The Life Cycle of Solar Mass Stars (Edexcel IGCSE Physics)

1. Nebula

• All stars form from a giant cloud of hydrogen gas and dust called a nebula

2. Protostar

• The force of gravity within a nebula pulls the particles closer together until it forms a hot ball of gas, known as a protostar
• As the particles are pulled closer together the density of the protostar will increase
  • This will result in more frequent collisions between the particles which causes the temperature to increase

3. Main Sequence Star

• Once the protostar becomes hot enough, nuclear fusion  reactions occur within its core
  • The hydrogen nuclei will fuse to form helium nuclei
  • Every fusion reaction releases heat (and light) energy which keeps the core hot

• Thermal expansion from fusion reactions occur within its core due to fusion and the force of gravity keeps the star in equilibrium
• At this point, the star is born, and it becomes a main-sequence star
• During the main sequence, the star is in equilibrium and said to be stable
  • The inward force due to gravity is equal to the outward pressure force which results from the expanding hot gases inside the star

4. Red Giant

• After several billion years the hydrogen causing the fusion reactions in the star will begin to run out
• Once this happens, the fusion reactions in the core will start to die down
• This causes the core to shrink and heat up
  • The core will shrink because the inward force due to gravity will become greater than the outward force due to the pressure dies down

• A new series of reactions will then occur around the core, for example, helium nuclei will undergo fusion to form beryllium
• As the core shrinks, more reactions will cause the outer part of the star to expand
• It will become a red giant
  • It is red because the outer surface starts to cool

5. White Dwarf

• The star will eventually become unstable and eject the outer layer of dust and gas
• The core which is left behind will collapse completely, due to the pull of gravity, and the star will become a white dwarf
• The white dwarf will be cooling down and as a result, the amount of energy it emits will decrease

The lifecycle of a solar mass star