The Life Cycle of Solar Mass Stars (AQA GCSE 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

Life Cycle of Stars

• Once a protostar is formed, its life cycle will depend on its mass
• The different life cycles are shown below:

Flow diagram showing the life cycle of a star which is the same size as the Sun (solar mass) and the lifecycle of a star which is much bigger than the Sun

• Once a star is born it is known as 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 from the fusion reactions

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 of the expanding gases as the fusion dies down

• A new series of reactions will then occur around the core, for example, helium nuclei will undergo fusion to form beryllium
• These 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. Planetary Nebula

• Once this second stage of fusion reactions have finished, the star will become unstable and eject the outer layer of dust and gas
  • The layer of dust and gas which is ejected is called a planetary nebula

6. White Dwarf

• 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

7. Black Dwarf

• Once the star has lost a significant amount of energy it becomes a black dwarf
• It will continue to cool until it eventually disappears from sight

The lifecycle of a solar mass star