life cycle of stars

• stars depend on one key process, fusion of elements to release their energy. their stability depends on two forces.

  1. gravitational acting inwards
  2. radiation and gas pressure acting outwards
• 1st stage:

  stars are formed from clouds of gas, hydrogen and helium, and dust.
• 2nd stage:

  protostars form when gravity collects this cloud together and as it is compressed it heats up.
• 3rd stage:

  in main sequence stars, hydrogen fuses to form helium. this creates enough radiation and gas pressure to balance the gravitational forces.
• 4th stage: (for low mass stars e.g the sun)

  the hydrogen fuel begins to run out and the star will begin to fuse helium into larger elements (e.g carbon). this increases the gas and radiation pressure. the star will expand and become a red giant.
• 4th stage: (for large mass stars)

  the hydrogen fuel begins to run out and the star will begin to fuse helium into larger elements (e.g iron). this increases the gas and radiation pressure. the star will expand and become a supergiant.
• 5th stage: (for low mass stars e.g the sun)

  the helium fuel begins to run out and the reaction will slow. the gas and radiation pressure will decrease. the star will shrink and become and white dwarf.
• 5th stage: (for large mass stars)

  the helium fuel begins to run out and the reaction will slow. the star becomes very unstable and will explode into a supernova, throwing gases and dust out into space. the high pressure and temperature in a supernova will cause the fusion of the heaviest elements.
• 6th stage: (for large mass stars)

  the remaining matter will collapse due to the gravitational force as the gas and radiation pressure will be small and shrink to form a neutron star or a black hole.