PHYSICS ASTRONOMY. TOTAL: 100 marks. Section A Please use the answer sheet provided for this section.

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1 93/09(a) Semester 2, 2009 Page 1 of 11 THE UNIVERSITY OF SYDNEY SCHOOL OF PHYSICS PHYSICS ASTRONOMY NOVEMBER 2009 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 were the brighter planets not labelled as stars in the constellations, despite being known since antiquity? (a) The planets are brighter than the brightest stars in the constellations. (b) The planets move between constellations from one night to the next. (c) The planets move between constellations over weeks or months. (d) The planets are always moving faster than the stars across the sky. (e) Unlike the constellations, the planets return to the same parts of the sky from year to year. Question 2 Why don t we see a solar eclipse every month? (a) The orbit of the Moon is tilted relative to the orbit of the Earth. (b) The Earth s rotation axis is tilted relative to the orbit of the Moon. (c) The Moon takes slightly longer than a month to go around the Earth. (d) The orbit of the Moon is elliptical. (e) The orbit of the Earth is elliptical.

2 93/09(a) Semester 2, 2009 Page 2 of 11 Question 3 How are magnetic fields generated in the solar system? (a) Energetic particles colliding with an atmosphere (b) Deeply buried bar magnets (c) Convection within an iron-rich liquid (d) Currents in hydrogen under extreme pressure (e) Both (c) and (d) Question 4 Which one of the following objects is most like the planetesimals that formed in the proto-solar nebula? (a) Asteroid (b) Earth (c) Saturn (d) Venus (e) Sun Question 5 What is the name given to the disc-shaped cloud of icy bodies believed to extend from about 30 AU out to 50 AU or more. It is believed that short-period comets originate from this region. (a) Oort cloud (b) Widmanstätten cloud (c) Apollo-Amors (d) Kirkwood region (e) Kuiper belt Question 6 Where does our best information about the density of other planets come from? (a) Missions that have returned samples to Earth (b) The gravitational forces measured by orbiting spacecraft (c) Experiments carried out by astronauts (d) Observations by sensitive radio telescopes (e) Careful measurements of transits across the face of the Sun

3 93/09(a) Semester 2, 2009 Page 3 of 11 Question 7 How would you describe the surface appearance of those icy satellites in the solar system with the most ancient surfaces? (a) Dark in color with few impact craters (b) Dark in color with many impact craters (c) Very bright and white with many impact craters (d) Generally showing long bright grooves and a modest number of impact craters (e) Generally having small, active volcanoes Question 8 The density of Comet Halley was found to be about 200 kg.m -3 (0.2 g.cm -3 ). What does this suggest about Comet Halley? (a) It is composed of about 50% rock and 50% frozen water and methane. (b) It is loosely packed ices with a small amount of rocky material. (c) It formed in the inner solar system and was ejected by Jupiter to the Kuiper belt. (d) It is really an S-type asteroid. (e) It was once a moon of Neptune and was ripped away by a large impact. Question 9 Two important stars in the constellation Carina are α Carinae (Canopus) and l Carinae. The apparent visual magnitudes of the two stars are 0.7 for α Carinae and for l Carinae. Which of the following statements most accurately describes the brightness of l Carinae? (a) 1.6 times fainter than α Carinae (b) 3.0 times brighter than α Carinae (c) 3.0 times fainter than α Carinae (d) 15.8 times brighter than α Carinae (e) 15.8 times fainter than α Carinae Question 10 What can you infer when you see a visible light spectrum with absorption lines in it? (a) The light passed through ionized atoms (b) Atoms are colliding with other atoms (c) Electrons absorbed the light and moved to lower atomic energy levels (d) Electrons absorbed the light and moved to higher atomic energy levels (e) The gas that produced the absorption lines was very hot

4 93/09(a) Semester 2, 2009 Page 4 of 11 Question 11 What is the physical mechanism by which energy is transported in the outer part of the Sun? (a) Conduction (b) Convection (c) Radiation (d) Nuclear fusion (e) Levitation Question 12 What factors determine the frequencies of oscillations in the Sun and similar stars? (a) The speed of sound inside the star (b) The size of the star (c) The age of the star (d) (a) and (b) only (e) (a), (b) and (c) Question 13 Why are red giant stars less common than main sequence stars? (a) Red giant do not form as often as main sequence stars. (b) Most stars have already passed through the red giant phase. (c) Helium requires very high temperatures in order to undergo fusion. (d) Only relatively cool stars become red giants. (e) The red giant stage is relatively short. Question 14 Which of the following statements is true of stars with masses between 0.4 and 4 solar masses? (a) They undergo thermonuclear fusion of hydrogen and helium, but never get hot enough to ignite carbon. (b) They undergo thermonuclear fusion of hydrogen, but never get hot enough to ignite helium. (c) They produce type-i supernovae after they exhaust their nuclear fuels. (d) They produce type-ii supernovae after they exhaust their nuclear fuels. (e) They undergo carbon detonation.

5 93/09(a) Semester 2, 2009 Page 5 of 11 Question 15 Some properties of different stars in our Galaxy are listed below: I II III IV V Metallicity roughly equal to the Sun s Typical location in the Galactic halo Orbit lying close to the plane of the Milky Way Surrounded by gas and dust Formation early in the history of the Galaxy Which of these properties are characteristic of Population II stars? (a) I and V (b) III, IV, and V (c) I, II, and IV (d) II and V (e) I and III Question 16 How do we know that the disc of the Milky Way is rotating? (a) There is a massive black hole at the Galactic Centre. (b) Much of the mass in the Milky Way is dark matter. (c) We see opposite Doppler shifts from gas clouds on either side of the Galactic Centre. (d) Nearby young star clusters and HII regions are located in a spiral arm pattern. (e) We can see Cepheids on the far side of the disc. 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) Location closest to the galaxy producing the lobes (d) Gravitational lensing of a single quasar (e) Light emitted direct from the accretion disk around a super-massive black hole

6 93/09(a) Semester 2, 2009 Page 6 of 11 Question 18 According to the current standard model for cosmology, large galaxies are formed by the merger of small systems. In this picture, the dwarf spheroidal galaxies (dsphs) are potentially a major building block of the Milky Way's halo. There is, however, a problem with this scenario. Why? (a) The amount of dark matter in the dsphs would significantly alter the structure of the Milky Way. (b) The chemical abundance of the dsphs differs from that of the Milky Way halo. (c) The dsphs mainly contain main sequence dwarf stars, which is not true for the Milky Way halo. (d) The process of galaxy merging has not been observed elsewhere in the universe. (e) There are no dsphs in the vicinity of the Milky Way. Question 19 What of the following is the main piece of evidence that the universe is expanding? (a) Redshifts are greater for more distant galaxies. (b) Light from distant quasars is sometimes bent by gravitational lensing. (c) The microwave background radiation has a blackbody spectrum. (d) There is not enough visible matter in clusters of galaxies to account for their hot x-ray gas (e) All the galaxies in our Local Group are all moving away from us. Question 20 In the early history of the universe, recombination refers to the time when the universe became transparent to photons. Which of the following correctly describes events at recombination? (a) Matter and antimatter annihilated to create a slight excess of matter (b) Protons and neutrons combined to produce deuterium and helium nuclei (c) Nuclei and electrons combined to produce neutral atoms (d) The first galaxies formed within clusters (e) The first stars formed within galaxies This is the end of Section A

7 93/09(a) Semester 2, 2009 Page 7 of 11 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) Tidal forces are heating Io, the innermost Galilean satellite of Jupiter. (b) The surface of Mars was once warmer than it is today. (c) Most long period comets come from the Oort Cloud rather than the Kuiper Belt. (d) Neutrinos are produced by nuclear reactions in the core of the Sun. (e) Surface temperature of a star is related to the core temperature. Question 2 Briefly describe ONE OBSERVATION supporting each of the following statements (two or three lines each): (a) The interstellar medium is permeated by dust particles with a size roughly comparable to the wavelength of visible light. (b) Recent star formation occurs in the spiral arms of galaxies. (c) There is a massive black hole at the centre of the Milky Way. (d) Quasars are relatively small objects, much smaller than their host galaxy. (e) Dark matter cannot be baryonic.

8 93/09(a) Semester 2, 2009 Page 8 of 11 Question 3 (a) For rocky planets, what basic property of the planet is the primary physical characteristic which permits continuing geological activity over billions of years? Briefly describe why this property is so critical. (b) Despite having a molten core, Venus does not support a significant magnetic field. Describe the most probable physical cause for this dramatic difference between Venus and Earth. (c) Earth's magnetosphere has protected it from the solar wind (unlike Venus). If this protection was lost, particles from the solar wind would break up molecules in the atmosphere and cause lighter gases, such as hydrogen, to be lost. Why would this loss be very serious? (d) Many of the moons in the outer solar system exhibit surprisingly young surfaces suggesting active geological histories. Briefly outline two main reasons why these moons are (or have been) geologically active. Question 4 On separate pages, draw two empty Hertzsprung-Russell diagrams (make each about half a page) and label the axes with appropriate values. (a) On the first H-R diagram, sketch the evolutionary path of the Sun up to the point when it settles into stable nuclear fusion in the core. Indicate any key events along the path and write one or two lines of text describing each. (b) Briefly describe the evolutionary changes the Sun undergoes during the time it is on the Main Sequence. (c) On the second H-R diagram, sketch the evolutionary path of the Sun after it leaves the Main Sequence. Indicate any key events along the path and write one or two lines of text describing each. (d) Briefly describe how the evolutionary history of a star with 0.3 times the mass of the Sun will differ from the Sun s history.

9 93/09(a) Semester 2, 2009 Page 9 of 11 Question 5 The illustration at left shows an artist s impression of a hot Jupiter a giant planet in a very close orbit around its parent star. The first such planet discovered was 51 Pegasi b which takes 4.23 days to orbit a sun-like star. The system is about 50 light years from Earth. The planet was found using the radial velocity method for detecting extra-solar planets. (a) Describe the two key properties of 51 Peg b that made it relatively easy to detect with the radial velocity method (b) Why were many astronomers at the time very surprised to find that planetary systems like 51 Peg exist? (c) What additional physical processes are believed to be in action to allow the existence of hot jupiters within our model for planetary system formation? (d) Briefly define what is meant by the term habitable zone around a star. (e) If we wish to use the radial velocity method to find planets in the habitable zone around stars, what sort of stellar systems should we search? Explain why we should choose these stars. Question 6 (a) Using a block diagram to illustrate the various stages, outline the process of gas recycling between stars and the interstellar medium in the galaxy. (b) Draw a diagram showing the four main components of an active galactic nucleus as given by the unified model. (c) Briefly outline the origin and evolution of the large-scale structure that is now apparent in the distribution of galaxies.

10 93/09(a) Semester 2, 2009 Page 10 of 11 Question 7 The graph below shows a plot of the relative abundance of elements found in the solar system (note the logarithimic vertical scale). (a) Why are Hydrogen and Helium by far the most abundant elements in the solar system? (b) Why is Iron (Fe) far more abundant than all the heavier elements, and even more abundant than many of the lighter elements? (c) Much of the diagram exhibits an up-down zigzag pattern with alternate atomic number. What is the origin of this pattern? (d) The three most important nuclear reactions in stars are: i) proton-proton chain ii) CNO cycle iii) triple-alpha process For each reaction, describe the types of stars in which it is the dominant reaction. (e) Briefly describe why, in general, heavier elements require higher core temperatures to sustain fusion reactions.

11 93/09(a) Semester 2, 2009 Page 11 of 11 Question 8 (a) Briefly outline three key observations supporting the Big Bang model of the universe. (b) In a few lines, explain what occurred in the Inflationary Era in the evolution of the universe. (c) Inflation explains two key problems in the Big Bang model. Outline these problems and explain how inflation solves them. (d) Briefly describe each of the significant components of the energy density of the universe as it is understood today, including their approximate percentage contributions This is the end of your questions