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What to do: 1) Read article and look at the diagrammatic summary of the Big Bang. 2) Draw a series of pictures illustrating the Big Bang Theory. Number, label and comment on each picture frame. Make your Big Bang Theory as understandable as possible. Use the time, density and temperature references given in the diagram.

It was the most fantastic explosion anyone can imagine. What was to become every piece of matter in the universe was packed into a tiny bit of matter, a super-atom. Sometime between 12 and 20 billion years ago that super-atom exploded in what astronomers call the Big Bang.

At that moment, time and the Universe began. The fireball explosion of the Big Bang sent matter rushing off in all directions. The Universe began to grow larger, or expand, and it has been expanding ever since.

The First Kinds of Matter

At first, nearly all of the Universe was a cloud of hot hydrogen. Some of the hydrogen fused and formed helium, as happens today in the hot cores of stars. But then the hydrogen became too cool for more fusion to take place. Only a few minutes after the Universe was born, nine-tenths of its mass became hydrogen and one-tenth became helium.

The Galaxies Form

By about 100,000 to a million years after the Big Bang, the hydrogen and helium formed giant clouds. Some parts of space had bigger clouds than others parts. The gravity of these bigger clouds attracted and swallowed up smaller clouds. Gravity also caused the bigger clouds to be packed tighter and tighter. These clouds of hydrogen and helium were the beginning of the galaxies.

Each new galaxy cloud had smaller clumps of hydrogen and helium. These smaller clouds then became the birthplace of stars and planets. Today there are billions of galaxies. All of them seem to be 10 billion years old or more.

The Expanding Universe

When we say that the Universe is expanding, we mean that the galaxies are rushing away from each other at great speed. Some astronomers thank that the Universe will keep on expanding forever. Others think that it may slow down and stop expanding one day. All the galaxies may then start tumbling back in, drawn by gravity. Billions of years later, they may all smash together and form another super-atom that will explode in a Big Bang. These astronomers think that the cycle of expansion and explosion may never end.

No one knows for certain how the Universe began. Maybe it has always existed- without a beginning or an end. We can study the stars through telescopes, make measurements, make guesses, send robot spacecraft to the planets or visit them ourselves. But scientists will never know all there is to know about the Universe. Science is an endless search for knowledge. Each new discovery scientists make about a planet, a star, or a galaxy makes us a little bit richer in our knowledge about the world.