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
Answer: b. The answer is not "a" because most of the gas and stars in
the disk are not in globular clusters.
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.
Answer: "c" is reasonable.
"a" is not sensible because gas can and does cool down
by emitting photons.
"b" is not sensible because the halo has old stars, which
do not make
ionization nebulae.
"c" is sensible because the spiral arms are where most new
new stars are made.
The most massive stars die after only
a couple million years and so are still in
or close to the spiral arm when they die.
"d" is not a very good answer, because "fountains" might
spray gas from the disk
into the halo.
3.) (Problem adapted from textbook:)
a.) Write several sentences to explain the differences between ionized
hydrogen,
atomic hydrogen, and molecular hydrogen.
Answer:
Ionized hydrogen: The hydrogen atoms have lost their electrons.
So, the gas
contains bare protons and electrons. Note that
the interstellar medium contains
more than just hydrogen; it contains
helium, carbon, etc.. Some of these atoms
have also lost some of their electrons.
Atomic hydrogen: The hydrogen atoms are neutral. Their electrons are
bonded
to them. Most atomic hydrogen is either warm (around 10,000 K)
or cool
(around 100 K).
Molecular hydrogen: The hydrogen atoms have bonded with each other to make
H2 molecules. If the interstellar medium is cool and
dense enough to form H2
molecules, then it will have been
cool and dense enough to have formed other
types of molecules, such as CO.
b.) What is the most common form of gas (i.e. ionized, atomic, or
molecular)
in the interstellar medium?
Answer: The most common form of interstellar gas is atomic hydrogen.
For example, most hydrogens are neutral rather than ionized or
bonded with
another atom to make a molecule. (A side note: it is
possible, and in fact
probable, that this gas doesn't take up the
majority of the space. That honor
goes to ionized gas.)
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?
Answer: Atomic hydrogen has an emission line whose wavelength is 21 cm.
4.) List 2 ways in which the galaxy changes (as time goes by)
as a result of the star-gas-star cycle.
Answer: (here are 3, though only 2 are required)
1.) The fraction of "metal" (defined in astronomy as any element
heavier
than lithium) atoms to hydrogen atoms in recently made stars
and the interstellar
medium increases as time goes by because of
the star-gas-star cycle.
2.) The numbers of brown dwarfs, white dwarfs, neutron stars
and black holes
increases.
3.) Assuming that there is no new gas coming into the galaxy,
the amount of mass
in the form of gas is decreasing (because
a small fraction is being converted into
brown dwarfs, white
dwarfs, neutron stars, and black holes.)
5.)
List 3 ways in which most stars in the halo are different from
most stars in the disk of the galaxy.
Answer:
1.) Most of the stars in the halo are very old. In contrast,
the disk of the galaxy
contains young, middle aged, and old
stars.
2.) The fraction of metal atoms to hydrogen atoms is usually
even smaller in halo
stars than in disk stars.
3.) The stars in the halo are on "random" orbits (I mean
that some orbit clockwise
while others orbit counterclockwise,
etc.) that take them far from the disk of the
galaxy,while the stars in the disk are orbiting
in the same direction and mostly
stay within the disk.
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
Answer: b
7.)
a.) What do we mean by dark matter? (I'm just looking for a
simple answer, here)
Answer: "Dark Matter" is mass that we know to exist, but that
we cannot see
because it either radiates very dimly or not at all.
b.) Explain why scientists think that the galaxy contains
dark matter.
Answer:
We can calculate the mass of stuff residing in the galaxy
within a sphere
of radius, r, from the orbital velocity of
the material at radius, r, as at makes its trek
around the
galaxy. A rotation curve refers to a plot of orbital velocity
as a function
of orbital radius. To calculate the mass of
the stuff residing within a sphere of
radius, r,
we just use Newton's version of Kepler's third
law (or some reworking of it),
the observed orbital velocity,
and the radius.
When
scientists do that to find the mass
of material in the galaxy
at various radii, they find that the calculated mass is much,
much larger than the mass they estimate from counting
stars, multiplying by the
mass/star, and adding in the mass of
the interstellar medium. The difference between
the amount of mass calculated from the rotation curve and the
amount of mass
estimated from stars and gas is called "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
Answer: d
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
Answer: b