Read Chapter 19
1.) Most of the gas and stars in the disk:
a.) orbit around the center of their local globular cluster, rather
than the center of the galaxy
b.) orbit the center of the galaxy; the material generally
travels in the same "sense" -- i.e. direction
c.) orbit the center of the galaxy; different stars and gas clouds
travel in different directions
d.) none of the above
2.) (Problem adapted from textbook:) Which one of the
following is sensible:
a.) Once gas has been heated by a supernova explosion or
stellar wind, it remains
hot indefinitely.
Thus, after all of the gas in the galaxy has been through
a supernova
explosion or wind, the galaxy will no longer
contain cold molecular cloud gas
(and stars) and so won't be able to form any new stars.
b.) Since the gas in the halo has a much lower density
than the gas in the galaxy's disk,
ionization nebulae (H II regions)
are much more common in the halo than in the disk.
c.) If you could watch a time-lapse movie of a spiral
galaxy over millions of
years, you would see many stars being
born inside of the spiral arms.
d.) The gas that is currently in the galactic disk has been in
the galactic disk since the galaxy
started to form.
3.) (Problem adapted from textbook:)
a.) Write several sentences to explain the differences between ionized
hydrogen,
atomic hydrogen, and molecular hydrogen.
b.) What is the most common form of gas (i.e. ionized, atomic, or
molecular)
in the interstellar medium?
c.) This chapter mentions that the hydrogen in
this gas ("this gas" = the most
common form of gas in the ISM) is
easy to observe at a particular wavelength.
What is it that wavelength?
4.) List 2 ways in which the galaxy changes (as time goes by)
as a result of the star-gas-star cycle.
5.)
List 3 ways in which most stars in the halo are different from
most stars in the disk of the galaxy.
6.) The spirals in spiral arms are:
a.) the remnants of smaller galaxies that merged when the Milky Way was
formed
b.) the portions of waves in which the gas density is greater than average
c.) streamers of gas ejected from the galactic center
d.) none of the above
7.)
a.) What do we mean by dark matter? (I'm just looking for a
simple answer, here)
b.) Explain why scientists think that the galaxy contains
dark matter.
8.) As the Milky Way formed:
a.) a spherical cloud of gas flattened into a disk-shaped cloud
b.) the galaxy's rotation speed increased
c.) stars were made, so that the earliest stars have different
orbits than the stars made later
d.) all of the above
e.) none of the above
9.) What is in the center of our galaxy and why do we think this?
a.) a star forming region is in the center of the galaxy and we think
this because we can see the
Herbig Haro objects created by the newly
formed stars' protostellar
jets.
b.) a very massive black hole is in the center of the galaxy and we
think this because we can
calculate the object's mass from the orbital motions
of stars near to it.
c.) an A-type main sequence star is in the center of the galaxy and we
think this
because we
can see the hydrogen absorption lines in its spectrum
d.) a neutron star is in the center of the galaxy and we think this
because we can see its pulses