The Life Cycle of Solar Mass Stars (Edexcel IGCSE Science (Double Award) (Modular): Physics Unit 2): Revision Note

The life cycle of solar mass stars

• All stars, including the Sun, began as a cloud of dust and gas

• Once a star has formed, it will spend its life going through a sequence of evolutionary stages, known as the life cycle of a star

Summary of the life cycles of stars

Star formation

• All stars follow the same initial stages:

nebula → protostar → main sequence star

Nebula

• Stars form from a giant interstellar cloud of gas and dust called a nebula

Protostar

• The force of gravity within a nebula pulls the particles closer together until a hot ball of gas forms, known as a protostar

• As the particles are pulled closer together the density of the protostar will increase

• This results in more frequent collisions between the particles which causes the temperature to increase

Main sequence star

• Once the protostar becomes hot enough, nuclear fusion  reactions occur within its core

• Once a star initiates fusion, it is known as a main sequence star

• During the main sequence, the star is in equilibrium and said to be stable

Evolution of a solar mass star

• After the main sequence, stars which are similar in size to the Sun go through the following evolutionary stages:

main sequence → red giant → white dwarf

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

• The star will begin to fuse helium which causes the outer part of the star to expand

• As the star expands, its surface cools and it becomes a red giant 

White dwarf

• Once the helium fusion reactions have finished, the star collapses and becomes a white dwarf

• The white dwarf cools down over time and as a result, the amount of energy it emits decreases

  • Once the star has lost a significant amount of energy and no longer emits light, it becomes a black dwarf