Main Sequence Stars (Oxford AQA IGCSE Combined Science Double Award)

Main Sequence Stars

• Once the protostar becomes hot enough, nuclear fusion  reactions occur within its core where its temperature and density are the greatest

  • The hydrogen nuclei will fuse to form helium nuclei

  • Every fusion reaction releases heat and light which keeps the core hot

• The greater the mass of the star, the hotter its core will be

• Hotter temperatures allow further fusion reactions to occur which produce heavier nuclei

• The Sun is an example of a star that is in the main sequence period of its life cycle

Our sun

Equilibrium in stars

• Stars are held together by a delicate balance of inward and outward forces

• Gravity acts inwards

  • This causes an attractive force which pulls the outer layers inwards

• Pressure acts outwards

  • This arises from an outward force which is exerted by the expanding hot gases inside the star

• During the main sequence, the star is in equilibrium as the forces acting on it are balanced and it is said to be stable

• The inward force due to gravity is equal to the outward force from the fusion reactions

Forces acting on a stable main sequence star