Stellar evolution

• Blue/white stars (mass larger than the sun):

  • stellar nebula
  • protostar
  • main sequence
  • red supergiant
  • supernova
  • neutron star/black hole
• Red/orange stars (similar mass to the sun):

  • stellar nebula
  • protostar
  • main sequence
  • red giant
  • planetary nebula
  • white dwarf
  • Black dwarf
• White/blue stars are the hottest and have a surface temperature of about 50000 C
• Red/ orange stars are relatively cool and have a surface temperature of about 2500C
• High mass stars are white/blue in colour
• Low mass stars are red/orange in colour
• • nebula

  Stars form from large clouds of dust and gas particles (nebulae) that are drawn together by gravitational forces over millions of years. As the particles get closer the temperature and pressure becomes so large that nuclear fusion of hydrogen nuclei to helium nuclei occurs.  This releases enormous amounts of energy in the form of heat and light.
• • star (main sequence)

  Fusion produces forces that make the star expand outwards, but gravitational force is always pulling the particles within the star inwards. When these two opposing forces become balanced a star is stable and called a main sequence star. It should stay this way for millions of years, at a constant size and temperature.
• red giant

  Eventually hydrogen fusion stops as the star runs out of fuel. Gravitational force is now bigger than the outward fusion force which causes the star to collapse inwards and compress. This causes it to heat up to even higher temperatures so that fusion of helium nuclei begins. The increased power output causes the star to expand greatly. The surface area is so large that it is cooler than before, so its colour changes to red and the star is called a red giant.
• white dwarf

  Eventually fusion stops when the star runs out of helium nuclei and the gravitational force causes the star to collapse inwards and compress again. This heats it up so it changes colour to emit white light. The star is squashed so greatly by the gravitational force to become a small and very dense white dwarf. (They are so dense that a teaspoon full would weigh more than a cruise liner). A white dwarf eventually cools down and change colour as it does so, eventually becoming black.
• After the stable period, a giant star expands into red supergiant. (It produces all the elements up to iron during nuclear fusion). When it finally runs out of nuclei to fuse it collapses due to the gravitational force, and then explodes – an exploding star is called a supernova.

   

  The explosion throws dust and gas back into space and so another nebula is formed. A dense core remains – called a neutron star, because it is made entirely from neutrons. If its mass is large enough it can compress further to become a black hole. (Their gravity is so strong that not even light can escape!)