Read Chapter 20
1.) List 3 ways in which elliptical galaxies differ from
spiral galaxies.
Answer:
1.) The stars in elliptical galaxies are on "random" orbits around
the center of the galaxy.
Thus, unlike the stars in the disk of
a spiral galaxy, they do not orbit in the same direction.
2.) Elliptical galaxies have very little gas
3.) Most of the stars in elliptical galaxies are old. In contrast,
spiral galaxies have old,
young, and middle-aged stars.
2.) Describe irregular galaxies.
Answer: The irregular galaxy category is a catch-all category
that includes galaxies that
don't fall into the elliptical,
lenticular, or spiral galaxies. They tend to have a blobby
appearance.
The blobs are regions that have many new stars.
Generally, irregular galaxies are making
new stars, thus these
galaxies must contain gas. So, irregular galaxies must have
enough
cold gas for the stars to form, in contrast with elliptical
galaxies, which don't have enough
cold gas for stars to form.
3.) How are galaxies distributed throughout the universe:
a.) Galaxies tend to be evenly spread throughout the universe
b.) Galaxies tend to reside near each other in groups or
clusters; these groupings are scattered
throughout the universe
c.) Most of the galaxies in the universe reside near each other in
a single large grouping; the
remaining galaxies are sparsely scattered throughout the universe
d.) None of the above
Answer: b
4.) "Main Sequence Fitting":
a.) Can be used to measure distance
b.) Compares the apparent brightness of one cluster of stars to that of
another
c.) Usually requires plotting measurements on
a Hertzsprung-Russell (H-R) diagram
d.) All of the above
e.) None of the above
Answer: d
5.) The "Tully-Fisher" Relation
a.) Says that a galaxy's rotation speed
is proportional to its
luminosity
b.) Can be used to find the apparent brightness of
elliptical galaxies
c.) Can only be applied accurately for galaxies within about
10,000 lightyears
d.) All of the above
e.) None of the above
Answer: a
6.)
a.)
What does the term "standard candle" mean in astronomy?
Answer:
A white dwarf star will explode in a supernova explosion
only if it acquires
enough material to make its total mass more than
1.4 times the mass of the Sun. So, all
white dwarf explosions have
the same mass (within an iota) and explode with the same energy.
Thus, their explosions have the same luminosity.
b.)
Write 2 or 3 sentences that explain why white dwarf supernovae
are good standard candles.
Answer:
We have learned the true (intrinsic) luminosities of
white dwarf supernovae by
1.) measuring the apparent brightnesses
of the nearest white dwarf supernovae that have been
observed,
2.) measuring the distances to their host galaxies, and then 3.) using
the formula:
apparent brightness = luminosity / (4 pi distance2)
to calculate their luminosities. So, we know
the luminosities of white dwarf supernovae. When a new
white dwarf supernovae happens,
we can measure its apparent brightness and then use the
above formula to calculate the distance
to the supernova explosion and its host galaxy
(now that we know its luminosity of the SN).
7.) (problem adapted from the textbook)
Using modern telescopes, we can see some of the
Cepheid variable stars in the galaxy M100.
Astronomers measured the apparent brightness of
one of the Cepheid variable stars; it is
3.8 x 10-19 watt/m2. From
the period and the
period-luminosity relationship, they calculated that the
luminosity is 1.2 x 1030 watts.
Use the apparent brightness vs luminosity formula
(it is presented in "Mathematical Insight 20.1")
to calculate the distance between the Earth and
the Cepheid variable star in M100 (i.e. the
distance between the Earth and M100).
Answer: distance = square root of (luminosity/
(4 x pi x apparent brightness))
So, the distance = 5.0 x 1023
meters = 5.3 x 107 lightyears.
8.) As we look at galaxies at further and further distances,
and ignoring the "noise" caused by random
motion, we find that:
a.) The more distant a galaxy is, the larger it looks to us
b.) The more distant a galaxy is, the bluer its light looks to us
c.) The more distant a galaxy is, the faster it is moving away from us
d.) All of the above
e.) None of the above
Answer: c
9.)
a.) Write the equation for Hubble's Law.
Answer: v = Ho times d
b.) Suppose that you lived in a different group of galaxies than our
own and suppose that you
observed other groups of galaxies.
Do you think that they would be stationary or moving
relative to your galaxy?
Answer: They would be moving
c.) -- if they are moving, which way would they be moving (toward or
away)?
or, if they are stationary, why are they stationary?
Answer: They would be moving away from my own galaxy.
10.) If you lived in a galaxy far, far away, and your civilization
were able to make the same types of observations
and calculations that
humans have made, (ignoring the "noise" caused by random motions)
you would think that
a.) Other galaxies are moving away from you at a speed proportional to
their distance from you
b.) Other galaxies are moving toward you at a speed proportional to their
distance from Earth
c.) The galaxies between you and Earth are moving toward you and the
galaxies on the far side
of you or Earth are moving away from you
d.) None of the above
Answer: a
11.)
What is a "cosmological redshift"?
Answer: The universe expands because the space occupied by the universe
expands. As the space expands,
it carries material with it. Also,
as the universe expands, it stretches the light within it.
The photons are stretched
to longer wavelengths
than their original wavelengths. Thus, they are "redshifted"
12.) Which one of the following is true?
a.) The age of the universe is approximately equal to Ho
b.) The age of the universe is approximately equal to Ho2
c.) The age of the universe is approximately equal to 1/Ho
d.) The age of the universe is approximately equal to 1/(Ho2)
Answer: c