Star Formation (CIE IGCSE Physics)

Extended tier only

• Stars go through a sequence of evolutionary stages, known as the life cycle of a star
• All stars follow the same initial stages:

nebula → protostar → stable star

Nebula

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

Protostar

• The gravitational attraction 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 increases
• This results in more frequent collisions between the particles which causes the temperature to increase

Stable star

• Once the protostar becomes hot enough, nuclear fusion reactions occur within its core
  • The hydrogen in the core of the star is converted into helium
  • Every fusion reaction releases heat and light which keeps the core hot

• Once a star initiates fusion, it is known as a stable star
• During this stage, the star is in equilibrium as the forces acting on it are balanced
• Gravitational forces act inwards
  • This is an attractive force which pulls the outer layers inwards
• Thermal pressure acts outwards
  • This is exerted by the expanding hot gases inside the star as energy is released during fusion

Balanced forces in a stable star

The outwards and inwards forces within a star are in equilibrium. The centre red circle represents the star's core and the orange circle represents the star's outer layers

• Once a stable star is formed, the final stages of its life cycle depend on its mass
• The different life cycles are shown below

Summary of the life cycles of stars

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 more massive than the Sun