The Life Cycle Of A Star

Unit 2

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Life Cycle of Stars

  • Stars initialy form from clouds of dust and gas.
  • The force of gravity makes them spiral together to form a protostar.
  • Gravitational energy is converted into heat energy - temperature rises.
  • When the temperature gets high enough hydrogen nuclei undergo nuclear fusion to form helium nuclei and give out massive amounts of heat and light.    A star is born. 
  • Smaller masses of gas and dust may pull together to make planets that orbit the star. The star immediately enters a long stable period.
  •  The heat created by the nuclear fusion provides an outward pressure which balances the inward force of gravity.
  • The star maintains its energy output for millions of years due to massive amounts of hydrogen it consumes.
  • In its stable period it is called a main sequence star.  This lasts for several(10) billion years
  • Eventually hydrogen begins to run out. Heavier elements like iron are made by by nuclear fusion of helium. Small stars become a Red Giant and big stars become Red Super Giant if big. It becomes red because the surface cools.
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The Life Cycle Of A Star - 2

Red giant

  • A small star like our sun will then begin to cool and contract into a white dwarf.
  • As light fades completely it becomes a black dwarf.

Super Red Giants

  • Big stars however start to glow brighty again and undergo more fusion and expand and contract forming elements as heavy as iron.
  • Eventually they expload in a supernova.
  • This throws our layers of dust and gas into space leaving a very dense core called a neautron star.
  • If the star is big enough it will become a black hole.
  • The dust and gas thrown off by the Supernova will form a Second Generation Star

The heavier elements are only made in the final stages of a big star just before the final supernova. The presence of heavier elements in the Sun and inner planets is clear evidence our world was formed this way.

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