Main Sequence Stars (Oxford AQA IGCSE Physics)
Revision Note
Written by: Caroline Carroll
Reviewed by: Lucy Kirkham
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
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