Introduction to astronomy and cosmology

Chapter 1: Astronomy, an Observational Science 1 1.1 Introduction 1 1.2 Galileo Galilei’s proof of the Copernican theory of the solar system 1 1.3 The celestial sphere and stellar magnitudes 4 1.3.1 The constellations 4 1.3.2 Stellar magnitudes 5 1.3.3 Apparent magnitudes 5 1.3.4 Magnitude calculations 6 1.4 The celestial coordinate system 7 1.5 Precession 9 1.6 Time 11 1.6.1 Local solar time 11 1.6.2 Greenwich mean time 11 1.6.3 The equation of time 12 1.6.4 Universal time 12 1.6.5 Sidereal time 13 1.6.6 An absolute time standard – cosmic time 14 1.7 A second major observational triumph: the laws of planetary motion 16 1.7.1 Tycho Brahe’s observations of the heavens 17 1.7.2 Johannes Kepler joins Tycho Brahe 20 1.7.3 The laws of planetary motion 20 1.8 Measuring the astronomical unit 23 1.9 Isaac Newton and his Universal Law of Gravity 25 1.9.1 Derivation of Kepler’s third law 30 1.10 Experimental measurements of G, the Universal constant of gravitation 32 1.11 Gravity today: Einstein’s special and general theories of relativity 33 1.12 Conclusion 36 1.13 Questions 36 Contents vii viii Contents Chapter 2: Our Solar System 1 – The Sun 39 2.1 The formation of the solar system 39 2.2 The Sun 43 2.2.1 Overall properties of the Sun 43 2.2.2 The Sun’s total energy output 45 2.2.3 Black body radiation and the sun’s surface temperature 46 2.2.4 The Fraunhofer lines in the solar spectrum and the composition of the sun 49 2.3 Nuclear fusion 50 2.3.1 The proton–proton cycle 53 2.4 The solar neutrino problem 57 2.4.1 The solar neutrino problem is solved 58 2.5 The solar atmosphere: photosphere, chromosphere and corona 59 2.5.1 Coronium 61 2.6 The solar wind 61 2.7 The sun’s magnetic fi eld and the sunspot cycle 62 2.7.1 Sunspots 62 2.7.2 The sunspot cycle 64 2.8 Prominences, fl ares and the interaction of the solar wind with the earth’s atmosphere 65 2.8.1 The aurora 66 2.9 Solar eclipses 67 2.9.1 Two signifi cant solar eclipses 69 2.9.2 The Shapiro delay 71 2.10 Questions 72 Chapter 3: Our Solar System 2 – The Planets 75 3.1 What is a planet? 75 3.2 Planetary orbits 77 3.2.1 Orbital inclination 78 3.3 Planetary properties 79 3.3.1 Planetary masses 79 3.3.2 Planetary densities 80 3.3.3 Rotation periods 80 3.3.4 Planetary temperatures 81 3.3.5 Global warming 83 3.3.6 Albedo 83 Contents ix 3.4 Planetary atmospheres 84 3.4.1 Secondary atmospheres 86 3.4.2 The evolution of the earth’s atmosphere 87 3.5 The planets of the solar system 87 3.5.1 Mercury 88 3.5.2 Venus 89 3.5.3 The Earth 92 3.5.4 The moon 94 3.5.5 Mars 102 3.5.6 Ceres and the minor planets 106 3.5.7 Jupiter 108 3.5.8 Saturn 113 3.5.9 Uranus 117 3.5.10 Neptune 120 3.5.11 Pluto 124 3.5.12 Eris 128 3.6 Comets 129 3.6.1 Halley’s comet 130 3.6.2 Cometary nuclei 131 3.7 Questions 132 Chapter 4: Extra-solar Planets 135 4.1 The radial velocity (Doppler wobble) method of planetary detection 135 4.1.1 Pulsar planets 138 4.1.2 The discovery of the fi rst planet around a sun-like star 139 4.2 Planetary transits 142 4.3 Gravitational microlensing 145 4.4 Astrometry 148 4.5 Discovery space 149 4.6 Selection effects and the likelihood of fi nding solar systems like ours 151 4.7 Questions 151 Chapter 5: Observing the Universe 153 5.1 Thinking about optics in terms of waves rather than rays 153 5.1.1 The parabolic mirror 153 5.1.2 Imaging with a thin lens 156 5.1.3 The achromatic doublet 159 x Contents 5.2 The human eye 161 5.3 The use of a telescope or pair of binoculars to see fainter objects 163 5.4 Using a telescope to see more detail in an image 164 5.4.1 An interesting worked example of the effects of diffraction 166 5.4.2 The effect of diffraction on the resolution of a telescope 167 5.5 The magnifi cation of a telescope 168 5.6 Image contrast 170 5.7 The classic Newtonian telescope 170 5.8 The Cassegrain telescope 172 5.9 Catadioptric telescopes 172 5.9.1 The Schmidt camera 172 5.9.2 The Schmidt–Cassegrain telescope 173 5.9.3 The Maksutov–Cassegrain telescope 174 5.10 Active and adaptive optics 174 5.10.1 Active optics 175 5.10.2 Adaptive optics 175 5.11 Some signifi cant optical telescopes 176 5.11.1 Gemini North and South telescopes 176 5.11.2 The Keck telescopes 177 5.11.3 The South Africa Large Telescope (SALT) 177 5.11.4 The Very Large Telescope (VLT) 178 5.11.5 The Hubble Space Telescope (HST) 179 5.11.6 The future of optical astronomy 180 5.12 Radio telescopes 181 5.12.1 The feed and low noise amplifi er system 182 5.12.2 Radio receivers 183 5.12.3 Telescope designs 184 5.12.4 Large fi xed dishes 186 5.12.5 Telescope arrays 188 5.12.6 Very Long Baseline Interferometry (VLBI) 189 5.12.7 The future of radio astronomy 191 5.13 Observing in other wavebands 193 5.13.1 Infrared 193 5.13.2 Submillimetre wavelengths 193 5.13.3 The Spitzer space telescope 195 5.13.4 Ultraviolet, X-ray and gamma-ray observatories 195 Contents xi 5.14 Observing the universe without using electromagnetic radiation 197 5.14.1 Cosmic rays 197 5.14.2 Gravitational waves 199 5.15 Questions 202 Chapter 6: The Properties of Stars 205 6.1 Stellar luminosity 205 6.2 Stellar distances 205 6.2.1 The parsec 207 6.3 Proper motion 208 6.3.1 Hipparcos and GAIA 208 6.4 The absolute magnitude scale 209 6.4.1 The standard formula to derive absolute magnitudes 210 6.5 Colour and surface temperature 212 6.6 Stellar photometry 214 6.7 Stellar spectra 214 6.7.1 The hydrogen spectrum 215 6.7.2 Spectral types 216 6.8 Spectroscopic parallax 217 6.9 The Hertzsprung–Russell Diagram 219 6.9.1 The main sequence 220 6.9.2 The giant region 220 6.9.3 The white dwarf region 222 6.9.4 Pressure broadening 222 6.10 The size of stars 223 6.10.1 Direct measurement 223 6.10.2 Using binary star systems to calculate stellar sizes 225 6.10.3 Using the Stephan–Boltzman law to estimate stellar sizes 226 6.11 The masses and densities of stars 227 6.12 The stellar mass–luminosity relationship 228 6.13 Stellar lifetimes 229 6.14 Questions 230 xii Contents Chapter 7: Stellar Evolution – The Life and Death of Stars 231 7.1 Low mass stars: 0.05–0.5 solar masses 231 7.2 Mid mass stars: 0.5–∼8 solar masses 232 7.2.1 Moving up the main sequence 233 7.2.2 The triple alpha process 234 7.2.3 The helium fl ash 236 7.3 Variable stars 237 7.4 Planetary nebula 239 7.5 White dwarfs 240 7.5.1 The discovery of white dwarfs 240 7.5.2 The future of white dwarfs 241 7.5.3 Black dwarfs 241 7.6 The evolution of a sun-like star 241 7.7 Evolution in close binary systems – the Algol paradox 243 7.8 High mass stars in the range 8 solar masses 243 7.9 Type II supernova 246 7.9.1 The Crab Nebula 247 7.9.2 Supernova 1987A 248 7.10 Neutron stars and black holes 250 7.11 The discovery of pulsars 252 7.11.1 What can pulsars tell us about the universe? 255 7.12 Pulsars as tests for general relativity 257 7.13 Black holes 259 7.13.1 The detection of stellar mass black holes 260 7.13.2 Black holes are not entirely black 262 7.14 Questions 262 Chapter 8: Galaxies and the Large Scale Structure of the Universe 265 8.1 The Milky Way 265 8.1.1 Open star clusters 266 8.1.2 Globular clusters 267 8.1.3 The interstellar medium and emission nebulae 268 8.1.4 Size, shape and structure of the Milky Way 269 8.1.5 Observations of the hydrogen line 271 8.1.6 A super-massive black hole at the heart of our galaxy 275 Contents xiii 8.2 Other galaxies 275 8.2.1 Elliptical galaxies 275 8.2.2 Spiral galaxies 277 8.2.3 Evidence for an unseen component in spiral galaxies – dark matter 279 8.2.4 Weighing a galaxy 280 8.2.5 Irregular galaxies 283 8.2.6 The Hubble classifi cation of galaxies 284 8.3 The universe 285 8.3.1 The cosmic distance scale 285 8.3.2 Using Supernova 1987A to measure the distance of the Large Magellanic Cloud 285 8.3.3 The Cepheid variable distance scale 287 8.3.4 Starburst galaxies 289 8.3.5 Active galaxies 291 8.3.6 Groups and clusters of galaxies 295 8.3.7 Superclusters 297 8.3.8 The structure of the universe 297 8.4 Questions 298 Chapter 9: Cosmology – the Origin and Evolution of the Universe 301 9.1 Einstein’s blunder? 301 9.2 Big Bang models of the universe 301 9.3 The blueshifts and redshifts observed in the spectra of galaxies 303 9.4 The expansion of the universe 304 9.4.1 A problem with age 306 9.5 The steady state model of the universe 308 9.6 Big Bang or Steady State? 309 9.7 The cosmic microwave background 309 9.7.1 The discovery of the cosmic microwave background 310 9.8 Infl ation 312 9.9 The Big Bang and the formation of the primeval elements 313 9.10 The ‘ripples’ in the Cosmic Microwave Background 313 9.11 How dark matter affects the cosmic microwave background 314 9.12 The hidden universe: dark matter and dark energy 316 9.12.1 Evidence for dark matter 317 9.12.2 How much non-baryonic dark matter is there? 319 9.12.3 What is dark matter? 319 xiv Contents 9.12.4 Dark energy 322 9.12.5 Evidence for dark energy 322 9.12.6 The nature of dark energy 324 9.13 The makeup of the universe 325 9.14 A universe fi t for intelligent life 326 9.14.1 A ‘multiverse’ 328 9.14.2 String theory: another approach to a multiverse 328 9.15 Intelligent life in the universe 329 9.15.1 The Drake equation 329 9.15.2 The Search for Extra Terrestrial Intelligence (SETI) 331 9.16 The future of the universe 332