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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.