The Life Cycle of Larger Stars (AQA GCSE Physics)

• A large star is one which is bigger than the Sun
• Stars that are larger than the Sun have much shorter lifespans - in the region of hundreds of millions of years (instead of billions)
  • This is because they burn through the fuel in nuclear fusion much quicker than smaller stars

• The life cycle of a star bigger than the Sun starts in the same way as a solar mass star

1. Nebula

• All stars form from a giant cloud of hydrogen gas and dust called a nebula

2. Protostar

• The force of gravity within a nebula pulls the particles closer together until it forms a hot ball of gas, known as a protostar
• As the particles are pulled closer together the density of the protostar will increase
  • This will result 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
  • The hydrogen nuclei will fuse to form helium nuclei
  • Every fusion reaction releases heat (and light) energy which keeps the core hot

4. Red Supergiant

• Eventually, the main sequence star will reach a stage when it starts to run out of hydrogen gas in its core
• 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 is greater than the outward force due to the pressure of the expanding gases

• A new series of fusion reactions will then occur around the core, for example helium nuclei will undergo fusion to form beryllium
• These fusion reactions will cause the outer part of the star to expand and it will become a super red giant
  • A super red giant is much larger than a red giant

5. Supernova

• Once the fusion reactions inside the red supergiant finally finish, the core of the star will collapse suddenly causing 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 will be ejected into space during the supernova explosion, forming a planetary nebula

6. Neutron Star (or Black Hole)

• In the case of the biggest 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

Lifecycle of a star much larger than our Sun

• 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 exploded out 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 up to Iron, this is proof that it must have once been made from the remains of a Supernova

A supernova