Edexcel IGCSE Physics 复习笔记 8.2.3 The Life Cycle of Larger Stars

• A large star is one that is much larger 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)

   

• The life cycle of a star larger 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

   

• Thermal expansion from fusion reactions occur within its core and the force of gravity keeps the star in equilibrium
• At this point, the star is born, and it becomes a main-sequence star
• During the main sequence, the star is in equilibrium and said to be stable
  • The inward force due to gravity is equal to the outward pressure force which results from the expanding hot gases inside the star

   

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 a supernova

6. Neutron Star (or Black Hole)

• At the centre of this explosion a dense body, called a neutron star will form
• In the case of the largest 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 stars much larger than our Sun