Chapter 12

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fozzy2016
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209112
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Chapter 12
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2013-03-23 23:22:31
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Astronomy 105
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  1. the interstellar clouds called molecular clouds are _____.
    the cool clouds in which stars form.
  2. What is a protostar?
    A star the is still in the process of forming.
  3. Most interstellar clouds remain stable in size because the force of gravity is opposed by _____ within the cloud.
    Thermal pressure.
  4. which part of the electromagnetic swpectrum generally gives us our best views froming in dusty clouds?
    Infrared.
  5. what kind of gas cloud is most likely to give birth to stars?
    A cold, dense gas cloud.
  6. What phenomena is not commonly associated with the star formation process?
    Intense ultraviolet radiation coming from a protostar.
  7. When does a protostar become a main-sequence star?
    When the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space.
  8. Approximately what core temperature is required before hydrogen fusion can begin in a star?
    10 million K.
  9. Which star spends the longest timein the protostellar phase of life?
    A 1 solar mass star.
  10. What is the approximate range of masses that newborn main sequence stars can have?
    0.1 to 150 solar masses.
  11. The cast majority of stars in a newly formed star cluster are _____.
    Less massive than the Sun.
  12. What statement about brown dwarfs is not true?
    Brown dwarfs eventually collapse to become white dwarfs.
  13. What can we learn about a star form a life track on an H-R diagram?
    The surface temperature and luminosity the star will have at each stage of its life.
  14. List the stages of life for a low-mass star in the correct order?
    Protostar, main-sequence star, red giant, panetary nebula, white dwarf.
  15. What happens when a main-sequence star exhausts its core hydrogen fuel supply?
    The core shrinks while the rest of the star expands.
  16. The main source of energy for a star as it grows in size to become a red giant is _____.
    Hydrogen fusion in a shell surrounding the central core.
  17. The overall helium fusion reaction is:
    Three helium muclei fuse to form one carbon nucleus.
  18. What is a helium flash?
    The sudden onset of helium fusion in the core of a low-mass star.
  19. What is a planetary nebula?
    Gas ejected from alow-mass star in the final stage of its life.
  20. The ultimate fate of our sun is to _____.
    Become a white dwarf that will slowly cool with time.
  21. What is the CNO cycle?
    A set of steps by which four hydrogen nuclei fuse into one helium nucleus.
  22. In order to predict whether a star will eventually fuse oxygen into a heavier element, you mainly want to know what fact about the star?
    Its mass.
  23. Why is iron significant to understanding how a supernova occurs?
    Iron cannot release energy either by fission or fusion.
  24. After a supernova explosion, the remains of the stellar core _____.
    May be either a neutron star or a black hole.
  25. Why is supernova 1987A particularly important to astronomers?
    It is the nearest supernova to have occured at a time when we were capable of studying it carefully with telescopes.
  26. Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
    The two starts should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
  27. Which two processes can generate energy to help a star maintain its internal thermal pressure?
    Nuclear fusion and gravitational contraction.
  28. Our Sun is considered to be a _____.
    Low-mass star.
  29. What process is required to allow a gravitational-collapsing gas cloud to continue to collapse?
    The cloud must radiate much of its thermal energy.
  30. Angular momentum plays an important role in star formation. What characteristic of a protostellar system is probably not strongly affected by the star's angular momentum?
    The onset of core hydrogen fusion.
  31. How does the surface tempurature and luminosity of a protostar compare to the surface tempurature and luminosity of the main-sequence star it becomes?
    A main-sequence star is hotter and dimmer than it was as a protostar.
  32. Consider a large molecular cloud that will give birth to a cluster of stars. What would you expect to be true?
    a few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.
  33. We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?
    Brown dwarfs would outnumber all ordinary stars.
  34. Where would a brown dwarf be located on an H-R diagram?
    Below and to the right of the lowest part of the main-sequence.
  35. Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?
    H-R diagrams of globular clusters.
  36. why is a 1 solar-mass red giant more luminous than a 1 solar-mass main sequence star?
    Fusion reactions are producing energy at a greater rate in the red giant.
  37. Carbon fusion occur in high-mass stars but not in low-mass stars because _____.
    The cores of low-mass stars never get hot enough for carbon fusion.
  38. What statement about various stages of core nuclear burning in a high-mass star is not true?
    Each successive stage lasts for approximately the same amount of time. 
  39. What event marks the beginning of a supernova?
    The sudden collapse of an iron core into a compact ball of neutrons.
  40. Suppose that the star Betelgeuse were to supernova tomorrow. What would it look like to the naked eye?
    Betelgeuse would remain a dot of light, but would suddenly become so bright, that for a few weeks, we'd be able to see this dot in the daytime.
  41. Observations show that elements with atomic mass numbers divisble by 4 tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case.
    At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.
  42. A spinning neutron star has been observed at the center of a _____.
    Supernova remnant.
  43. What is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?
    Planetary nebula formation is more common.
  44. You discover a binary star system in which one star is a 15 MSun main-sequence star and the other is a 10 MSun giant. How do we think that a star system such as this might have come to exist?
    The giant must once have been the more massive star, but is now less massive because it transferred some of its mass to its companion.

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