Chapter 15, Problem # 1
Lets take a rough approximation on the length of recorded history to be 10,000
years. The first convincing evidence that some nebulae are in fact remote
galaxies similar to our own (and thus that we live in an isolated galaxy which
is just one of many such systems) was Hubble's measurement of the distance to
the Andromeda galaxy, which was published in 1925. Thus, people have known
about distant galaxies for about 75 years. So, the length of time they have
not known about them is 10,000 - 75 = 9,925 years or 9,925/10,000 =
.99 = 99 % of recorded human history.
During most of recorded history, people thought that the universe was far
smaller than we now know it to be. Although we often do not know exactly
how large people thought the universe was, we know that they generally
thought that the Earth, planets, and stars they could see were the entire
extent of the universe. Also, they often believed that the stars and
planets were relatively small objects, and thus that they were rather
close to the Earth. Therefore, they viewed the "total universe" made up
of the Earth, planets, and stars to be probably smaller than what we have
more recently calculated to be the size of our solar system. And we now know
that our solar system in only a very small piece of our galaxy which is only
one among the many galaxies that are spread throughout the vastness of the
Chapter 15, Problem # 5
The present constellations will not be recognizable in Earth's sky 100 million
years from now, because the differential rotation of the galaxy, combined
with the random motions of the stars, causes the stars in our galaxy to
continually (but very slowly) change their positions relative to each other.
100 million years is a sufficiently long time for these slow changes in
the relative positions of the stars to disrupt the patterns of stars which
we call the constellations. (Note that the time for the Sun to make one
complete rotation about the center of the galaxy is 240 million years,
so 100 million years is a substantial fraction of that time.)
(a) If our solar system was very close to the galactic center, the night sky
would be full of very bright stars, since there is an extremely
high density of young, bright stars near the galactic center. Also, we would
see dark clouds of dust and glowing clouds of gas lying along the plane of
the galactic disk. We might also see the brightly glowing accretion disk
which would surround the black hole which may lie at the center of the galaxy.
(b) If our solar system was moved far above the galactic disk to a location
in the halo, the night sky would have very few bright stars, since the halo
contains a low density of stars and most of the stars which are there are
old, low luminosity stars. However, if you looked toward the center of the
galaxy, you would see an amazing view of the galaxy's disk and bulge. You
could see the disk's spiral arms, young star clusters, and gas and dust
clouds. At the center of the disk, you would see the bright bulge made up of
older red stars.
(c) If our solar system was at the center of a globular cluster, the night
sky would contain a large number of reddish main sequence stars distributed
uniformly across the sky, and producing combined starlight that was ten
times brighter than the full moon. Behind those stars, in the direction of
the galactic center, you would see a view of the galactic disk and bulge as
described in part (b).
(d) If our solar system was located near the center of the Orion star-forming
region, the night sky would be dominated by glowing ionized gas which would
be visible in all directions. You would also see some darker regions of
dust. Also, you might see some young bright stars which were just beginning
to emerge from their "cocoons" of dust and debris.
(e) If our solar system was moved to the edge of the milky way disk, most of
the night sky would be relatively dark, containing only a few stars. However,
in the direction of the galactic center, you would see a band of stars, gas,
and dust, since you would be looking at the galactic disk edge-on. This
band would extend only partway around the sky.
It would be unlikely to find a Sun-like star in the Orion Nebula, because
the stars there are younger than the Sun. If a star with the Sun's
mass formed in the Orion Nebula at the same time as the other stars present
there, it would not yet have had time to evolve onto the main sequence.
It would also be unlikely to find a Sun-like star in the halo, since most of
the halo stars are older than the main sequence lifetime of the Sun.
Chapter 15, Problem # 10
(a) If spectra showed that a faint distant galaxy was reddish with a spectrum
of K-type stars, you would expect it to be an elliptical galaxy. If spectra
showed that the galaxy was bluish with B- and A-type stars, you would expect
it to be an irregular galaxy.
(b)The dominance of young stellar features in irregular galaxies does not
prove that the galaxies themselves formed recently. It simply means that so
much star formation is occurring in these galaxies at the present time that
the light from the young stars being created dominates over the light of
whatever population of older stars might also exist within the galaxy.
The prominent light from star-forming regions comes from massive, hot,
blue stars, because these stars are far brighter that the less massive
stars which are forming in these regions at the same time.
Massive, hot, blue stars are not see outside the star-forming regions, because
their lifetimes are so short that they die before they have moved very far
from their birthplaces.
Chapter 16, Problem # 4
(a) If galaxies showed equal numbers of redshifts and blueshifts (uniformly
mixed in all directions), this would imply that the universe was static and
that the observed redshifts and blueshifts were due to the random motions
of the galaxies within the universe.
(b) If all the galaxies showed blueshifts with the size of the blueshift
increasing in proportion to the distance to the galaxy, this would imply
that the universe was collapsing.
(c) If all the galaxies on one half of the sky showed redshifts while
all the galaxies on the other half of the sky showed blueshifts, this would
imply that our galaxy is moving through a static universe (at a speed
greater than that of the random motions of the galaxies). This would
cause all the galaxies in front of us (in the direction our galaxy is
moving toward) to appear blueshifted, while all the galaxies behind us
(in the direction our galaxy is moving away from) would appear redshifted.
This configuration of blueshifted and redshifted galaxies could also
occur if our galaxy was sitting still while the rest of the universe was
flowing past it.
Chapter 16, Problem # 10
The current theory of the power source of quasars is that there is a
supermassive black hole at the center of a quasar. This rotating black
hole is surrounded by an accretion disk made of material which is rotating
around the black hole and falling toward it. The acceleration of this material
causes it to emit huge amounts of radiation.
The evidence for supermassive black holes in quasars includes the observation
that quasars' emission regions are very small, only about 10 times the
diameter of the solar system, yet they have very high luminosities of about
1012 to 1015 times the luminosity of the Sun. Thus,
normal stellar processes cannot be responsible for producing the quasar's
emission. Also, scientists have used the width of emission lines from the
hot gas near the quasars' cores to determine the velocity of this gas, which
then can be used to estimate the mass of the central portion of the quasars.
Comparing this mass to the energy output of the quasars, scientists find
that the energy source of the quasar is so efficient that it releases 10-30
percent of the mass-energy in the matter in the central region of the quasar.
Stellar fusion is again ruled out by these observations, since it is not
efficient enough. However, a rotating black hole can convert 20-30 percent
of the mass it accretes into energy. Thus, the evidence points to a
massive black hole as the likely candidate for the energy source of a
The evidence for massive black holes in some nearby galaxies includes the
observed central light peaks in these galaxies which could be caused by
either a black hole or by a very dense star cluster. More convincing is
the fact that spectroscopic measurements have shown sharp increases in
stellar velocities near the galactic nuclei, indicating a very large mass
concentrated within that small region. Combining these two measurements,
scientists find that the mass to light ratio within the nuclei of
these galaxies is higher than the ratio for ordinary stellar material,
indicating that the concentration of mass at the center of these galaxies
is not emitting light as stars do. Thus, a massive black hole is
the best candidate for what lies at the center of these galaxies.