Astrophysics

• Universe


  A large collection of billions of galaxies
• Galaxy


  A large collection of billions of stars
• Solar system


  A collection of planets orbiting a common star. Our solar system is in the Milky Way galaxy.
• Gravitational field strength


  The force per unit mass on a body in a gravitational field, measured in Newtons per kilogram (N/kg)
• Gravitational field strength


  Varies with the mass and size of the body, different on other planets and the moon compared to the Earth
• Orbits


  • Moons orbit planets (slightly elliptical with near constant orbital speed)
  • Planets and comets orbit the Sun (orbits of planets slightly elliptical with near constant orbital speed, orbits of comets highly elliptical)
  • Artificial satellites orbit the Earth
• Comets have a greater speed nearer to the star (when r is smaller) because the ice inside them melts as they get closer (& warmer) - causing their mass to decrease
• Stellar evolution


  1. Nebula (cloud of dust & gas)

  2. Clumping due to gravity

  3. Nuclear fusion (hydrogen nuclei fuse to form helium, releasing energy)

  4. Main sequence star (outward pressure balances inward pressure due to gravity)

  5. Collapse (hydrogen used up, no outward pressure from fusion)

  6. Red giant (if similar mass to Sun)

  7. White dwarf (if similar mass to Sun)

  8. Red supergiant (if larger mass than Sun)

  9. Supernova (if larger mass than Sun)

  10. Neutron star or black hole (if exceptionally massive)
• Stellar classification


  Stars classified according to their colour, which is related to their surface temperature (hotter stars bluer, cooler stars redder)