydrogen in a shell around a dead core.

Question 12

We can determine the age of a star cluster by measuring the color of the reddest red giant stars in the cluster.

Question 13

Binary stars can evolve to become novae and supernovae because slight differences in mass can mean large differences in main-sequence lifetimes.

Question 14

The more massive a star,the more hydrogen it has to burn,and the longer its main-sequence lifetime will be.

Question 15

Stars with masses similar to the Sun can lose more than 30 percent of their mass before they become white dwarfs.

Question 16

A Type Ia supernova is as luminous as 10 billion Suns.

Question 17

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

Question 18

Stars evolve primarily because they run out of fuel in their cores.

Question 19

The evolutionary cutoff between low- and high-mass stars occurs at approximately:

A) 0.5 M _$\bigodot$
B) 1 M _$\bigodot$
C) 4 M _$\bigodot$
D) 8 M _$\bigodot$

Question 20

A main-sequence star is unique because:

A) hydrostatic equilibrium exists at all radii
B) energy transport occurs via convection throughout much of its interior
C) hydrogen burning occurs in its core
D) it emits strong surface winds

Question 21

The luminosity of a star depends on:

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

Question 22

The Sun will likely stop being a main-sequence star in:

A) 5,000 years
B) 5 million years
C) 500 million years
D) 5 billion years

Question 23

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

A) 3 percent
B) 10 percent
C) 30 percent
D) 70 percent

Question 24

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

A) 0.5 M _$\bigodot$
B) 1 M _$\bigodot$
C) 3 M _$\bigodot$
D) 10 M _$\bigodot$

Question 25

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 remains of a star
D) A type of young,medium-mass star

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) This is a trick question.Hydrogen actually burns increasingly slower with time

Question 27

A low-mass 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

Question 28

What would you need to measure about a planetary nebula (see the image below)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

Question 29

When a G2 star leaves the main sequence:

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

Question 30

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

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

Question 31

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

Question 32

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

A) luminosity
B) chemical composition
C) magnetic field strength
D) rotation rate

Question 33

When a star depletes its core supply of hydrogen,__________ dominates in the core and __________ dominates in the atmosphere.

A) pressure; pressure
B) pressure; gravity
C) gravity; gravity
D) gravity; pressure

Question 34

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

Question 35

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 as ash in the core.

Question 36

About how long will a 2 M _$\bigodot$ star live as a main-sequence star?

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

Question 37

Degenerate refers to a state of matter at:

A) high temperature
B) high density
C) high luminosity
D) high mass

Question 38

Asymptotic giant branch (AGB)stars have high-mass loss rates because:

A) they are rotating quickly
B) they have weak magnetic fields
C) they have strong winds
D) they have low surface gravity

Question 39

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) Main sequence
B) Red giant
C) Horizontal branch
D) White dwarf

Question 40

The figure below plots the chemical composition of the Sun as a function of radius for three different times.Which graph represents the correct chemical composition just prior to the Sun becoming a red giant?

A) Figure A
B) Figure B
C) Figure C
D) No figure is correct

Question 41

When a low-mass star becomes an AGB star and has a temperature of 3300 K,in which wavelength range will it shine the brightest?

A) Visible
B) Infrared
C) X-ray
D) Radio

Question 42

A 1 M _$\bigodot$ star in a binary system could create the following chemical element and eject it into the interstellar medium:

A) carbon
B) helium
C) iron
D) All of the above

Question 43

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 elements processed in the core of the post-asymptotic giant branch star
D) primarily helium from the post-asymptotic giant branch star

Question 44

How many times longer does a 1.0 M _$\bigodot$ main-sequence star live compared to a 2.1 M _$\bigodot$ main-sequence star?

Question 45

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

Question 46

A Type Ia supernova has a luminosity of approximately:

A) 10 thousand L _$\bigodot$
B) 10 million L _$\bigodot$
C) 10 billion L _$\bigodot$
D) 10 trillion L _$\bigodot$

Question 47

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

A) one star is always larger 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 ages
D) the more massive binary star always gets more mass from the less massive binary star when both are main-sequence stars

Question 48

Which point on the figure below represents the location on the H-R diagram where the star is expelling mass creating a planetary nebula?

A) A
B) B
C) C
D) D

Question 49

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 which exceeds the Chandrasekhar limit
D) The collision of members of a binary system

Question 50

Which figure below represents the youngest star cluster?

A) Figure A
B) Figure B
C) Figure C
D) Figure D

Question 51

What is "degenerate" in the degenerate core of a white dwarf?

Question 52

Which figure below represents the oldest star cluster?

A) Figure A
B) Figure B
C) Figure C
D) Figure D

Question 53

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

A) 0.8 M _$\bigodot$
B) 1.4 M _$\bigodot$
C) 2.3 M _$\bigodot$
D) 5.5 M _$\bigodot$

Question 54

Which point on the figure below represents the location on the H-R diagram where the star is on the horizontal branch?

A) A
B) B
C) C
D) D

Question 55

You observed three different star clusters and found that the main-sequence turnoff stars in Cluster 1 had spectral type A,the main-sequence turnoff stars in Cluster 2 had spectral type B,and the main-sequence turnoff stars in Cluster 3 had spectral type G.Which star cluster is the youngest?

A) Cluster 1
B) Cluster 2
C) Cluster 3
D) It is impossible to determine their ages given only the spectral types.

Question 56

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 repulse each other strongly because they each contain six protons

Question 57

What ionizes the gas in a planetary nebula (see the image below)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

Question 58

The Ring Nebula (image below)is a planetary nebula that currently has a radius of 0.4 pc and an expansion velocity of 250 km/s.Approximately how long ago did its parent star die and eject its outer layers?

A) 1,600 years ago
B) 3,200 years ago
C) 5,400 years ago
D) 28,000 years ago

Question 59

In what two ways does temperature affect the rate of nuclear reactions?

Question 60

Suppose you measured H-R diagrams for the two star clusters pictured below. Which of the following statements is TRUE?

A) Cluster A is younger than Cluster B.
B) Cluster A is closer to us than Cluster B.
C) Both a and b are TRUE.
D) Both a and b are FALSE.

Question 61

Describe the triple-alpha process of nuclear fusion?

Question 62

Why are novae thought to be recurrent?

Question 63

Using the figure below,describe how the composition of the Sun varies by radius over time?

Question 64

What is the shortest phase of evolution for a one solar mass star that we can visibly see?

Question 65

When the Sun becomes an AGB star,its radius will be approximately 100 R _$\bigodot$ .If its mass at this point will be approximately the same as it is now,how will its surface gravity as an AGB star compare to its present surface gravity as a main-sequence star? Note that g = GM/R².

Question 66

Explain the significance of Roche lobes in a binary system.

Question 67

Why does helium burning in the core of a giant star not cause the star to become more luminous?

Question 68

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

Question 69

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

Question 70

Suppose you observe three star clusters.Cluster 1 has a main-sequence turnoff point at spectral type G,Cluster 2 has a turnoff point at spectral type A,and Cluster 3 has a turnoff point at spectral type B.Which cluster is youngest and which is oldest? Explain why.What is the approximate age of the oldest cluster?

Question 71

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 72

What types of chemical elements can low-mass stars contribute to the enrichment of the interstellar medium and how are they produced?

Question 73

The figure below shows the H-R diagram of stars from the cluster 47 Tucanae.Explain what the main-sequence turnoff is and how it can be used to determine the age of the cluster.