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E:\2012-2013\SSU\PHS 207spring 2013\3rd test 4-8 Galaxies the Universe & Cosmology\ Answers (5pg)
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Milky Way Galaxy ESSAY QUESTIONS
1. Why are Cepheid variable stars important in our study of the Milky Way galaxy?
By monitoring a Cepheid star its distance from us can be calculated. Since we assume
that nearly all the stars in a cluster are nearly the same age, we can determine the
cluster’s age and distance ad some range of the galaxy.
2. What is the difference between an open cluster, an association, and a globular
cluster? In which components of the Galaxy does each lie?
In the disk portion of a galaxy
An open cluster is a group of up to a few thousand stars that are roughly the same age
and held together by gravitational attraction.
An association is a group of stars that are NOT gravitational bound together but follow
the same path through space.
In the nuclear bulge and halo
globular clusters contain 100,000 to 1 million tightly packed stars and contain some of
the oldest stars.
See Table 6.1 on page 152 for more specifics.
2.
What is the difference between a population I star and a population II star?
Population I stars are hottest (more blue) , most luminous, youngers, in spiral arms,
and contain heavier elements than the population II stars.
3.
Why does the average chemical composition of stars now forming differ from that of
older stars?
The younger stars contain heavier elements created in massive stars and gathered
from the supernovae of those stars. The older stars were created before the
supernovae released the heavier elements.
4.
How do theories of the origin of the Milky Way explain its halo?
A spiral galaxy like our Milky Way contain three visible parts: the nuclear bulge, a disk,
and a halo that surrounds the nuclear bulge. The halo with its globular clusters support
the idea that the Milky Way grew through the accretion of smaller galaxies.
5.
What evidence do we have that the center of our galaxy is a powerful source of
energy?
There are radio, infrared, and X-ray and gamma rays emanating from the center of the
Milky Way. There are probably visible energy too, but we can’t see them through the
molecular clouds and dust between the center and us.
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Galaxy ESSAY Questions
1.
Explain why good distance indicators must be luminous objects. Why must they be
unambiguously recognizable?
We need light or an energy source for luminosity that we can detect to know that the
object is present or detect its shadow like absorption nebulae.
From an object’s spectra we can determine its temperature & radial velocity (Doppler
shift) and its distance by Hubble’s law.
If there is any identification ambiguity, then the data might not be valid and unveritable
by other astronomers.
2.
Why can't evolution go from elliptical to spiral to irregular?
Elliptical galaxies contain mostly older stars while spiral and irregular galaxies contain
younger stars. Nature progresses physically from younger to older and not in the
reverse direction.
3.
What evidence do we have that large galaxies cannibalize small galaxies?
The Milky Way is believed to have merge or cannibalized small galaxies because of the
diversity in age and composition of the many star clusters in it. If the galaxy had all at
one time then the age and composition would be more uniform.
4.
Why is the number of galaxies in the Local Group uncertain?
There may be some galaxies in our Local Group that includes the Milky Way that can
not be detected because the galaxy ‘s luminosity is not detected or could be blocked by
brighter stars in the path to our detection.
AGN ESSAY questions
1. Our galaxy is not a radio galaxy, but it does emit radio energy. Explain
where this radio energy comes from.
The radio signal is theorized to be the result of a black hole when electrons
are accelerated to nearly the speed of light. Our galaxy, the Milky Way, has
a black hole, but it does not dominate the energy sources.
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2. Why should we expect quasars to be small? How small should we expect them to be?
Quasars are small (less than 1 light day) because its luminosity can change too quickly for a
larger object to achieve. They are believed to be the compact center of a massive black hole.
3. What observational evidence do we have that quasars are the centers of very distant
galaxies?
No other known phenomena can provide sufficient luminosity for us to detect at such
distances.
4. Describe a model of a quasar and show how the model explains the observed properties
of a quasar.
Models of a Quasar from
http://www.antonine-education.co.uk/Image_library/Physics_5_Options/Astrophysics/quasar_3.gif
From http://heasarc.gsfc.nasa.gov/docs/cgro/images/epo/gallery/agns/agn_inferred.gif
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5. What evidence do we have that particuliar galaxies are powered by black holes at their
centers?
No other know source could emit so much energy.
Universe & Cosmology ESSAY questions
1.
What is the difference between homogeneity and isotropy?
Each term describes uniformity in a different way.
Homogeneity is the distribution of matter is the same everywhere at any distance.
Isotropic is the distribution is uniform in anyl direction.
2.
What evidence do we have that the universe is expanding?
Astronomers (especially amateur ones) keep finding stars and galaxies that are moving
away from us, so the Universe is expanding.
3.
How can the Big Bang still be visible? In what form has it been detected?
According to present theory the radio frequency cosmic background radiation being
detected is the result of the cooling down of the Big Bang.
4.
How might we tell whether the Universe is open or closed?
If the Universe were closed the expansion of the Universe would be slowing down by
the force of gravity. Therefore the Universe is NOT closed.
Since the Universe expansion is not slowing , the Universe is open or flat.
5.
Why would a value of 100 km/sec/Mpc for the Hubble constant cause a revision
in theories of stellar evolution?
The age of the Universe is 1/H where H is the Hubble Constant. If the H were 100
km/sec/Mpc, then the known is around 10 billion years old. Yet we know that many
stars much older than 10 billion years and that others are 2nd and 3rd generation
stars that could NOT have developed in just 10 billion years
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7.
How could the inflationary universe solve the horizontal problem?
The horizontal problem points out that the observed uniformity is not possible in just
the Big Bang theory.
The inflationary theory allows the Big Bang theory to expand the observed uniformity
(the cosmological principle) to meet the objection of the horizontal problem. This
inflation to even out the properties of the Universe occurred in a very small time after
the Big Bang occurred. Look at the depiction on page 195 entitled “History of the
universe from the Big Bang to the present.)
8.
How do we know there was a big bang?
We don’t know for sure that there was a Big Bang, but at present the Big Bang theory
explains what has been observed.
THE END