The Life Cycle of Stars (Oxford AQA IGCSE Physics)

Life Cycle & Mass

• Stars have a life-cycle which means they change over time

  • The exact stages of the life cycle are determined by the mass of the star

• All stars start in a stellar nebula, become a protostar and then a main sequence star

• During the main sequence

  • Hydrogen nuclei combine in nuclear fusion reactions to produce light and heat

  • Stars can maintain their energy output for millions of years because of the large amounts of hydrogen available in space

  • Once all of the hydrogen has reacted, the star moves onto the next stage in its life cycle

  • This is dependent on the mass of the star

Summary of the life cycles of stars

Life Cycle of High-Mass 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)

The life cycle of a high-mass star

Red supergiant

• After several million years, the hydrogen causing 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 point in space that not even light can escape from

Life Cycle of Low-Mass Stars

• After the main sequence, a smaller mass star, like our Sun, finishes its life cycle in the following evolutionary stages:

Red giant → planetary nebula → white dwarf → black dwarf

The life cycle of a low-mass star

Red giant

• After several billion years the hydrogen causing 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

• This causes the core to shrink and heat up

  • The core will shrink because the inward force due to gravity will become greater than the outward force due to the pressure of the expanding gases as the fusion dies down

• A new series of reactions will then occur around the core, for example, helium nuclei will undergo fusion to form heavier elements, such as carbon

• These reactions will cause the outer part of the star to expand to become a red giant 

  • It is red because the outer surface starts to cool

Planetary nebula

• Once this second stage of fusion reactions has finished, the star will become unstable and eject the outer layer of dust and gas

  • The layer of dust and gas which is ejected is called a planetary nebula

White dwarf

• The core which is left behind will collapse completely, due to the pull of gravity, and the star will become a white dwarf

• The white dwarf will be cooling down and as a result, the amount of energy it emits will decrease

Black dwarf

• Once the star has lost a significant amount of energy it becomes a black dwarf

• It will continue to cool until it eventually disappears from sight