Space

• The solar system is not to scale in diagrams because the sizes of the sun and planets are very different, and the distances involved are extremely large
• Main parts of the solar system

  • Sun
  • 8 main planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)
  • Dwarf planets (including Pluto)
  • Moons
  • Asteroids
  • Comets
• Moons


  Natural satellites which orbit planets
• Most planets in the solar system have at least one moon, and some have a large number of moons
• Galaxy


  Massive group of stars, e.g. the Milky Way contains hundreds of billions of stars
• The solar system is just a tiny part of the Milky Way galaxy, and there are hundreds of billions of galaxies in the universe
• How stars like the sun form

  1. Cloud of dust and gas (nebula) collapses due to gravity

  2. Collapsing cloud becomes a protostar

  3. Protostar undergoes nuclear fusion, releasing energy

  4. Gravity and fusion energy reach equilibrium, forming a stable star
• Forces acting on a star

  • Gravity acts inwards, tending to make the star collapse
  • Energy from nuclear fusion acts outwards, tending to make the star expand
• The two opposing forces (gravity and fusion energy) are balanced, so the star is in equilibrium
• Nebula


  Cloud of dust and gas
• Star formation


  1. Gravity causes cloud of dust and gas to collapse

  2. As particles move faster, temperature rises to millions of degrees Celsius

  3. This early stage is called a protostar
• Nuclear fusion


  Hydrogen nuclei join together to form helium nuclei
• Main sequence star


  • Nuclear fusion releases a huge amount of energy
  • Stars can stay in the main sequence for a very long time
• Lifecycle of a star about the same size as the Sun

  1. Hydrogen in the star begins to run out

  2. Outward force due to fusion energy is less than inward force due to gravity, causing the star to collapse inwards

  3. Collapse causes temperature to increase

  4. Helium nuclei fuse together to create heavier elements

  5. Star expands to form a red giant

  6. Red giant stops fusing helium

  7. Star shrinks and forms a white dwarf

  8. White dwarf gradually cools down and forms a black dwarf
• Lifecycle of a much larger star than the Sun

  1. Star leaves the main-sequence stage and expands into a red supergiant

  2. Helium nuclei fuse together to produce heavier elements

  3. Nuclear fusion cannot make any element heavier than iron

  4. Star stops undergoing nuclear fusion

  5. Star explodes in a supernova

  6. Supernova temperature is high enough to produce elements heavier than iron

  7. Supernova explosion distributes these heavy elements throughout the universe

  8. Remains of the star can form a neutron star or a black hole
• Elements heavier than iron are only produced in a supernova
• Neutron star consists of neutrons densely packed together
• Black hole has such a large gravity that not even light can escape
• Planets


  • Orbit the Sun
  • Orbit is more like an ellipse than a circle
• Circular orbits


  • Force of gravity holds the object in orbit
  • Velocity changes but speed remains constant
• Velocity


  Speed of an object in a given direction
• If the speed of a satellite increases

  The radius of its orbit decreases
• This is also true for planets and moons
• The Earth's orbit around the Sun is virtually circular
• Moon


  • Natural satellite of the Earth
  • Force of gravity holds the moon in its orbit
• Artificial satellite


  • Man-made
  • Can be geostationary (orbit once every 24 hours)
  • Other artificial satellites orbit more frequently
• Redshift


  The observed increase in the wavelength of light from distant galaxies
• Redshift


  Provides evidence for the Big Bang theory
• The universe contains hundreds of billions of galaxies
• In the 1900s, astronomers detected that the light from very distant galaxies has an increased wavelength compared to light from closer galaxies
• Observing light from the Sun through a prism

  1. Produces a spectrum with dark lines

  2. Dark lines are caused by certain elements in the Sun absorbing light of specific wavelengths
• Redshift


  The observed shift of spectral lines towards longer, red wavelengths
• Redshift


  Indicates that the galaxies are moving away from each other
• Galaxies further away


  Have a bigger redshift and are moving faster
• Nearby galaxies


  Have a small redshift and are moving more slowly
• Very distant galaxies


  Have a very large redshift and are moving away very fast
• Big Bang theory


  The theory that the universe began from a very small region that was extremely hot and dense, which then expanded into the universe we see today
• Around 1998, astronomers noticed that the rate of expansion of the universe is actually increasing, rather than gradually slowing down as expected
• Scientists have suggested that the universe contains matter and energy that cannot be detected, called dark matter and dark energy, which could explain the accelerating expansion of the universe