Download PHYSICS 1500 - ASTRONOMY TOTAL: 100 marks Section A Please

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THE UNIVERSITY OF SYDNEY
SCHOOL OF PHYSICS
PHYSICS 1500 - ASTRONOMY
NOVEMBER 2006
Time allowed: TWO Hours
TOTAL: 100 marks
Section A
Please use the answer sheet provided for this section.
20 multiple choice questions (1 mark each)
Question 1
A solar eclipse that occurs when the moon's umbra does not reach the Earth's surface
is called
(a)
(b)
(c)
(d)
(e)
a total solar eclipse.
a partial solar eclipse.
an annular solar eclipse.
a penumbral solar eclipse.
an umbral solar eclipse.
Question 2
Condensation in the solar nebula probably led to the formation of
(a)
(b)
(c)
(d)
(e)
icy grains beyond the present orbit of Jupiter.
metallic grains near the present orbit of Mercury.
silicate grains near the present orbit of Earth.
all of the above.
none of the above.
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Question 3
The oxygen in Earth’s atmosphere
(a)
(b)
(c)
(d)
(e)
was manufactured inside stars.
was added to the atmosphere by plant life.
has grown more abundant since the origin of the Earth.
all of the above.
none of the above.
Question 4
Atmospheric belts and zones
(a)
(b)
(c)
(d)
(e)
have been observed only in the atmosphere of Jupiter.
are caused by a planet's magnetic field.
are caused by rising and sinking gases in the atmosphere.
are more obvious in Saturn’s atmosphere than Jupiter’s atmosphere.
explain the formation of Cassini's division.
Question 5
Uranus and Neptune do not contain liquid metallic hydrogen because they
(a)
(b)
(c)
(d)
(e)
are not massive enough.
do not contain enough hydrogen.
rotate too slowly.
are too far from the sun.
have magnetic fields that are much too weak.
Question 6
Most Kuiper Belt Objects (KBOs) lie within 50 AU of the sun because
(a)
(b)
(c)
(d)
(e)
this was the extent of the original solar nebula from which the planets
formed.
gravitational forces due to Neptune constrain the KBO orbits.
more distant KBOs appear as long period comets.
passing stars deflect more distant KBOs into interstellar space.
Pluto deflects more distant KBOs inward.
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Question 7
If you go outside in the early evening, you see different stars in summer and winter
because of:
(a)
(b)
(c)
(d)
(e)
the Earth’s rotation about its own axis.
the Earth’s orbit about the Sun.
the changing tilt of the Earth’s axis.
the rotation of the system of stars.
the statement is not true; from a given place on the Earth, we always see the
same stars.
Question 8
The primary means of transporting energy through the outer part of our Sun, as
evidenced by its "granular appearance" at the surface is:
(a)
(b)
(c)
(d)
(e)
radiation.
conduction.
convection.
levitation.
nuclear reactions.
Question 9
Younger stars have more heavy elements than older stars because
(a)
(b)
(c)
(d)
(e)
old stars destroy heavy elements as they age.
young stars burn their nuclear fuels faster.
heavy elements were made in previous generations of stars.
heavy elements haven't had time to settle to the core of these younger stars.
all of these.
Question 10
Which of the following stars has the highest average density?
(a)
(b)
(c)
(d)
(e)
a supergiant star.
a main sequence star.
a red giant star.
a white dwarf.
a brown dwarf.
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Question 11
There is a lower mass limit for stars on the main sequence because
(a)
(b)
(c)
(d)
(e)
low mass stars form from the interstellar medium very rarely.
low mass objects are composed primarily of solids, not gases.
pressure does not depend on temperature in degenerate matter.
the lower limit represents stars with zero radius.
there is a minimum temperature for hydrogen fusion.
Question 12
We observe the radio emission from neutron stars to be pulsed because:
(a)
(b)
(c)
(d)
(e)
the rotation axis is not aligned with the magnetic axis.
the electrons travel in spirals around magnetic field lines.
the surface of the neutron star expands and contracts with a regular period.
neutron stars are created in supernova explosions.
the radio waves must travel through the interstellar medium.
Question 13
Far ultraviolet (FUV) observations need to be made in space because
(a) there are very few FUV sources in the Universe.
(b) FUV light is completely absorbed by the ozone layer.
(c) Earth’s atmosphere emits strongly in the FUV, washing out any
extraterrestrial signals.
(d) the instruments need to be kept very cool.
(e) FUV light is reflected back into space by Earth’s atmosphere.
Question 14
HII regions are
(a) regions in the interstellar medium comprised of twice-ionised hydrogen.
(b) regions in the interstellar medium comprised of molecular hydrogen, H2 .
(c) regions in the interstellar medium detected by the 21 cm (1420 MHz) radio
signal.
(d) emission nebulae in which hydrogen is ionised by a nearby hot star.
(e) regions in the interstellar medium where the abundance of hydrogen is twice
that measured for the Sun.
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Question 15
Why does the disk of our galaxy appear blue?
(a) all the red starlight is scattered out of our view.
(b) the blue light is recombination radiation from HII regions.
(c) the blue light is emission from elements in the interstellar medium heavier
than hydrogen.
(d) it is illuminated by the Magellanic clouds and other satellite galaxies.
(e) it is illuminated by the youngest and hottest newly-forming stars.
Question 16
A Population II star can be identified by
(a)
(b)
(c)
(d)
(e)
the lack of metal absorption lines in their spectra.
the abundance of metal absorption lines in their spectra.
its red appearance in a visual image.
strong metal emission lines in their spectra.
unusually strong ultraviolet emission.
Question 17
We know our galaxy has a black hole at its centre because
(a) we see relativistic radio jets.
(b) we see broad emission lines similar to those associated with active galaxies.
(c) we see narrow emission lines but no broad emission lines, consistent with a
Type 2 Seyfert.
(d) we measure the rapid motion of individual stars orbiting close to the galactic
centre.
(e) the overall radiative luminosity of our galaxy is unusually high.
Question 18
Spiral galaxies generally appear bluer than elliptical galaxies because
(a) ellipticals contain more emission nebulae emitting Balmer lines.
(b) spirals contain more gas and dust and hence more young stars.
(c) ellipticals contain more gas and dust and hence their starlight suffers more
interstellar extinction.
(d) ellipticals are more distant and the starlight suffers cosmological redshift.
(e) spirals contain more Population II stars than ellipticals.
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Question 19
Why is it difficult to measure the Hubble constant?
(a)
(b)
(c)
(d)
(e)
the Hubble law only holds for the most distant galaxies.
it is difficult to measure the radial velocity of galaxies.
it is difficult to measure the distance to galaxies.
it is difficult to measure the angular sizes of galaxies.
type Ia supernovae are not really very good distance calibrators.
Question 20
The cosmological principle states that
(a)
(b)
(c)
(d)
(e)
the Universe is non-static and evolving with time.
the expansion of the Universe is accelerating.
the Universe started with a big bang.
the Universe is anisotropic and inhomogeneous.
the Universe appears the same in every direction and at every location.
------ This is the end of Section A. ------
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Section B
Please use the booklet provided for this section.
THIS SECTION HAS EIGHT (8) QUESTIONS
ANSWER ALL QUESTIONS
(10 marks each)
Question 1
Briefly describe ONE OBSERVATION supporting each of the following statements
(one or two lines each):
(a)
(b)
(c)
(d)
(e)
Water once existed on the surface of Mars.
The ‘seas’ (mare) on the surface of the Moon are younger than the lunar
highlands.
The planets of the solar system formed in an accretion disk around the infant
Sun.
Nuclear reaction rates inside stars increase rapidly with temperature.
Stars lose significant amounts of mass during their lifetime.
Question 2
Briefly describe ONE OBSERVATION supporting each of the following statements
(one or two lines each):
(a)
(b)
(c)
(d)
(e)
Some stars are orbited by planets with masses comparable to Jupiter.
Red dwarf stars are much more common than red giant stars.
Stars are forming in the Orion Molecular Cloud.
Dark matter cannot be baryonic.
Interstellar dust particles scatter light more effectively at shorter
wavelengths.
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Question 3
(a)
Pluto has recently had its status downgraded to become a ‘dwarf planet’.
Briefly outline the arguments for this change based on
(i) its mass and size
(ii) the characteristics of its orbit
(b)
Spectral absorption lines produced by ionised Calcium are very prominent at
the blue end of the solar spectrum. Hydrogen lines are also present, but
much weaker. Given that there are around 500,000 Hydrogen atoms for
every Calcium atom in the solar atmosphere, explain the very strong
Calcium absorption.
(c)
The Sun produces energy using the proton-proton cycle reaction, converting
hydrogen into helium. Briefly describe:
(i) the reactions involved in the proton-proton cycle.
(ii) why this reaction produces energy.
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Question 4
The diagrams below show Hertzsprung-Russell (colour-magnitude) diagrams for two
star clusters.
(a)
For both diagrams, reproduce a very simple sketch of them in your answer
book and indicate where you would find the Supergiant, Giant and Main
Sequence stars and the White Dwarfs (even if they are not apparent in the
cluster).
(b)
The Sun is a yellow dwarf star on the Main Sequence. On each of your
diagrams, mark the approximate position the Sun would occupy if it were a
member of each cluster.
(c)
Briefly describe and contrast the range of stars plotted in each cluster.
(d)
Briefly discuss the observational ‘selection effects’ apparent in the diagrams
for each cluster.
(e)
From your knowledge of stellar evolution and the age of our galaxy, estimate
the approximate age of each cluster and briefly justify your answer.
Question 5
(a) List and briefly describe one advantage and one disadvantage of
observations with radio telescopes.
(b)
List and briefly describe three different components of the material making
up the interstellar medium.
(c)
Using 3 or 4 dot points, outline briefly the main stages of star formation.
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Question 6
(a) Sketch the rotation curve of our Galaxy and explain how it provides
evidence for the presence of dark matter.
(b)
List and briefly describe three observational properties of active galaxies that
distinguish them from normal galaxies.
(c)
Briefly explain the origin of the cosmic microwave background radiation in
the context of the Big Bang theory.
Question 7
The illustration below shows the future of Triton, the largest moon of Neptune.
Triton’s irregular orbit is taking it inexorably closer to its parent planet. In the far
future the moon will have come so close to Neptune that Triton will be torn to pieces.
Answer the following questions by carefully analyzing the diagram and drawing on
your astronomical knowledge.
(a) Name the key astrophysical ‘objects’ in this illustration.
(b)
Briefly describe the physical characteristics (approximate size,
composition,…) of these key astrophysical ‘objects’.
(c)
Why do you think is Triton’s orbit decaying, causing it to spiral in towards
Neptune?
(d)
Why will Triton be torn to pieces? What forces produce this result?
(e)
Very briefly speculate on the likely future evolution of this system.
Question 8
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(a)
Briefly explain how star formation in a typical molecular cloud eventually
ceases.
(b)
How would estimates of the mass of nearby galaxies change if astronomers
discovered that Cepheid variables were more intrinsically luminous than
originally thought?
(c)
Consider the figure below showing the observed spectrum of a quasar at a
redshift z = 5.8. At that redshift, light in the Lyman- line from hydrogen
gas (i.e. the n = 1 to 2 transition in energy levels) is observed at a
wavelength of 8200 Å (820.0 nm).
Explain qualitatively why there is a sharp drop in the observed intensity at
wavelengths less than 8200 Å (820.0 nm).
wavelength (Å)
(d)
Atomic nuclei began to form about 100 seconds after the Big Bang and
continued to form for perhaps 30 minutes.
(i)
Name at least two atomic nuclei that were formed during this phase of
the early universe.
(ii)
Why did these nuclei not form earlier?
(e)
Briefly explain the meaning of the ‘epoch of reionisation’.
------ This is the end of your questions. ------