life cycle of stars

• • starts as cloud of dust and gas
• • force of gravity pulls dust and gas together to form prostar
  • temps rising and protostar becomes denser and more particals collide
  • when temp is high enough, hydrogen nuclei in core under go nuclear fusion to form helium nuclei
  • gives out huge amounts of energy which keeps the core of the star hot
• • star enters long stable period where radioactive pressure is balanced againts the gravitational force,
  • this is the main sequence and typically lasts several billions of years
• • eventually the hydrogen atoms run in the core
  • fusion stops and so does the radiation pressure so forces are unbalanced
  • temp increases until outer layers are hot enough to start going under going fusion
  • star begins to expand again until the outer layers balance
  • causes outer layers to cool and the star becomes a red giant
  • process continues until elements become to heavy to diffuse