Lesson Summary

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Star cluster RCW 108. Photo courtesy of NASA. Public domain.


  • Hydrogen and helium formed shortly after the Big Bang. Heavier elements were created after a few hundred thousand years.
  • Stars contain heavy elements such as iron.
  • The "Inside-out" theory states that the center of the galaxy formed first, followed by the outer parts.
  • The "Outside-in" theory state that the outer parts of the galaxy formed first, with the center created at another time.
  • Stars contain mostly hyrogen and helium, but also have iron, carbon and oxygen.
  • There are four stages of stars: Protostar, main sequence, red giant/supergiant and white dwarf/black hole.
  • The Earth formed with heavy elements inside of it.
  • Heavy elements are found within stars today
  • Recent evidence shows that there is a lot of matter in the universe that we cannot directly detect. This matter is called dark matter

  • The rate of expansion of the universe is increasing. The cause of this increase is unknown; one possible explanation involves a new form of energy called dark energy.
Last modified: Wednesday, 7 September 2011, 4:30 PM