2.5 physics

• Solar system


  A set of planets and bodies that orbit the Sun (our nearest star)
• Solar system


  • Kept in orbit by gravitational forces
  • Consists of eight planets, four inner rocky ones and four outer gaseous ones
  • Separated by a band of rocky material and debris called the asteroid belt
• Formation of the solar system

  1. Cloud of dust and gas, rich in hydrogen, begins to contract under gravity

  2. Gas is compressed, making it hotter

  3. Cloud begins to spin

  4. Protostar formed at the centre of the spinning cloud

  5. Protostar continues to get hotter until nuclear fusion occurs, forming the Sun

  6. Cloud continues spinning around the Sun and accretion of material due to gravitational forces gradually leads to the formation of planets
• Inner planets


  Formed from the accumulation of generally denser non-volatile material, have solid surfaces that can be walked on
• Outer planets (gas giants)

  Made up of accumulated gaseous material from the dust and gas cloud, much larger (radii in the order of 10^4 km) than the inner rocky planets (radii in the order of 10^3 km), although the inner rocky planets are denser, the gas giants are far more massive (10^26-10^27 kg) than they are (10^23-10^25 kg)
• Pluto


  Classified as a dwarf planet due to its small mass (~10^22 kg) and very irregular, elliptical orbit
• Moons


  • Some planets have moons which orbit them individually, moons have a mass that is generally <10^23 kg
• Comets


  Large balls of rock and ice from space, as they near the Sun, heat from the Sun melts the ice and vaporizes it, creating a vapour 'tail', have highly elliptical and irregular orbits
• Asteroids


  Large rocks moving through space, mainly found in the asteroid belt between Mars and Jupiter (mass of belt ~= 10^21 kg), can be remnants of former planets that never formed completely or were projected into space during collisions of planets in the early solar system
• Galaxies


  Composed of hundreds of millions of solar systems each orbiting the galaxy's centre of mass, which is thought to be a supermassive black hole for most galaxies
• Life cycle of stars

  1. Born from a cloud of dust and gas in a galaxy, gravitational attraction between the particles draws them together so the cloud becomes denser, temperature and pressure of the cloud increases as the particles move together, eventually the pressure gets so great that the particles can fuse together, nuclear fusion occurs as hydrogen nuclei fuse together to form helium nuclei, releasing a large amount of energy in the form of heat and radiation, the heat energy released causes gaseous expansion which is felt as an outward force, opposing collapsing forces of the cloud due to gravity, radiation pressure provides another outward force, eventually an equilibrium is established, where the fusion-derived expansional forces balance with the forces promoting gravitational collapse

  2. Eventually the star runs out of gas nuclei that can undergo fusion, the outward gas pressure decreases so the system is no longer in equilibrium, gravitational collapse results, and the star nears the end of its life, the mechanism with which this collapse takes place depends on the size of the star
• Massive stars


  Become red supergiants when they begin to collapse, eventually heavier elements begin to fuse in order to sustain the fusion process and maintain equilibrium, once all possible fusion of elements has occurred, an iron core forms in the star, fusion ceases and the star contracts under gravity, the star is too massive to be stable, so it collapses in on its centre and explodes into a supernova leaving a neutron star, during collapse into a supernova, much heavier elements such as Uranium are formed and distributed into space via the supernova explosion
• Very massive stars


  Collapse to form a red supergiant with such force that they form a black hole, regions of such high density that even light can't escape the force of gravity
• Low mass stars


  Form a red giant which eventually collapses to produce a planetary nebula before cooling to become a white dwarf
• Astronomical unit (AU)


  Unit of measurement for distances in space, 1 AU = 150,000,000 km = 1.5x10^11 m
• Light year (l-y)


  Unit of measurement for distances in space, 1 light year is the distance that light will travel in 1 year, 1 light year = 9.46x10^15 m
• Hertzsprung-Russell (H-R) diagram


  Diagrams that help to classify stars by comparing their temperature and brightness (luminosity), stars lying on the diagonal of H-R diagrams are in the main sequence stages of their life
• Paths of star life cycles on H-R diagrams

  Can be shown by drawing arrows, for a star the size of our sun, they will become red giants, then white dwarfs