PMT notes

• Solar System


  One star (the Sun), plus the eight planets and the dwarf planets that orbit around the Sun, and natural satellites (moons) that orbit planets
• The sun lies at the centre of our solar system, it is heliocentric
• Planets in the solar system

  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • (Pluto)
• Planets

  • Smaller planets are made of primarily rock, then the larger planets are primarily gas
  • All planets orbit the Sun on the same plane
  • All planets rotate, just at different speeds
  • Some planets rotate in the opposite direction or on a skewed axis to the other planets, and this may be due to past collisions throwing its axis off balance
  • Larger planets have rings, as their gravitational field is so strong it attracts debris
• Geocentric model

  Initially, Earth was at the centre, the planets, our moon, and the sun, orbited the Earth
• Heliocentric model


  The sun at the centre, with the planets orbiting the Sun
• Evidence for heliocentric model

  • Mars' "retrograde motion"
  • Galileo observing moons orbiting Jupiter showed not everything orbited the Earth
  • Kepler showed that the planets orbited in ellipses, and not circles
• Planetary Orbits

  1. Gravitational force causes the planet to change direction constantly (it moves in a circle around the sun)

  2. This means the velocity is always changing

  3. Hence the force causes the planet to accelerate without increasing its speed
• For a stable orbit


  If the planet moves closer to the sun (orbital radius decreases), the gravitational attraction to the sun increases, so the orbital speed of the planet increases
• Life Cycle of Star

  1. Dust and gas cloud is present in a galaxy

  2. The gravitational attraction between the gas/dust particles draws them together

  3. The cloud becomes more concentrated, as the particles get closer

  4. The temperature and pressure of the cloud increases as the particles get pushed so close together

  5. Eventually the pressure gets so great that the gas/dust particles are able to fuse together

  6. Fusion occurs as the light (mainly hydrogen gas) nuclei fuse together to form helium nuclei, creating a large amount of energy

  7. This release opposes the collapsing of the cloud due to gravity

  8. So eventually an equilibrium forms, where the energy released due to fusion balances the pressure of gravitational collapse

  9. Eventually the star runs out of gas to fuse, so it collapses

  10. If Massive, the star will collapse, increasing the pressure + temperature of the core, meaning heavier elements can fuse. Once all the fusion has happened, it is too massive to be stable, so the star collapses, rebounds on its centre and produces a supernova

  11. What remains is either a neutron star or black hole

  12. If Normal-sized, the same process happens, less fusion occurs however (less fuel to fuse), the star collapses, and produces a planetary nebula, a lower scale supernova, and a white dwarf remains

  
• Red Shift

  Light appears red shifted from galaxies which are moving away from Earth, evidence of an expanding universe
• As the universe expands, the distance from the galaxies also expands, so light from a galaxy has its wavelength "red-shifted" as it appears to move away from us, and the frequency appears to decrease
• Evidence for the Big Bang


  • Red Shift
  • CMB (Cosmic Microwave Background radiation)
• The Big Bang accounts for all the experimental evidence, so it is the most accepted model currently
• There is much about the universe that is not understood, for example dark mass and dark energy