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

93/09(a)
Semester 2, 2010
Page 1 of 10
_______________________________________________________________________________________
THE UNIVERSITY OF SYDNEY
SCHOOL OF PHYSICS
PHYSICS 1500 - ASTRONOMY
NOVEMBER 2010
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
At a given time of night, we see different stars in the summer night sky and the winter
night sky. Which of the following factors causes this?
(a) Earth’s rotation about its own axis
(b) Earth’s orbit about the Sun
(c) Changing tilt of the Earth’s axis
(d) Rotation of the system of stars
(e) None, because the statement is not true.
Question 2
Why don’t we see a lunar eclipse every month?
(a) Earth’s rotation axis is tilted relative to the orbit of the Moon.
(b) The orbit of the Moon is elliptical.
(c) The Moon takes slightly longer than a month to go around the Earth.
(d) Earth’s orbit is elliptical.
(e) The orbit of the Moon is tilted relative to the orbit of the Earth.
93/09(a)
Semester 2, 2010
Page 2 of 10
_______________________________________________________________________________________
Question 3
Which of the following ‘facts’ is true about the oxygen in Earth’s atmosphere?
(a) Oxygen was manufactured inside stars.
(b) Oxygen has been added to the atmosphere by plant life.
(c) Oxygen has grown more abundant in the atmosphere since the Earth was
formed.
(d) all of the above
(e) none of the above
Question 4
The moon appears to be no longer geologically active. Which of the following best
explains this observation?
(a) It is rotating too slowly.
(b) Radioactive elements were never abundant on the Moon.
(c) It is too small.
(d) The crust is too thin.
(e) It does not have water on its surface.
Question 5
Estimates of the density of Comet Halley vary significantly, but the average is about
0.6 g.cm-3 (600 kg.m-3) – just over half the density of water. What does this imply
about Comet Halley?
(a) Halley is composed of about 50% rock and 50% frozen water and methane.
(b) Halley is loosely packed ices with a small amount of rocky material.
(c) Halley formed in the inner solar system and was ejected by Jupiter to the
Kuiper belt.
(d) Halley is really an S-type asteroid.
(e) Halley was once a moon of Neptune and was ripped away by a large impact.
Question 6
Most planets discovered around nearby stars are relatively massive and lie very close
to their parent stars. Why is this?
(a) These are clearly the most common type of planet.
(b) Smaller, rocky planets always lie closer to the star but are undetectable.
(c) Few planetary systems have giant planets more than 1 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.
93/09(a)
Semester 2, 2010
Page 3 of 10
_______________________________________________________________________________________
Question 7
Compared to the rest of the solar surface, what are two important properties of
sunspots?
(a) Lower temperature and intense magnetic fields
(b) Higher temperature and pressure
(c) Lower brightness and weak magnetic fields
(d) Higher temperature and weak magnetic fields
(e) Higher pressure and intense magnetic fields
Question 8
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 hot that most of the hydrogen is ionised and the atoms
cannot absorb energy.
(c) These stars are so cool that nearly all of the electrons in the hydrogen atom
are in the ground state.
(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.
Question 9
Consider two main-sequence stars that have different masses. Compared to the low
mass star, how would you describe the more massive star?
(a) Lower luminosity and older
(b) Higher luminosity and shorter lifetime
(c) Higher luminosity and longer lifetime
(d) Lower luminosity and longer lifetime
(e) Higher luminosity and younger
Question 10
Which of the following nuclear fuels does a one-solar-mass star consume over the
course of its entire evolution?
(a) Hydrogen
(b) Hydrogen and helium
(c) Hydrogen, helium and carbon
(d) Hydrogen, helium, carbon and oxygen
(e) Hydrogen, helium, carbon, oxygen and iron
93/09(a)
Semester 2, 2010
Page 4 of 10
_______________________________________________________________________________________
Question 11
Canopus (α Carinae) is a star of spectral class F0 and luminosity class II. Based on
this information, which of the following statements are true?
I. Canopus has a surface temperature less than the sun.
II. Canopus has a diameter that is greater than that of the sun.
III. Canopus is more luminous than the sun.
IV. Canopus is located near the upper left hand corner in the HR diagram.
(a)
(b)
(c)
(d)
(e)
I and II
II and IV
II, III, and IV
II, and III
I, II, III, and IV
Question 12
Why are neutron stars expected to spin rapidly?
(a) They conserved angular momentum when they collapsed.
(b) They have high orbital velocities.
(c) They have high densities.
(d) They have high temperatures.
(e) The energy from the supernova explosion that formed them made them spin
faster.
Question 13
Which of the following statements correctly describes the extinction of starlight due
to the interstellar medium?
I.
Extinction is greater in the ultraviolet than at visible wavelengths.
II.
Extinction is greater in the infrared than at visible wavelengths.
III.
Extinction is caused by ionized hydrogen.
IV.
Extinction is caused by dust particles.
(a)
(b)
(c)
(d)
(e)
I & III
II & III
I & IV
II & IV
only IV
93/09(a)
Semester 2, 2010
Page 5 of 10
_______________________________________________________________________________________
Question 14
Why do younger stars have more heavy elements than older stars?
(a) Old stars destroy heavy elements as they age.
(b) Young stars burn their nuclear fuels faster.
(c) Heavy elements were made in previous generations of stars.
(d) Heavy elements haven't had time to settle to the core of these younger stars.
(e) All of these statements are true.
Question 15
Why does the bulge of a spiral galaxy appear yellow in colour?
(a) All the blue starlight is scattered out of our line of sight.
(b) The light is dominated by recombination radiation from HII regions.
(c) Emission comes from elements in the interstellar medium heavier than
hydrogen.
(d) The light is dominated by older stars.
(e) The light is dominated by the youngest and hottest newly-formed stars.
Question 16
Which of the following types of galaxies contain large clouds of gas and dust, both
young and old stars, but have no obvious spiral arms or nucleus?
(a) Irregular
(b) S0
(c) E7
(d) Sa
(e) E0
Question 17
What is the origin of the hot spots in double-lobed radio galaxies?
(a) Nuclear fusion of hydrogen
(b) The interaction of high speed particles from the jets with the intergalactic
medium
(c) Gravitational lensing of the black hole at the centre of the galaxy
(d) Gravitational lensing of a single quasar
(e) Light emitted direct from the accretion disk around a super-massive black
hole
93/09(a)
Semester 2, 2010
Page 6 of 10
_______________________________________________________________________________________
Question 18
In the unified model of active galaxies, which of the following factors explains the
difference between Seyfert I and Seyfert II galaxies?
(a) The rate at which the central black hole is consuming material
(b) The thickness of the torus that obscures the accretion disk
(c) The mass of the central black hole that powers the nucleus
(d) The orientation of the accretion disk and torus to our line of sight
(e) The rate of supernova explosions
Question 19
According to the standard model, which elements were created in the Big Bang?
(a) Hydrogen only
(b) Hydrogen, helium and lithium only
(c) Elements from hydrogen up to carbon and oxygen
(d) Elements from hydrogen up to iron
(e) Elements from hydrogen up to uranium
Question 20
How does the inflation theory resolve the ‘flatness problem’ in the Big Bang model?
(a) Matter is created and destroyed until the density reaches equilibrium at its
critical value.
(b) It predicts a non-zero cosmological constant.
(c) It predicts that large-scale structure is distributed more or less evenly.
(d) The rapid expansion smooths out any initial spacetime curvature.
(e) The rapid expansion smooths the non-zero cosmological constant.
This is the end of Section A.
93/09(a)
Semester 2, 2010
Page 7 of 10
_______________________________________________________________________________________
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 formed from the glancing impact of a large body on the
proto-Earth.
(b) The planets formed from a single gaseous disk.
(c)
Some meteors that strike the Earth’s atmosphere were once part of larger
bodies.
(d) Energy is transported by convection in the outer part of the Sun.
(e)
Red dwarf stars are much more common than red giant stars.
(10 marks)
Question 2
Briefly describe ONE OBSERVATION supporting each of the following statements
(two or three lines each):
(a)
The Milky Way formed, at least in part, by pulling apart smaller galaxies and
absorbing their stars.
(b) The Magellanic Clouds are small galaxies outside the Milky Way.
(c)
Light is bent by gravity.
(d) The power source for the giant lobes of radio galaxies lies in the centres of
the galaxies.
(e)
The Big Bang actually happened.
(10 marks)
93/09(a)
Semester 2, 2010
Page 8 of 10
_______________________________________________________________________________________
Question 3
Celestial bodies adopt a size and internal structure governed by the competing forces
of gravity pulling inwards, balanced against some other force pushing outwards.
(a)
What is the ‘other force’ in each of the following objects? In each case, very
briefly explain how it arises.
(i)
(ii)
(iii)
(iv)
(b)
Terrestrial planets
‘Normal’ stars (like the Sun)
White Dwarfs
Black Holes
Briefly explain why, when a star runs out of its primary fuel, the core gets
hotter, not cooler.
(10 marks)
Question 4
The majority of stars, when plotted on the Hertzsprung-Russell diagram, fall along a
band known as the Main Sequence, ranging from hot/luminous through to cool/faint.
(a)
What is the main source of energy for stars lying on the Main Sequence?
(b)
What physical property is paramount in determining where a newly-formed
star will appear on this sequence? Why is this property so important?
(c)
After the main sequence, stars of intermediate mass pass through a Red Giant
phase before a dramatic event propels them onto the Horizontal Branch. What
is this event, and what is happening in the core of the star to cause this
change?
(d)
Name the most likely end-points in the evolutionary life of stars with initial
masses
(i)
(ii)
(iii)
1 solar mass
10 solar masses
40 solar masses
In each case, briefly explain how the final stages of its evolution lead to this
state.
(10 marks)
93/09(a)
Semester 2, 2010
Page 9 of 10
_______________________________________________________________________________________
Question 5
(a)
Sketch edge-on and face-on views of a spiral galaxy, labelling the disk,
nuclear bulge and the halo components. Also indicate where the Population I
and Population II stars reside and typical locations of galactic and globular star
clusters.
(b)
Apart from location in the galaxy, list two other properties that differ
markedly between Population I and Population II stars.
(c)
The interstellar medium is inhomogeneous, existing in several distinct
‘phases’. Briefly describe what is meant by this statement.
(d)
What is the ‘winding problem’ in the spiral structure of galaxies? What does
it imply about the nature of spiral arms?
(10 marks)
Question 6
(a)
How can we explain the observation that stars in the outer parts of spiral
galaxies orbit the centre faster than expected?
(b)
The Hubble Law is a relation between velocity and distance. Carefully
explain how the Hubble Law can be used to determine the distance to a
galaxy.
(c)
What evidence do we have that the expansion of the universe is accelerating?
(d)
What is the cosmological principle? Provide one piece of evidence in support
of this principle.
(10 marks)
93/09(a)
Semester 2, 2010
Page 10 of 10
_______________________________________________________________________________________
Question 7
The image at right shows HH30, a newly
formed star still hidden by gas and dust.
NASA
Answer the following questions by carefully
analysing the image and drawing on whatever
you may know of such systems.
(a)
Briefly describe the key astrophysical
‘features’ of this illustration.
(b)
Sketch a simple Hertzsprung-Russell
diagram with appropriately labelled
axes. Indicate roughly where this
object would lie on the diagram.
(c)
Make an estimate of how long it is
since this object began to condense in
its parent gas cloud? Justify your
estimate.
(d)
How would you observe this object?
(10 marks)
Question 8
Suppose you were given access to a large ground-based optical telescope with a high
resolution spectrograph and optical and near-infrared imaging instruments.
(a)
You observe a nearby edge-on spiral galaxy using both the optical and
infrared imaging instruments. Briefly describe the difference in the views of
the galaxy provided by the two instruments.
(b)
You observe the same galaxy using the spectrograph, moving the
observation point of the instrument along the disk of the galaxy. What
would you expect the spectra to reveal about motions of stars within the
galaxy?
(c)
If the telescope is moved to observe a giant elliptical galaxy at the same
distance as the spiral (a member of the same cluster of galaxies), what would
the spectrograph observation reveal about motions of stars within the
elliptical galaxy?
(d)
If the telescope is moved to observe a much more distant giant elliptical
galaxy, what would the spectrograph observation reveal?
(10 marks)
This is the end of your questions.