Topic 7: Astronomy

• Structure of the Solar System:

  • 1 star (the Sun)
  • 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)
  • dwarf planets (Pluto)
  • moons (natural satellites)
  • asteroids
  • dust/gas
• Observing Space Timeline:

  1. naked eye (thousands of years)
  2. telescopes (simple, on Earth)
  3. better telescopes (complex, on Earth)
  4. advanced telescopes (in orbit/space)
• Telescope images are clearer in space as not all frequencies and wavelength of EM waves reach Earth.

  By placing telescopes in space, all wavelength of the EM spectrum are available which allows more detailed images and more discoveries
• Geocentric Model - the Earth is at the centre of the universe and all other astronomical objects orbit around it
• Heliocentric model - the Sun is the centre of the solar system as after the invention of the telescope, the Geocentric model was disproved
• The size of gravity (gravitational field strength) around an object depends on its mass.

  The greater the mass, the greater gravity is
• Orbits around the Sun:

  • planets orbit the sun in an oval shape (elliptical)
  • moons orbit planets in a circular shape (circular)
  • comets orbit the sun in a more oval shape (highly elliptical)

  The sun is not at the centre of the orbit
• Circular Orbits:

  • velocity is a vector
  • since direction is changing, so is velocity
  • speed remains constant
• The relationship between the speed of an object and its radius is a negative correlation so a smaller radius equals faster speed
• The Doppler Effect - the apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer
• Light received from distant galaxies appears to be shifted to the red end of the visible light spectrum (red light has lower frequency and higher wavelength)

  This shows that galaxies are moving away from us and the universe is expanding.

  The further a galaxy, the greater the red-shift is (moving faster away)
• Big Bang Theory:

  • the universe began around 14 billion years ago
  • all matter and energy existed as a tiny dot of infinite density and temperature
  • suddenly began expanding and continues today (average density decreases)

  Evidence for this includes red-shift which shows that other galaxies are constantly moving away from us (space is expanding)
• Steady State Theory:

  • the universe is creating matter as it expands meaning it maintains a constant average density
  • it has always existed

  This is a disproved theory despite having red-shift as evidence because it cannot explain the existence of Cosmic Microwave Background (CMB) radiation
• During an explosion, energy (radiation) is released equally in all directions, similar to the Big Bang Theory.

  This energy is detected as CMB radiation which is equally spread throughout the universe
• Main Star Life Cycle:

  Nebula (cloud of dust and gas)--1-> Protostar --2-> Main Sequence Star (stable as forces are balanced) --3-> Red Giant/Supergiant

  1. Gravity pulls the gas together (potential -> kinetic -> thermal energy)
  2. Thermal energy means nuclear fusion begins
  3. Hydrogen runs out, the star expands and cools
• There are two endings of a star life cycle depending on the mass of the main sequence star:

  • low mass main sequence (e.g. Sun) --> red giant --> white dwarf (planetary nebula) --> black dwarf
  • large mass main sequence --> red supergiant --> supernova (explodes) --> neutron star or a black hole if massive
• Main sequence stars are stable because the force of gravity that pushes the core inwards (hot enough to fuse hydrogen into helium) equals the force of radiation pressure (nuclear fusion pushing out - thermal expansion)