Life cycle of stars

• Nebula


  A big cloud of dust and gas
• Star formation


  1. Gravity pulls dust and gas together to form a protostar

  2. Protostar gets bigger and denser as more particles collide and join

  3. Increased gravity and collisions raise the temperature

  4. Nuclear fusion of hydrogen to helium starts, releasing energy and forming a main sequence star
• Main sequence star


  • Outward pressure from nuclear fusion is balanced by inward pressure of gravity, allowing for a long stable period
• Star life cycle after main sequence

  1. Star runs out of hydrogen fuel

  2. Gravity contracts the star into a small, hot, dense ball

  3. Nuclear fusion of heavier elements starts, causing the star to expand again
• Red giant


  • Formed from a small to medium sized main sequence star
  • Becomes unstable and expels outer layers
  • Leaves behind a hot, dense, solid core called a white dwarf
• White dwarf


  • Gives off lots of light so appears white
  • Gets cooler and darker over time as it emits energy, becoming a black dwarf
• Red supergiant


  • Formed from a very large main sequence star
  • Undergoes cycles of expansion and contraction with more nuclear fusion
  • Eventually explodes in a supernova
• Supernova


  1. Ejects heavy elements across the universe

  2. If the star was very big, it condenses into a neutron star

  3. If the star was truly massive, it collapses into a black hole
• Black hole


  So dense that its gravity pulls in any nearby light, appearing as an empty space
• Stars initially form from clouds of dust and gas that come together under gravity
• The Wealth of Nations was written in 1776
• Marginal utility is the additional utility gained from consuming an additional product, and total utility is the sum of marginal utilities