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                               Astronomy 105 - Homework 5 – Chapter 12 and 13
                                               Complete before May 25th at 9:00 AM

Chapter 12
1. The interstellar clouds called molecular clouds are                 10. Which of the following lists the stages of life for a low-mass
_________.                                                             star in the correct order?
   A) the hot clouds of gas expelled by dying stars                        A) Protostar, main-sequence star, red giant, planetary
   B) another name for Herbig-Haro objects                                       nebula, white dwarf
   C) clouds that contain little hydrogen and instead are made             B) Protostar, main-sequence star, red giant, supernova,
        predominantly of complex molecules such as carbon                        neutron star
        dioxide and sulfur dioxide                                         C) Protostar, main-sequence star, planetary nebula, red
   D) the cool clouds in which stars form                                        giant
                                                                           D) Main-sequence star, white dwarf, red giant, planetary
2. What is a protostar?                                                          nebula, protostar
   A) A star in its final stage of life
   B) A star that has planets                                          11. When a main-sequence star exhausts its core hydrogen
   C) An intermediate-mass star                                        fuel supply _________.
   D) A very young star (relative to its lifespan)                         A) the core shrinks while the rest of the star expands
                                                                           B) the entire star shrinks in size
3. What two physical forces oppose each other in a star-                   C) the core immediately begins to fuse its helium into
forming cloud?                                                                 carbon
    A) Gravity and thermal pressure.                                       D) the star becomes a neutron star
    B) Gravity and degeneracy pressure
    C) Thermal pressure and radiation pressure.                        12. The main source of energy for a star as it grows in size to
    D) Stellar winds and gravity.                                      become a red giant is _________.
                                                                          A) hydrogen fusion in the core
4. Which part of the electromagnetic spectrum generally gives             B) helium fusion in the core
us our best views of stars forming in dusty clouds?                       C) hydrogen fusion in a shell surrounding the central core
   A) Infrared              B) Optical.                                   D) gravitational contraction
   C) Ultraviolet.          D) Blue.
                                                                       13. The overall helium fusion reaction is _________.
5. What kind of gas cloud is most likely to give birth to stars?          A) two helium nuclei fuse to form one beryllium nucleus
   A) A hot, dense gas cloud.                                             B) three helium nuclei fuse to form one carbon nucleus
   B) A cold, dense gas cloud.                                            C) two hydrogen nuclei fuse to form one helium nucleus
   C) A cold, low-density gas cloud.                                      D) Four helium nuclei fuse to form one oxygen nucleus
   D) A hot, low-density gas cloud.
                                                                       14. What is a helium flash?
6. What temperature is the threshold for hydrogen fusion in a             A) A sudden brightening of a low-mass star, detectable
star?                                                                        from Earth by observing spectral lines of helium
    A) 10 billion K.      B) 10 thousand K                                B) The ignition of helium shell burning in a high-mass star
    C) 10 trillion K      D) 10 million K.                                   with a carbon core
                                                                          C) The sudden onset of helium fusion in the core of a low-
7. What can we learn about a star from a life track on an H-R                mass star
diagram?                                                                  D) It is another name for the helium fusion reaction.
   A) What surface temperature and luminosity the star will
       have at each stage of its life.                                 15. What is a planetary nebula?
   B) When the star was born.                                             A) Interstellar gas from which planets are likely to form in
   C) The star's current stage of life.                                      the not-too-distant future
   D) Where the star is located                                           B) Gas created from the remains of planets that once
                                                                             orbited a dead star
8. The vast majority of stars in a star cluster are _________.            C) Gas ejected from a low-mass star in the final stage of its
   A) about the same mass as our Sun                                         life
   B) very high-mass, type O and B stars                                  D) The remains of a high-mass star that has exploded
   C) red giants
   D) less massive than the Sun                                        16. The ultimate fate of our Sun is to _________.
                                                                          A) become a black hole
9. When does a star become a main-sequence star?                          B) explode in a supernova
   A) A star becomes a main-sequence star when it becomes                 C) become a rapidly spinning neutron star
      luminous enough to emit thermal radiation.                          D) become a white dwarf that will slowly cool with time
   B) A star becomes a main-sequence star when a piece of a
      molecular cloud first begins to contract into a star.
   C) A star becomes a main-sequence star when the rate of
      hydrogen fusion within the star's core becomes high enough to
      balance the rate at which the star radiates energy into space.
   D) A star becomes a main-sequence star the instant that hydrogen
      fusion first begins in the star's core.
17. Why is iron significant to understanding how a supernova       25. According to our modern understanding, what is a nova?
occurs?                                                               A) The sudden formation of a new star in the sky
   A) Iron is the heaviest of all atomic nuclei, and thus no          B) The explosion of a massive star at the end of its life
        heavier elements can be made.                                 C) An explosion occurring on the surface of a white dwarf
   B) Iron cannot release energy either by fission or fusion, so          in a close binary system
        a star with an iron core has no way to generate               D) A rapidly spinning neutron star
        additional energy to counteract the crush of gravity.
   C) Supernovae often leave behind neutron stars, which are       26. Suppose that a white dwarf is gaining mass because of
        made mostly of iron.                                       accretion in a binary system. What happens if the mass
   D) The fusion of iron into uranium is the reaction that         someday reaches the 1.4 solar mass limit?
        drives a supernova explosion.                                 A) The white dwarf will collapse in size, becoming a
                                                                           neutron star.
18. After a supernova explosion, the remains of the stellar core      B) The white dwarf will explode completely as a white
_________.                                                                 dwarf supernova.
   A) will always be a neutron star                                   C) The white dwarf will undergo a nova explosion.
   B) may be either a neutron star or a black hole                    D) The white dwarf will collapse to become a black hole.
   C) will always be a black hole
   D) may be either a white dwarf, neutron star, or black hole     27. A neutron star is _________.
                                                                      A) the remains of a star that died by expelling its outer
19. Why is Supernova 1987A particularly important to                      layers in a planetary nebula
astronomers?                                                          B) the remains of a star that died in a massive star
    A) It provided the first evidence that supernovae really              supernova (if no black hole was created)
       occur.                                                         C) a star made mostly of elements with high atomic mass
    B) It was the first supernova detected in nearly 400 years.           numbers, so that they have lots of neutrons
    C) It is the nearest supernova to have occurred at a time         D) an object that will ultimately become a black hole
       when we were capable of studying it carefully with
       telescopes.                                                 28. A typical neutron star is more massive than our Sun and
    D) It occurred only a few light-years from Earth.              about the size of _________.
                                                                      A) the Moon                               B) Earth
20. In order to predict whether a star will eventually fuse           C) a small asteroid (10 km in diameter)
oxygen into a heavier element, you mainly want to know what           D) Jupiter
fact about the star?
    A) The star's luminosity.                                      29. Pulsars are thought to be _________.
    B) The star's mass.                                               A) rapidly rotating neutron stars
    C) The star's constellation.                                      B) accreting white dwarfs
    D) The star's heavy element abundance.                            C) unstable high-mass stars
                                                                      D) accreting black holes
21. Our Sun is considered to be a _________.
   A) low-mass star        B) intermediate-mass star               30. What is the basic definition of a black hole?
   C) high-mass star       D) brown dwarf                             A) A black hole is a dead star that has faded from view.
                                                                      B) A black hole is an object with gravity so strong that not
22. Where would a brown dwarf be located on an H-R                       even light can escape.
diagram?                                                              C) A black hole is any object made from dark matter.
   A) It would be located in the upper right of the H-R               D) A black hole is a compact mass that emits no visible
       diagram.                                                          light.
   B) It would be located above and to the left of the highest
       part of the main sequence.                                  30. Based on current understanding, the minimum mass of a
   C) It would be located below and to the right of the lowest     black hole that forms during a massive star supernova is
       part of the main sequence.                                  roughly _________.
   D) It would be located in the lower left of the H-R                 A) 0.5 solar masses
       diagram.                                                        B) 1.4 solar masses
                                                                       C) 10 solar masses
Chapter 13                                                             D) 3 solar masses

23. The white dwarf that remains when our Sun dies will be         32. Imagine that our Sun were magically and suddenly replaced
mostly made of ______.                                             by a black hole of the same mass (1 solar mass). What would
   A) hydrogen.            B) helium.                              happen to Earth in its orbit?
   C) neutrons.            D) carbon.                                 A) Nothing --- Earth's orbit would remain the same.
                                                                      B) Earth would orbit faster, but at the same distance.
24. What is an accretion disk?                                        C) Earth would slowly spiral inward until it settled into an
   A) Any flattened disk, such as the disk of the Milky Way                orbit about the size of Mercury's current orbit.
      Galaxy                                                          D) Earth would almost instantly be sucked into oblivion in
   B) A disk of hot gas swirling rapidly around a white dwarf,             the black hole.
      neutron star, or black hole
   C) A disk around a young star in which planets can form
   D) A disk of material found around every white dwarf in the
      Milky Way Galaxy

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