The Life Cycle of Larger Stars (AQA GCSE Physics): Revision Note

Larger Stars

• After the main sequence, a larger star finishes its life cycle in the following evolutionary stages:

red supergiant → supernova → neutron star or black hole

Red supergiant

• After several million years, the hydrogen that fuels 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 to form carbon

  • This is followed by further fusion reactions in which nitrogen and oxygen are formed

  • Heavier elements up to iron are also formed

• This causes the outer part of the star to expand

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

Supernova

• Once the fusion reactions inside the red supergiant cannot continue, the core of the star will collapse suddenly

• The outer layers are blown away in a gigantic explosion 

  • This is called a supernova

• At the centre of this explosion, a dense body called a neutron star will form

• The outer remnants of the star are ejected into space during the supernova explosion, forming new clouds of dust and gas (nebula)

  • The nebula from a supernova may form new stars with orbiting planets

    • The heaviest elements (elements heavier than iron) are formed during a supernova and are ejected into space

    • These nebulae may form new planetary systems

Neutron star (or black hole)

• In the case of the most massive stars, the neutron star that forms at the centre will continue to collapse under the force of gravity until it forms a black hole

  • A black hole is an extremely dense object which has a gravitational pull so strong that not even light can escape from it

Lifecycle of a star much larger than our Sun

Supernovae

• A supernova is a bright and powerful explosion that happens at the end of a massive star's life

  • It occurs when the star is bigger than the Sun

• The explosion releases a large amount of energy

• During a supernova, all of the elements which were produced by the fusion reactions are expelled along with neutrons

• The neutrons combine with the elements to form even heavier elements

• These elements are ejected into the universe by the supernova explosion and form new planets and stars

  • Since Earth contains many heavy elements, including ones heavier than iron, this is proof that it must have formed from the remains of one or more supernovae

A supernova