Life Cycle of Stars (WJEC GCSE Physics): Revision Note

Life Cycle of Stars

• All stars, including the Sun, began as a cloud of dust and gas

• Once a star has formed, it will spend its life going through a sequence of evolutionary stages, known as the life cycle of a star

Summary of the Life Cycles of Stars

Flow diagram showing the life cycle of a star which is the same size as the Sun (solar mass) and the lifecycle of a star which is much more massive than the Sun

Star Formation

• All stars follow the same initial stages:

Nebula → protostar → main sequence star

1. Nebula

• Stars form from a giant interstellar cloud of gas and dust called a nebula

2. Protostar

• The force of gravity 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 will increase

• This results in more frequent collisions between the particles which causes the temperature to increase

3. Main Sequence Star

• Once the protostar becomes hot enough, nuclear fusion  reactions occur within its core

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

• During the main sequence, the star is in equilibrium and said to be stable

The Life Cycle of a Low Mass Star

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

Red giant → planetary nebula → white dwarf

4. 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

• The star will begin to fuse helium which causes the outer part of the star to expand

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

5. Planetary Nebula

• Once the helium fusion reactions have 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

6. 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

The Life Cycle of a High Mass Star

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

Red supergiant → supernova → neutron star (or black hole)

• The key differences between a lower mass and higher mass star at this stage are:

  • A higher mass star will stay on the main sequence for a shorter time before it becomes a red supergiant

  • A lower mass star fuses helium into heavy elements, such as carbon, whereas a higher mass star fuses helium into even heavier elements, such as iron

4. 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 which causes the outer part of the star to expand

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

5. Supernova

• Once the fusion reactions inside the red supergiant cannot continue, the core of the star will collapse suddenly and cause a gigantic explosion 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 forming new clouds of dust and gas (nebula)

  • The heaviest elements are formed during a supernova, and these are ejected into space

  • These nebulae may form new planetary systems

6. 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