The Universe and Solar System

• Astronomy


  The branch of science that studies what is beyond Earth, also known as outer space
• Universe


  Everything that physically exists, including everything on Earth and everything in outer space. It is all of space and time and contains all forms of matter and energy
• Galaxy


  • A huge, rotating collection of celestial bodies which include: stars, gas, dust and planets
  • Classified into three major categories based on their shape

  
• Planetary system


  A set of gravitationally bound celestial bodies in orbit around a star or star system

  
• Orbit


  The closed path of a celestial body as it travels around another celestial object

  
• Solar System


  An example of a planetary system, in the Milky Way galaxy, consisting of the Sun which holds together all the planets and other celestial bodies using its gravitational force

  
• Star


  A massive celestial body composed of hot gases, mostly being hydrogen, that emits huge amounts of energy such as light and heat because nuclear fusion is taking place in its core
• Sun


  An example of a star, only a fraction of whose light reaches Earth, but is enough to keep water in its liquid state and provide enough energy to sustain life on Earth

  
• Planet


  • A large, non-luminous, round celestial object that orbits a star
  • Dwarf planets are spherical but do not dominate their orbit

  
• Types of planets


  • Terrestrial planets (part of the Inner Solar System, have hard and rocky surfaces)
  • Gas giants (part of the Outer Solar System, composed mostly of gases and liquids)

  
• Moon


  A type of non-luminous, natural satellite that travels around a planet or dwarf planet in an orbit

  
• Asteroid


  Small, irregularly-shaped celestial objects composed of rock and metal that orbit a star, varying in size up to 950 km in diameter, with most in the Asteroid Belt between Mars and Jupiter

  
• Meteoroid


  Pieces of metal or rock much smaller than an asteroid, ranging from dust particles to car or building size, that can be pulled by Earth's gravity into the atmosphere and burn up, creating a meteor
• Comet


  Large chunks of ice and dust that orbit the Sun, ranging from less than 100 m to more than 40 km across, that form a gaseous tail and a dust tail when close enough to the Sun

  
• Light year


  The distance light can travel in a year, equal to 9,461,000,000,000 km, used to measure distances in space because kilometres would be impractical
• Light takes time to travel, so we always see the Moon as it was 1.5 seconds ago