Download Ch_16-18_Example_Exam

NAME: __________________________
Spring 2014 – ASTR-1020: Stellar Astronomy
Exam 4 – Chapters 16, 17, & 18
Circle your answer clearly AND print the CAPITAL LETTER to the left of the question.
There are 30 multiple choice (3 pts. each) and 1 discussion (10 pts. each) question. Good luck.
Multiple Choice
Identify the choice that best completes the statement or answers the question.
____
1. 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
e. color
____
2. 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.
____
3. 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.
a. 10 percent
b. 33 percent
c. 50 percent
d. 75 percent
e. 100 percent
____
4. You observe a 0.8 M white dwarf in a binary orbit around a main-sequence star of mass 1.4 M. Which of
the following is most likely the original mass of the star that became the white dwarf?
a. 0.5 M
b. 1 M
c. 0.8 M
d. 1.4 M
e. 3 M
____
5. 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.
e. Helium builds up everywhere in the star’s interior.
____
6. Place the following evolutionary stages in order from youngest to oldest.
a.
b.
c.
d.
e.
1, 2, 3
2, 3, 1
3, 2, 1
3, 1, 2
2, 1, 3
____
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. carbon burning occurs in its core
d. it emits strong surface winds
e. hydrogen burning occurs in its core
____
8. Which star spends the longest time as a main-sequence star?
a. 0.5 M
b. 1 M
c. 3 M
d. 6 M
e. 10 M
____
9. 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
____ 10. Degenerate refers to a state of matter at:
a. low density
b. high density
c. low luminosity
d. high luminosity
e. high temperature
____ 11. 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
e. hydrogen burning to carbon in a shell surrounding its core
____ 12. 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.
____ 13. 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
____ 14. What is the name of the nuclear reaction illustrated here?
a.
b.
c.
d.
e.
the proton-proton chain
the CNO cycle
beta decay
the triple-alpha process
the alpha-beta reaction
____ 15. 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
____ 16. What would you need to measure about 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
____ 17. In the CNO cycle, carbon is used a catalyst for the fusion of hydrogen to helium. This means that:
a. three helium nuclei fuse to form carbon
b. carbon facilitates the reaction but is not consumed in it
c. carbon boosts the energy from the reaction, which is why massive stars are luminous
d. carbon breaks apart into three helium nuclei
e. the reaction produces carbon nuclei in addition to helium
____ 18. As a high-mass main-sequence star evolves off the main sequence, it follows a: __________ on the H-R
diagram.
a. nearly vertical path
b. path of constant radius
c. roughly horizontal path
d. path of declining luminosity
e. path of increasing temperature
____ 19. The luminosity of a Cepheid star varies in time because:
a. the entire star pulsates from its core to its surface
b. the outer envelope of the star pulsates in radius
c. the star rotates too quickly
d. the star is too massive to be stable
e. the star undergoes large surface temperature fluctuations
____ 20. The main difference between Cepheid stars and RR Lyrae stars is:
a. their masses
b. that Cepheids form at much greater distances from Earth
c. that RR Lyrae were discovered much earlier than Cepheids
d. their pulsation mechanisms
e. that Cepheids obey a period-luminosity relation, but RR Lyraes do not
____ 21. If you measure the average brightness and pulsation period of a Cepheid variable star, you can also determine
its:
a. age
b. rotation period
c. distance
d. mass
e. composition
____ 22. The nuclear reaction that releases the most energy per kilogram is:
a. silicon fusing to iron
b. oxygen fusing to silicon
c. carbon fusing to magnesium
d. helium fusing to carbon
e. hydrogen fusing to helium
____ 23. Massive stars explode when they:
a. accrete mass from their binary star companion
b. generate uranium in their cores
c. merge with another massive star
d. run out of nuclear fuel in their core, and the cores collapse
e. lose a lot of mass in a stellar wind
____ 24. Type I and Type II supernovae can be distinguished by what combination of observations?
a. light curves and the detection of energetic cosmic rays
b. light curves and the detection of neutrons
c. light curves and the detection of radio pulses
d. spectra and light curves
e. spectra and X-ray emission
____ 25. Essentially all the elements heavier than iron in our Milky Way were formed:
a. by supernovae
b. during the formation of black holes
c. by fusion in the cores of the most massive main-sequence stars
d. during the formation of planetary nebulae
e. during the initial stages of the Big Bang
____ 26. A neutron star contains a mass of up to 3M in a sphere with a diameter approximately the size of:
a. an atomic nucleus
b. an apple
c. a school bus
d. a city
e. the Earth
____ 27. We can identify only a fraction of all the radio pulsars that exist in our galaxy because:
a. gas and dust efficiently block radio photons
b. few swing their beam of synchrotron emission in our direction
c. most have evolved to become black holes, which emit no light
d. massive stars are very rare
e. neutron stars have tiny radii, and are hard to detect even with large telescopes
____ 28. What characteristic of a star cluster is used to determine its age?
a. the chemical composition of stars in the cluster
b. the luminosity of the faintest stars in the cluster
c. the color of the main sequence turnoff in the cluster
d. the total number of stars in the cluster
e. the apparent diameter of the cluster
____ 29. What is the meaning of the Schwarzschild radius around a black hole?
a. It is the radius at which an orbiting object would show a precession.
b. It is the radius at which gravitational redshift can be detected.
c. It is the radius at which the black hole’s spin equals the speed of light.
d. It is the radius at which the escape velocity equals the speed of light.
e. It is the radius at which a body falling onto the black hole would move at half the speed of light.
____ 30. Even if a black hole emitted no light, we can still detect it:
a. from sound waves produced by material falling onto the black hole
b. by tides produced on the Earth’s oceans
c. through its Hawking radiation
d. through its gravitational effect on surrounding gas or stars
e. by looking for dark patches on the sky where the black hole swallows background light
Short Answer (10 pts.)
1. Describe the structure of a red giant star just before the helium flash takes place. How does this compare to
the structure of a horizontal-branch star?