Download PHYSICS 1500 - ASTRONOMY TOTAL

93/09(a)
Semester 2, 2011
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THE UNIVERSITY OF SYDNEY
SCHOOL OF PHYSICS
PHYSICS 1500 - ASTRONOMY
NOVEMBER 2011
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
Why is the ecliptic not lined up with the celestial equator?
(a) The ecliptic is a fixed circle in the sky, but the celestial equator is different
for observers at different latitudes.
(b) The Earth's orbit is not a circle, but an ellipse.
(c) The Earth's axis is tilted by about 23 degrees from the plane of its orbit.
(d) The pull of the other planets makes the Earth wobble significantly during the
course of a year.
(e) The alignment between the ecliptic and the celestial equator varies throughout
the year, coming into alignment twice per year.
Question 2
The Earth's core is more dense than the crust or mantle. In terms of the Earth’s
history, what is the most likely explanation of this fact?
(a) The metals accreted first and then the silicates.
(b) Gravity has compacted the core and made it denser.
(c) The entire planet was molten at one time, allowing light and heavy elements
to separate.
(d) The sun heated the early Earth’s crust, causing it to expand.
(e) An unusually iron-rich planetesimal formed the core of the Earth.
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Question 3
Neither Mars nor Venus have conditions to support a rich biosphere. Which of the
following is not a contributing factor to this situation?
(a) Mars is close to the asteroid belt, which resulted in widespread cratering in
the heavy bombardment period.
(b) Venus is a little close to the sun, resulting in a runaway greenhouse effect in
the atmosphere.
(c) Mars is slightly too small to maintain a liquid core and therefore lacks a
strong magnetosphere, shielding its atmosphere from the stellar wind.
(d) Venus has only a weak magnetic field resulting from its very low spin,
allowing the solar wind to strip hydrogen from its atmosphere.
(e) Mars has too thin an atmosphere to allow open bodies of water to exist on its
surface.
Question 4
Which of the following statements about planetary and satellite geology is true?
(a) Saturn’s moon Enceladus has a young surface which is reformed by
cryovolcanism - a form of active geology at low temperatures.
(b) The Earth’s Moon has areas of very young surface from recently active lava
flows which form the darker ‘mare’ on the surface.
(c) Mercury lacks a protective magnetosphere and therefore has a young surface
due to constant erosion by the solar wind.
(d) Mars has a young surface due to eruptions of Olympus Mons, the largest
volcano in the Solar System.
(e) Ganymede, the largest of Jupiter’s Galilean satellites, has a record of heavy
cratering preserved in its rocky surface.
Question 5
Why are the rings of Uranus and Neptune very narrow?
(a) There are gravitational interactions between ring particles.
(b) A density wave confines them.
(c) The planet’s magnetic field holds them in place.
(d) Uranus and Neptune are much smaller than Saturn.
(e) They are shepherded by small moons.
Question 6
The Nebular Hypothesis suggests that the solar system grew from a flattened
accretion disk around the primordial sun. Observations claiming to support this are
listed below. Which of them is not true?
(a) The planets have approximately co-planar orbits.
(b) The physical composition of the planets is consistent with the chemistry of
the disk's expected temperature gradient.
(c) The planets all orbit in the same direction.
(d) Oort Cloud comets typically have prograde orbits in the plane of the ecliptic.
(e) Many solar system objects show signs of heavy bombardment.
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Question 7
The overwhelming majority of exoplanets known today have been discovered by the
Radial Velocity and Transit techniques. Which of the following types of planets
would you expect to be discovered most readily using these techniques?
(a) hot planets in orbits which graze the stellar surface
(b) large planets in wide orbits
(c) massive planets in close orbits
(d) cold planets beyond the ice line
(e) hot Jupiter-sized objects at orbital distances similar to Jupiter
Question 8
The stars Deneb (α Cygni) and Fomalhaut (α Piscis Austrini) both have an apparent
visual magnitude of about +1.2. Deneb is much further away, at a distance of almost
1500 light years, compared to just 25 light years for Fomalhaut. What is the
difference in luminosity between Deneb and Fomalhaut?
(a) Deneb is 3600 times more luminous than Fomalhaut
(b) Deneb is 460 times more luminous than Fomalhaut
(c) Deneb is 60 times more luminous than Fomalhaut
(d) Deneb is 7.7 times more luminous than Fomalhaut
(e) Deneb is the same luminosity as Fomalhaut
Question 9
Many of the most prominent stars in the night sky are of spectral type B, yet none of
the 100 closest stars is of this type. Which of the following statements best explains
this observation?
(a) B stars have high luminosities.
(b) B stars have large masses.
(c) B stars have relatively long lifetimes.
(d) The solar system is situated in an atypical region, devoid of O and B stars.
(e) Blue light is strongly absorbed by dust in the plane of the Milky Way.
Question 10
The spectra of two stars indicate that they are of the same spectral type. However,
Star A has a very broad line profile for the Hydrogen-alpha (Hα) absorption line
while Star B has a very narrow line profile. What does this tell us about the two stars?
(a) Star A is cooler than star B.
(b) Star A is more massive than star B.
(c) Star A has a denser atmosphere than Star B.
(d) Star A is older than star B.
(e) Star A is moving away from us faster than Star B.
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Question 11
What properties would you expect to be approximately common among stars within
an open star cluster?
(a) age
(b) initial chemical composition
(c) luminosity
(d) age and initial chemical composition
(e) age, initial chemical composition and luminosity
Question 12
Why does the radio emission we observe from neutron stars come as pulses?
(a) The neutron star’s rotation axis is not aligned with the magnetic axis.
(b) The radiation is produced by electrons travelling in spirals around magnetic
field lines.
(c) The surface of the neutron star expands and contracts with a regular period.
(d) Neutron stars are created in supernova explosions.
(e) The radio waves must travel through the interstellar medium.
Question 13
The following are some possible properties of Population II stars:
I.
Contain more heavy metals than population I stars
II.
Primarily old lower mass stars
III.
Located in globular clusters
IV.
Primarily found in the disk of the galaxy
What of the following is the correct combination of properties for Population II stars?
(a) I and II
(b) II
(c) IV
(d) II and III
(e) I, II, and III
Question 14
What are HII regions?
(a) Regions in the interstellar medium comprised of twice-ionised hydrogen.
(b) Regions in the interstellar medium comprised of molecular hydrogen.
(c) Regions in the interstellar medium detected by observations of 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
Which of the following statements best describes the Large and Small Magellanic
clouds?
(a) They are clouds of dense gas orbiting the Milky Way.
(b) They are small satellite galaxies orbiting the Milky Way.
(c) They are clouds of dense gas currently passing by the Milky Way.
(d) They are emission nebulae in the galactic halo.
(e) They are emission nebulae in the nuclear bulge.
Question 16
Why is the Galactic Centre directly observable at infrared (IR) and radio wavelengths
but not at optical or ultraviolet (UV) wavelengths?
(a) Black holes emit radiation at IR and radio wavelengths.
(b) There is an unusually large amount of dust and neutral gas at the Galactic
Centre.
(c) The stars orbiting the Galactic Centre emit very little optical or UV radiation.
(d) Light at IR and radio wavelengths can penetrate the Galactic disk more easily
than optical or UV radiation
(e) Sgr A* is a relativistic jet.
Question 17
In astronomy, what is a ‘standard candle’?
(a) Any astrophysical source that appears as bright as the Sun.
(b) Any astrophysical source that is intrinsically as bright as the Sun.
(c) A standard measure of luminosity, in units of the solar luminosity.
(d) A standard measure of luminosity, in units of the Milky Way's luminosity.
(e) A source of known intrinsic luminosity used to calibrate distances.
Question 18
What is the origin of the cosmological redshift?
(a) Photons losing speed as they travel from very distant sources.
(b) Light bending around intervening galaxies and intergalactic matter.
(c) Photons losing energy because of the expansion of spacetime.
(d) Photon frequencies increased by the expansion of spacetime.
(e) The changing expansion rate of the universe.
Question 19
Why do we think there was a slight imbalance between matter and antimatter in the
early universe?
(a) Some matter has survived annihilation to this day.
(b) Some antimatter has survived annihilation to this day.
(c) It is predicted by inflation theory.
(d) It is predicted by the existence of dark energy.
(e) Dark matter cannot be baryonic.
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Question 20
What may be observed when light from a distant quasar travels past a cluster of
galaxies on its way to us?
(a) A double-lobed radio galaxy
(b) Superluminal expansion
(c) Multiple images from gravitational lensing
(d) A blazar
(e) Gravitational waves
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
(two or three lines each):
(a)
The Earth and Moon probably formed from a collision early in the history of
the solar system.
(b) Almost all comets are gravitationally bound to the Sun (i.e. they do not
originate from interstellar space)
(c)
Mars was geologically active in the past.
(d) The Sun has a magnetic field that varies across its surface.
(e)
Some binary systems contain black holes.
(10 marks)
Question 2
Briefly describe ONE OBSERVATION supporting each of the following statements
(two or three lines each):
(a)
Nuclear reaction rates inside stars increase rapidly with temperature.
(b) Red dwarf stars are more common than red giant stars.
(c)
There is dust in the interstellar medium.
(d) The Milky Way’s halo contains dark matter.
(e)
The expansion of the universe is accelerating.
(10 marks)
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Question 3
(a)
All of the planets are expected to be warm or hot in their interiors compared to
their surfaces. What is the source of this energy for:
(i)
(ii)
(iii)
Earth and Venus?
Saturn?
Jupiter's moon Io?
(b)
In the case of the Earth, maintaining a molten metal core has been essential to
the preservation of our biosphere. Why?
(c)
In the case of Io, what are the observable consequences of this internal heat?
(d)
Stars are very hot in their interiors – in this case because of nuclear reactions
in the core. Briefly describe three (3) important physical consequences of this
heat source.
(10 marks)
Question 4
(a)
With the aid of a diagram, briefly explain the origin and significance of tides
on Earth.
(b)
Orbital resonances occur when two or more objects have periods that are
integer multiples of some common interval.
(i)
Give one example from within our solar system in which orbital
resonances create or sustain stable orbits. Briefly explain the details of
the resonance.
(ii)
Give one example from within our solar system in which orbital
resonances prevent stable orbits. Briefly explain the details of the
resonance.
(iii)
Give one example from within our solar system where the rotational
period (spin) of an object is dynamically linked with its orbital period.
Briefly explain the details of the resonance.
(10 marks)
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Question 5 (a) The largest fully steerable radio telescope in the world is the 110 m
diameter dish at Greenbank in West Virginia (pictured below).
Briefly discuss the relative merits of
observing an astronomical source with the
Greenbank telescope versus observing the
same source with a radio interferometer
composed of two 10 m diameter dishes,
spaced 110 m apart.
(b)
Using a simple sketch, highlight common features of galaxies containing
active galactic nuclei (AGN), as revealed by observations with radio
telescopes.
(c)
Describe some of the advantages offered by optical fibres in modern optical
astronomical instrumentation.
(10 marks)
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Question 6 The diagram below is an HR diagram (also called a colour-magnitude
diagram) of the star cluster NGC 104 (better known as 47 Tucanae).
(a)
On the vertical scale of this diagram, why is apparent magnitude used in place
of absolute magnitude or luminosity relative to the Sun?
(b)
Describe the different types of stars plotted in this diagram.
(c)
In what way is this diagram not representative of the general population of
stars in the disk of the Milky Way?
(d)
What does the diagram reveal about the age of the star cluster? How do you
know?
(10 marks)
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Question 7 The illustration below shows an artist’s impression of a cataclysmic
binary star system comprised of a red giant star and a white dwarf.
Answer the following questions by carefully analyzing the diagram and drawing on
whatever you may know of such systems.
(a)
Briefly describe the key astrophysical ‘features’ of this illustration.
(b)
Why would this system stand out for study amongst the billions of other stars
in the Milky Way?
(c)
Using dot-points, outline a likely history for this system up to the time
illustrated in the diagram.
(d)
Very briefly speculate on the likely future evolution of this system.
(10 marks)
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Question 8
The two diagrams below illustrate observations of the Cosmic Background Radiation
by the COBE and WMAP spacecraft.
Figure (a)
Figure (b)
(a)
Briefly explain what is shown in Figure (a) and its significance.
(b)
Briefly explain what is shown in Figure (b) and its significance.
(c)
What era (or eras) in the history of the universe is (are) reflected in these
results?
(d)
How do these images relate to the distribution of matter in the universe today?
(10 marks)
This is the end of your questions.