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 2007
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 land mass of the Earth is more concentrated in the northern hemisphere.
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)
(b)
(c)
(d)
(e)
The metals accreted first and then the silicates.
Gravity has compacted the core and made it denser.
The entire planet was molten at one time.
The sun heated the earth more in the past than now.
An iron-rich planetesimal formed the core of the Earth.
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Question 3
What is the main reason that no liquid water exists on the surface of Mars now?
(a)
(b)
(c)
(d)
(e)
The surface atmospheric pressure is too low.
The surface atmospheric pressure is too high.
No water in any form exists on Mars now.
The temperature never gets high enough for water to melt.
Water exists today only as a vapour in the thin atmosphere.
Question 4
Europa’s surface has few craters because
(a)
(b)
(c)
(d)
(e)
it is protected from impacts by Jupiter's gravity.
it does not have a solid surface.
its surface erases craters nearly as fast as they are formed.
its surface is not strong enough to support craters.
it keeps one face always pointed toward Jupiter which screens it from
incoming meteorites.
Question 5
The rings of Uranus and Neptune are very narrow because
(a)
(b)
(c)
(d)
(e)
of gravitational interactions between ring particles.
a density wave confines them.
the magnetic field holds them in place.
Uranus and Neptune are much smaller than Saturn.
they are shepherded by small moons.
Question 6
Planets discovered around nearby stars are likely to be relatively massive and lie very
close to their parent stars. This is because
(a) these are clearly the most common type of planet.
(b) smaller, rocky planets are expected to lie closer to the star but are
undetectable.
(c) few planetary systems have giant planets more than 2 AU from the star.
(d) detection of less massive planets is currently very difficult.
(e) detection of planets less massive than Jupiter is currently impossible.
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Question 7
The star Canopus ( Carinae) has an apparent visual magnitude of –0.7 and the star
Deneb ( Cygni) has an apparent visual magnitude of +1.3. Canopus is
approximately _________________ than Deneb.
(a)
(b)
(c)
(d)
(e)
2.0 times fainter
2.0 times brighter
6.3 times fainter
6.3 times brighter
15.8 times fainter
Question 8
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. One fact that helps explain this observation is:
(a)
(b)
(c)
(d)
(e)
B stars have large luminosities.
B stars have relatively long lifetimes.
the solar system is situated in an atypical region, devoid of O and B stars.
the statement is not true. About 10% of the nearest stars are B stars.
blue light is less absorbed by dust in the plane of the Milky Way.
Question 9
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 (H) absorption line
while Star B has a very narrow line profile. What does this tell us about the two stars?
(a)
(b)
(c)
(d)
(e)
Star A is hotter than star B.
Star A is more massive than star B.
Star A has a higher-density atmosphere than Star B.
Star A is older than star B.
Star A is moving away from us faster than Star B.
Question 10
Why don't we see Hydrogen Balmer lines in the spectra of stars with surface
temperatures of 3200 K?
(a) There is no hydrogen in stars this cool.
(b) These stars are so cool that nearly all of the electrons in the hydrogen atom are
in the ground state.
(c) The stars are hot enough that most of the hydrogen is ionized and the atoms
cannot absorb energy.
(d) Stars of this temperature are too cool to produce an absorption spectrum.
(e) Stars of this temperature are too hot to produce an absorption spectrum.
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Question 11
Scheat ( Pegasi) is a star of spectral class M2 and luminosity class II. Based on this
information which of the following are true?
I. Scheat has a surface temperature less than the sun.
II. Scheat has a diameter that is greater than that of the sun.
III. Scheat is more luminous than the sun.
IV. Scheat is located near the upper left hand corner in the HR diagram.
(a)
(b)
(c)
(d)
(e)
I & II
II & IV
II, III, & IV
I, II, & III
I, II, III, & IV
Question 12
Neutron stars are expected to spin rapidly because
(a)
(b)
(c)
(d)
(e)
they conserved angular momentum when they collapse.
they have high orbital velocities.
they have high densities.
they have high temperatures.
the energy from the supernova explosion that formed them made them spin
faster.
Question 13
The Large and Small Magellanic clouds are
(a)
(b)
(c)
(d)
(e)
clouds of dense gas orbiting the Milky Way.
small satellite galaxies orbiting the Milky Way.
clouds of dense gas currently passing by the Milky Way.
emission nebulae in the galactic halo.
emission nebulae in the nuclear bulge.
Question 14
What are cosmic rays?
(a) highly energetic particles propagating through interstellar space and mostly
produced during supernova explosions.
(b) highly energetic particles propagating through interstellar space and mostly
produced in globular clusters.
(c) highly energetic particles that escape from the cores of stars.
(d) intense light rays illuminating the interstellar medium and mostly produced
during supernova explosions.
(e) intense light rays illuminating the interstellar medium and mostly produced by
a pulsar.
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Question 15
HII regions are
(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 the observing 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.
Question 16
The Galactic Centre is observable at infrared (IR) and radio wavelengths but not at
optical or ultraviolet (UV) wavelengths because
(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 these wavelengths can penetrate the galactic disk more easily than
optical or UV radiation
(e) Sgr A* is a relativistic jet.
Question 17
What is a standard candle?
(a)
(b)
(c)
(d)
(e)
any astrophysical source that appears as bright as the Sun.
any astrophysical source that is intrinsically as bright as the Sun.
a standard measure of luminosity, in units of the solar luminosity.
a standard measure of luminosity, in units of the Milky Way's luminosity.
a source of known intrinsic luminosity used to calibrate distances.
Question 18
The cosmological redshift is due to
(a)
(b)
(c)
(d)
(e)
photons losing speed as they travel from very distant sources.
light bending around intervening galaxies and intergalactic matter.
photon wavelengths being stretched by the expansion of spacetime.
photon frequencies being stretched by the expansion of spacetime.
The changing expansion rate of the universe.
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Question 19
The critical density of the Universe is
(a) the minimum density of baryonic matter the Universe may have if it is open
and infinite.
(b) the maximum density of baryonic matter the Universe may have if it is open
and infinite.
(c) the minimum density of baryonic matter the Universe may have to allow the
formation of new stars and galaxies.
(d) the minimum density of matter and energy the Universe may have if it is open
and infinite.
(e) the maximum density of matter and energy the Universe may have if it is open
and infinite.
Question 20
Why do we think there was a slight imbalance between matter and antimatter in the
early Universe?
(a)
(b)
(c)
(d)
(e)
because some matter has survived annihilation to this day.
because some antimatter has survived annihilation to this day.
because it is predicted by inflation theory.
because it is predicted by the existence of dark energy.
because dark matter cannot be baryonic.
------ 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)
Pluto does not satisfy the new definition of a planet.
(b)
Many of the small satellites of Jupiter are captured asteroids.
(c)
Mars was geologically active in the past.
(d)
Comets can be broken apart by gravitational tidal forces.
(e)
Energy is transported by convection in the outer part of the Sun.
Question 2
Briefly describe ONE OBSERVATION supporting each of the following statements
(two or three lines each):
(a)
Stars form with disks surrounding them.
(b)
Stars spend most of their lives on the Main Sequence.
(c)
Recent star formation occurs in the spiral arms of galaxies.
(d)
The Milky Way’s halo contains dark matter.
(e)
Elliptical galaxies have less cool gas than spiral galaxies.
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Question 3
(a)
The atmospheres of Venus, Earth and Mars initially had similar
compositions, roughly 95% CO2 and 5% N2, and roughly equivalent
thicknesses. Briefly highlight the current differences between the three
atmospheres and explain why these differences arose.
(b)
In a few lines describe the process which led to the formation of the Earth’s
Moon. How did it differ from the formation of the major moons of Jupiter?
(c)
Briefly describe what is meant by the term gravitational resonance. Specify
two examples of Jupiter playing a part in a gravitational resonance.
(10 marks)
Question 4
The diagrams below show two different Hertzsprung-Russell (colour-magnitude)
diagrams for stars from the same section of sky and the same data set. The diagram
on the left shows 200 stars within 65 parsecs of the Sun. The diagram on the right
shows about 3300 stars with a median distance of about 240 parsecs, but with many
stars beyond 1000 parsecs.
(a)
Reproduce a simple sketch of the diagram on the right 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 diagram).
Continued on next page…
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(b)
Briefly discuss the observational ‘selection effects’ apparent in each of the
diagrams.
(c)
Reproduce a second, simple sketch of the diagram on the right. Mark the
approximate position the Sun would occupy if it were visible in this part of
the sky.
(d)
On the sketch from (c), add a path representing the evolution of the Sun
from the time of its arrival on the Main Sequence to its ultimate fate.
(e)
Label points along the path where significant changes occur in the core of
the star. Briefly describe the state of the core at these points.
(10 marks)
Question 5
(a)
Provide two sketches of a typical spiral galaxy - one from edge-on and one
face-on. Label the disk, bulge and the halo. Briefly describe the basic
characteristics of stellar orbits in these three components of the galaxy.
(b)
List and briefly describe three different components of the interstellar
medium and how they are observed.
(c)
Briefly summarise the main stages of the star gas star cycle.
(10 marks)
Question 6
(a)
List and briefly describe three observational properties of active galaxies
that distinguish them from normal galaxies.
(b)
What is the cosmological principle? How is it important to our
understanding of the Universe and what evidence do we have that it is
correct?
Briefly explain the origin of the cosmic microwave background radiation in
the context of the Big Bang theory.
(c)
(10 marks)
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Question 7
The illustration shows an artist’s impression
of a planet known to be orbiting the star
HD209458 every 3.5 days. The star is of
spectral type F8 and just slightly more
massive than the Sun.
Answer the following questions by carefully
analysing the diagram and drawing on your
astronomical knowledge.
(a)
Describe the general properties of
the star.
(b)
Describe the likely properties of the
planet.
(c)
How was the planet likely to have
been discovered?
(d)
In what ways is this system similar
or different to our solar system?
(e)
In what ways is the birth or
evolution of this system similar or
different to our solar system?
(10 marks)
Question 8
(a)
Suppose you observe a star orbiting the galactic centre at a speed of
1,000 km.s-1 in a circular orbit with a radius of 20 light-days. Using
v2 = G M r ,
estimate the mass of the object that the star is orbiting. Give your answer in
units of solar masses (use G = 6.67 x 10-11 N.m2.kg-2, c = 3.0 x 108 m.s-1 and
1 solar mass = 2.0 x 1030 kg).
(b)
In a few lines, define what is meant by dark matter and dark energy and
describe what types of observations support their existence.
(c)
Olber's paradox is a statement of the observation that the night sky is dark.
Explain why this is evidence for the Big Bang.
(d)
Briefly outline how the first atomic nuclei formed during the first few
minutes of the Big Bang.
(e)
Briefly describe what is meant by the term epoch of reionisation.
(10 marks)
------ This is the end of your questions. ------