Chapter 17 KEY

* marks the right answer

1.  Gravity would cause main-sequence stars to collapse if it were not for the opposing ______.
a.  strong nuclear forces.
b.  magnetic fields
*c.  thermal pressure.
d.  stellar winds.

2.  The interstellar clouds called molecular clouds are:
a.  clouds that contain little hydrogen and instead are made predominantly of complex molecules such as carbon dioxide and sulfur dioxide.
b.  the hot clouds of gas expelled by dying stars.
*c.  the cool clouds in which stars form.

3.  What is a protostar?
a.  An intermediate-mass star.
*b.  A very young star (relative to its lifespan).
c.  A star in its final stage of life.
d.  A star that has planets.

4.  The vast majority of stars in a newly formed star cluster are ______.
a.  about the same mass as our Sun.
*b.  less massive than the Sun.
c.  red giants.
d.  very high-mass, type O and B stars.

5.  When does a star become a main-sequence star?
a.  A star becomes a main-sequence star when it becomes luminous enough to emit thermal radiation.
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.

6.  Which of the following statements about brown dwarfs is NOT true?
a.  Brown dwarfs form like ordinary stars but are too small to sustain nuclear fusion in their cores.
b.  All brown dwarfs have masses less than about 8% that of our Sun.
c.  Brown dwarfs are supported by a type of pressure, called degeneracy pressure, that does not depend on the object's temperature.
*d.  Brown dwarfs will eventually collapse to become white dwarfs.

7.  Which of the following lists the stages of life for a low-mass star in the correct order?
a.  protostar, main-sequence star, red giant, supernova, neutron star
b.  main-sequence star, white dwarf, red giant, planetary nebula, protostar
c.  protostar, main-sequence star, planetary nebula, red giant
*d.  protostar, main-sequence star, red giant, planetary nebula, white dwarf

8.  When a main-sequence star exhausts its core hydrogen fuel supply:
a.  the core immediately begins to fuse its helium into carbon.
b.  the entire star shrinks in size.
c.  the star becomes a neutron star.
*d.  the core shrinks while the rest of the star expands.

9.  The overall helium fusion reaction is:

a.  Four helium nuclei fuse to form one oxygen nucleus.
b.  two helium nuclei fuse to form one beryllium nucleus.
*c.  three helium nuclei fuse to form one carbon nucleus.
d.  two hydrogen nuclei fuse to form one helium nucleus.

10.  What is a  helium flash?

*a.  The sudden onset of helium fusion in the core of a low-mass star.
b.  It is another name for the helium fusion reaction.
c.  A sudden brightening of a low-mass star, detectable from Earth by observing spectral lines of helium.
d.  The ignition of helium shell burning in a high-mass star with a carbon core.

11.   Which of the following statements about horizontal branch stars is NOT true?

*a.  Horizontal branch stars have inert (non-burning) carbon cores.
b.  In a particular star cluster, all horizontal branch stars have about the same luminosity.
c.  Horizontal branch stars have already been red giants.
d.  Both hydrogen fusion and helium fusion are occurring in horizontal branch stars.

12.    What is a planetary nebula?

a.  Gas created from the remains of planets that once orbited a dead star.
b.  The remains of a high-mass star that has exploded.
c.  Interstellar gas from which planets are likely to form in the not-too-distant future.
*d.  Gas ejected from a low-mass star in the final stage of its life.

13.     The ultimate fate of our Sun is to _____.

a.  explode in a supernova.
b.  become a black hole.
*c.  become a white dwarf that will slowly cool with time.
d.  become a rapidly spinning neutron star.

14.   What is the CNO cycle?

a.  The CNO cycle is the process by which carbon is fused into nitrogen and oxygen
b.  It is the series of fusion reactions that have produced all the carbon, nitrogen, and oxygen in the universe.
c.  The CNO cycle is the process by which helium is fused into carbon, nitrogen, and oxygen.
*d.  The CNO cycle is series of nuclear reactions with the final result being the fusion of four hydrogen nuclei into one helium nucleus.

15.    Why is iron significant to understanding how a supernova occurs?

a.  Iron is the heaviest of all atomic nuclei, and thus no heavier elements can be made.
*b.  Iron cannot release energy either by fission or fusion, so a star with an iron core has no way to generate additional energy to counteract the crush of gravity.
c.  The fusion of iron into uranium is the reaction that drives a supernova explosion.
d.  Supernovae often leave behind neutron stars, which are made mostly of iron.

16.        After a supernova explosion, the remains of the stellar core ______.

a.  will always be a neutron star.
b.  will always be a black hole.
c.  may be either a white dwarf, neutron star, or black hole.
*d.  may be either a neutron star or a black hole.

17.     Why is Supernova 1987A particularly important to astronomers? 

a.  It was the first supernova detected in nearly 400 years.
b.  It occurred only a few light-years from Earth.
*c.  It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.
d.  It provided the first evidence that supernovae really occur.

18.  Our Sun is considered to be a ______.

a.  brown dwarf
*b.  low-mass star
c.  high-mass star
d.  intermediate-mass star

19.  Close binary star systems are thought to form when _____.

*a.  a protostar is spinning so fast that it splits in two.
b.  the protostellar disk around a protostar has enough material to form a second star.
c.  a protostar emits two jets, each of which turns into a star.
d.  two interstellar gas clouds happen to contract very close to one another.

20.  Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?

a.  A main-sequence star is hotter and brighter than it was as a protostar.
b.  A main-sequence star is cooler and brighter than it was as a protostar.
*c.  A main-sequence star is hotter and dimmer than it was as a protostar.
d.  A main-sequence star is cooler and dimmer than it was as a protostar.

21.  Where would a brown dwarf be located on an H-R diagram?

a.  It would be located in the lower left of the H-R diagram.
b.  It would be located in the upper right of the H-R diagram.
*c.  It would be located below and to the right of the lowest part of the main sequence.
d.  It would be located above and to the left of the highest part of the main sequence.

22.  What process distributes most of the carbon in the universe?

a.  Supernovae
b.  Hot star winds
c.  Winds from main-sequence stars.
*d.  Winds from low-mass red giant stars.

23.  Which event marks the beginning of a supernova? 

a.  The onset of helium burning after a helium flash.
*b.  The sudden collapse of an iron core into a compact ball of neutrons.
c.  The sudden initiation of the CNO cycle.
d.  The beginning of neon burning in an extremely massive star.

24.  A spinning neutron star has been observed at the center of a ______.

a.  red supergiant.
b.  planetary nebula.
*c.  supernova remnant.
d.  protostar.