Evolution Of Low-Mass Stars Test Questions & Answers Ch12

Chapter 12: Evolution of Low-Mass Stars

12.1 The Life of a Main-Sequence Star Follows a Predictable Path

12.1a Categorize stellar types by mass.

12.1b Describe the changes in a star’s structure during its lifetime on the main sequence.

12.1c Explain why the products of fusion (that is, helium created during main-sequence hydrogen fusion) do not immediately fuse into heavier elements.

12.2 A Star Runs Out of Hydrogen and Leaves the Main Sequence

12.2a Compare and contrast the behavior of normal and degenerate gases.

12.2b Understand how shell burning around an inert core drives a star up and to the right (for example, the red giant branch) of the H-R diagram.

12.3 Helium Begins to Burn in the Degenerate Core

12.3a Understand why the ignition of fusion in degenerate material leads to an explosion.

12.3b Explain how the horizontal branch is formed.

12.4 The Low-Mass Star Enters the Last Stages of Its Evolution

12.4a Compare and contrast the internal structure of red giant and asymptotic giant branch stars.

12.4b Understand how mass loss from an AGB star creates a planetary nebula and white dwarf.

12.4c Explain the sources of radiation from planetary nebulae and white dwarfs.

12.5 Binary Stars Sometimes Share Mass, Resulting in Novae and Supernovae

12.5a Explain how the transfer of mass occurs in a binary system of stars.

12.5b Describe the sequence of evolutionary stages in a close binary system with mass transfer.

12.5c Differentiate the causes and characteristics of novae and Type Ia supernovae.

12.6 Star Clusters Are Snapshots of Stellar Evolution

12.6a Relate the location of the cluster’s main-sequence turnoff on an H-R diagram to the cluster’s age.

Working It Out 12.1

Working It Out 12.1a Use the mass-luminosity relationship to relate how changing a star’s mass changes its luminosity.

Working It Out 12.1b Estimate the main-sequence lifetime of a star given its mass.

Chapter 12: Evolution of Low-Mass Stars

MULTIPLE CHOICE

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

a. 0.5 solar masses (M_). c. 4 solar masses (M_).

b. 1 solar masses (M_). d. 8 solar masses (M_).

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

a. temperature c. mass

b. pressure d. radius

3. In stellar fusion, the term ash refers to

a. 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. a dusty product of fire from oxidation occurring in the outer layers of the star.

4. 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.

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

a. 5,000 years. c. 500 million years.

b. 5 million years. d. 5 billion years.

6. The following figure plots the chemical composition of the Sun as a function of radius for three different times. Which graph represents the correct chemical composition at the end of its main-sequence lifetime?

a. Figure A c. Figure C

b. Figure B d. No figure is correct.

7. 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.

8. The luminosity of a main-sequence star depends on its

a. mass, its age, and its distance. c. age.

b. mass. d. mass and its age.

9. About how long will a 2-M_ star live as a main-sequence star?

a. 20 million years c. 20 billion years

b. 2 billion years d. 200 million years

10. What is the current composition of the Sun’s core?

a. 100 percent hydrogen

b. 65 percent hydrogen; 35 percent helium

c. 50 percent hydrogen; 50 percent helium

d. 35 percent hydrogen; 65 percent helium

11. A more massive main-sequence star means __________ nuclear reactions in the core, __________ luminosity, and a __________ lifetime.

a. slower; lower; shorter c. faster; lower; shorter

b. slower; lower; longer d. faster; higher; shorter

12. Which star spends the least time as a main-sequence star?

a. 0.5 M_ c. 3 M_

b. 1 M_ d. 10 M_

13. Which star spends the most time as a main-sequence star?

a. 0.5 M_ c. 3 M_

b. 1 M_ d. 10 M_

14. 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.

15. When a G2 star leaves the main sequence, its luminosity

a. and surface temperature both stay the same.

b. and surface temperature both decrease.

c. increases and its surface temperature decreases.

d. and surface temperature both increase.

16. Degenerate refers to a state of matter at high

a. temperature. c. luminosity.

b. density. d. mass.

17. What material is degenerate in a low-mass star as it evolves off the main sequence?

a. hydrogen c. neutrons

b. protons d. electrons

18. What is true about degenerate matter in a low-mass star as it evolves off the main sequence?

a. It occurs in the envelope.

b. It doesn’t expand as temperature increases.

c. Higher temperature causes higher pressure.

d. All are valid choices.

19. 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

20. As a red giant star evolves, hydrogen shell burning proceeds increasingly faster because of

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.

21. 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.

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

a. triple-alpha reaction; carbon c. triple-alpha reaction; oxygen

b. proton-proton chain; lithium d. proton-proton chain; iron

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

a. A c. C

b. B d. D

24. 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 c. horizontal branch

b. red giant d. white dwarf

25. 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. c. gravity.

b. density. d. temperature.

26. A star’s position along the horizontal branch phase is determined primarily by its

a. luminosity. c. magnetic field strength.

b. chemical composition. d. rotation rate.

27. Low-mass stars above what mass will skip the helium flash and just begin stable helium burning?

a. 0.5 M_ c. 1 M_

b. 0.75 M_ d. 2 M_

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

a. 3 percent c. 50 percent

b. 10 percent d. 70 percent

29. 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 remaining core of a star

d. a type of young, medium-mass star

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

a. A c. C

b. B d. D

31. A single 1-M_ star will eventually eject significant amounts of which of the following chemical elements into the interstellar medium?

a. hydrogen c. iron

b. nickel d. all of the above

32. As a solar-type star evolves off the main sequence, its core

a. and envelope expand. c. shrinks and envelope expands.

b. and envelope shrink. d. expands and envelope shrinks.

33. Asymptotic giant branch (AGB) stars have high mass-loss rates because they

a. are rotating quickly. c. have strong winds.

b. have weak magnetic fields. d. have low surface gravity.

34. What ionizes the gas in a planetary nebula (see the following image) 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

35. The gas in a planetary nebula is composed of

a. primarily hydrogen from the surrounding interstellar medium.

b. primarily hydrogen from the postasymptotic giant branch star.

c. hydrogen and elements processed in the core of the postasymptotic giant branch star.

d. primarily helium from the postasymptotic giant branch star.

36. The correct order of evolutionary stages for a solar mass star is

a. main sequence, asymptotic branch, horizontal branch, white dwarf.

b. asymptotic branch, main sequence, horizontal branch, white dwarf.

c. horizontal branch, main sequence, asymptotic branch, horizontal branch, white dwarf.

d. main sequence, horizontal branch, asymptotic branch, white dwarf.

37. Consider the following image of this planetary nebula. What measurements are needed to determine how long ago the parent star died?

a. mass of the white dwarf c. nebula’s temperature and radius

b. mass and radius of the white dwarf d. nebula’s radius and expansion velocity

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

a. visible c. X-ray

b. infrared d. radio

39. Which stage of a star’s evolution is the longest lived?

a. main sequence c. asymptotic giant branch

b. giant branch d. postasymptotic giant branch

40. The Ring Nebula (see the following image) is a planetary nebula that currently has a radius of 0.37 parsecs (pc) and an expansion velocity of 30 kilometers per second (km/s). Approximately how long ago did its parent star die and eject its outer layers?

a. 1,600 years ago c. 54,000 years ago

b. 12,000 years ago d. 280,000 years ago

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

a. 0.8 M_. c. 2.3 M_.

b. 1.4 M_. d. 5.5 M_.

42. A Type Ia supernova has a luminosity of approximately

a. 10,000 solar luminosities (L_). c. 10 billion solar luminosities (L_).

b. 10 million solar luminosities (L_). d. 10 trillion solar luminosities (L_).

43. What occurs if a white dwarf exceeds the Chandrasekhar limit?

a. Nothing; the white dwarf just gets bigger.

b. The white dwarf contracts to a smaller, hotter object.

c. A nova explosion occurs.

d. A Type Ia supernova explosion occurs.

44. One of the members in a binary system of low-mass stars 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 lifetimes.

d. the more massive binary star always gets more mass from the less massive binary star when both are main-sequence stars.

45. 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

46. The run away fusion in a Type Ia supernova will consume a large part of a white dwarf’s mass and produce

a. hydrogen and carbon. c. iron and nickel.

b. hydrogen and helium. d. helium and carbon.

47. What situation can result in a Type Ia supernova?

a. Two white dwarf’s merge.

b. Main-sequence star transfers enough mass to a white dwarf.

c. Red giant star transfers enough mass to a white dwarf.

d. All choices are valid.

48. 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 oldest?

a. cluster 1

b. cluster 2

c. cluster 3

d. It is impossible to determine their ages given only the spectral types.

49. Which of following figures represents the oldest star cluster?

a. Figure A c. Figure C

b. Figure B d. Figure D

50. Which of the following figures represents the youngest star cluster?

a. Figure A c. Figure C

b. Figure B d. Figure D

51. Suppose you measured H-R diagrams for the two star clusters pictured here.

Which of the following statements is TRUE?

a. Cluster A is younger and closer than Cluster B.

b. Cluster A is older and closer to us than Cluster B.

c. Cluster A is younger and farther than Cluster B.

d. Cluster A is older and farther than Cluster B.

1. Using the following figure, describe how the composition of the Sun varies by radius over time.

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

3. How many times longer does a 0.7-M_ main-sequence star live compared with a 1-M_main-sequence star?

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

5. Calculate the main-sequence lifetimes of the following stars of different spectral types:

B0 (18 M_), B5 (6 M_), A5 (2 M_), F5 (1.3 M_), and M0 (0.5 M_). What trend do you

notice in your results?

6. Examine the following figure. Calculate the approximate luminosity a 10 solar mass main-sequence star will have compared with the sun.

7. 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?

8. What particles are degenerate in a white dwarf?

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

10. Describe the triple-alpha process of nuclear fusion.

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

12. What is the shortest phase of evolution for a 1 solar mass star that we can visibly see?

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

14. When the Sun becomes an AGB star, its radius will be approximately 100 R_. 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 with its current surface gravity as a main-sequence star? Note that g GMR².

15. Label the various regions of an asymptotic giant branch star in the following figure.

16. Label the various regions of a horizontal branch star in the following figure.

17. Explain the significance of Roche lobes in a binary system.

18. Why are novae thought to be recurrent?

19. Why are Type Ia supernovae standard candles with luminosities that can vary by as much as a factor of 2?

20. 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?

21. The following figure 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.