Star Formation (AQA GCSE Physics): Revision Note

The Sun

• The Sun is a star which lies at the centre of the Solar System

• It consists mostly of two elements, hydrogen and helium

• The Sun and all the planets in the Solar System formed from a massive cloud of dust and gas called a nebula

A nebula

A nebula is a cloud of gas and dust in space (Image courtesy of NASA)

• Gravitational attraction pulled this cloud together into a giant ball

• As the nebula collapsed, the centre of this ball got very dense and hot and began to rotate

• This hot, dense ball was the Sun as a protostar

• Eventually, when the Sun's core became hot enough, hydrogen nuclei started to undergo nuclear fusion to form helium nuclei

Our Sun

The Sun is the star at the centre of our Solar System

Star Formation

• All stars are thought to form in the same way and follow the same initial stages:

nebula → protostar → main sequence star

Nebula

• Stars form from a giant cloud of hydrogen 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

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 of the star hot

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

• The star will remain stable in this stage for the majority of its life

Equilibrium in stars

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

• Gravity acts inwards

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

• Pressure acts outwards

  • This arises from an outward force exerted by the expanding hot gases inside the star as a result of energy released by fusion reactions

• When the two forces are balanced, the star is in equilibrium, and it is said to be stable

• The inward force due to gravitational collapse is equal to the outward force due to fusion energy

Balanced forces in a main sequence star

The outward and inward forces within a main sequence star are in equilibrium. The centre red circle represents the star's core, and the orange circle represents the star's outer layers

• If the temperature of a star increases, the outward pressure will also increase

  • This occurs when the energy released by fusion reactions increases

  • This will cause the star to expand

• If the temperature drops, the outward pressure will also decrease

  • This occurs when the energy released by fusion reactions decreases

  • This will cause the star to contract