The "Big Bang"

• Scientists have come to call their theory of the event that began the universe the "Big Bang." All the matter and energy in the universe started as a tiny point, rapidly expanding in volume, forming subatomic protons and neutrons, then atoms. After about a billion years, atoms of hydrogen and helium, the only elements in existence then, clumped together under the force of gravity to form stars and galaxies.
  
  

The Supernova Phenomenon

• Stars produce enormous amounts of light energy from a process called nuclear fusion. They first use the lighter elements as fuel, beginning with hydrogen and helium and progressing through the heavier elements. The process stops with iron, though on Earth, we have dozens of elements heavier than iron. It requires an explosion at the end of a star's life, called a supernova, to produce these elements. According to the supernova theory, when stars explode, they leave behind pockets of gas and dust loaded with heavy elements.
  
  

Debris Collects

• Under the solar system development theory, about 4.5 billion years ago, a pocket of gas and dust such as that created by a supernova collected under the force of its own gravity and began to spin. The pocket contained mostly hydrogen, with traces of heavier elements up to uranium. Most of the hydrogen condensed in the center, with heavier elements forming rings just outside it, and more hydrogen outside of those rings. Our sun formed as a sphere of hydrogen and, after acquiring enough size and mass, started fusion reactions under the force of its own gravity.
  
  

Planets Form

• After our sun began its fusion reactions, the terrestrial planets -- Mercury, Venus, Earth and Mars -- condensed from the heavier elements immediately surrounding it. While these four planets have very different environments as a result of their varying proximities to the sun, they have similar, rocky compositions. Further out, the planets astronomers call "gas giants" -- Jupiter, Saturn, Uranus and Neptune -- formed from hydrogen, helium and traces of heavier elements. Astronomers theorize it took about 500 million years for the planets to form from the original rings of dust and gas.