Lesson Summary

• Hydrogen and helium formed shortly after the Big Bang. They formed first because they are the lightest, simplest elements Heavier elements were created after a few hundred thousand years.

• Stars contain heavy elements such as iron. Heavier elements fuse together in a star's center, through the process of nuclear fusion. 

• Galaxies are composed of millions to billions of stars 

• There are spiral, elliptical and irregular galaxies in our universe.

• Stars contain mostly hyrogen and helium (lightest elements), but more massive stars also have heavier elements such as iron, carbon and oxygen.

• There are four stages of stars: protostar, main sequence, red giant/supergiant and white dwarf/neutron star/black hole. The exact life cycle a star follows depends on its mass. 

• Our solar system formed in a nebula disk, with temperatures & densities higher closer to the sun. Planets closer to the sun (Mercury, Venus, Earth and Mars) are rocky and have heavier elements. Planets farther from the sun are gaseous and icy and made of mostly light elements. This is because of density and temperature differences in the nebula disk. 

• Heavy elements are found within stars today and are created through the process of nuclear fusion

• Recent evicence shos that there is a lot of matter in the universe we cannot directly detect. This matter is called dark matter. 

• Scientists theorize that dark matter and energy may help explain the more rapid expansion of our universe today. 

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