People have been observing space for thousands of years. many models of how the sun and planets are organized were proposed. One such model was proposed by Ptolemy, an Egyptian astronomer, about 2,000 years ago. His model is called the geocentric model of the universe. He proposed that the Earth was the centre of the universe and that the sun, moon and planets were in orbit around the Earth. For 1,500 years, this model was accepted as an explanation for how the universe worked.

However, in 1543 a Polish priest and astronomer, Nicolaus Copernicus, proposed an alternative model of the universe. He called it the heliocentric model. In this model the sun is at the center of the solar system and the planets are in orbit around it. This model was proven by Galileo Galilei in about 1615.He used a telescope that he built that was more powerful than any telescope that had ever been made before.

• The Branch of astronomy that deals with the formation of the universe is called cosmology.

• Astronomers believe that the universe began about 14 billion years ago in a 'Big Bang'

• At that time, the entire universe was inside a bubble that was much smaller than a pinhead. It was extremely hot and dense

• Then it suddenly exploded. The universe began to be formed. Time, space and matter all began with the Big Bang

• In a fraction of a second, the universe grew from the size of a single atom to bigger than the size of a galaxy. It kept on growing at a huge rate and is still expanding today.

• Within the first few seconds, protons, neutrons and electrons were made. However it was too hot for these to combine to form atoms

• After 300,000 years, the temperature had dropped enough to allow atoms of smaller elements, for example hydrogen and helium to form.

• After 1 billion years, these giant clouds of atoms were pulled together by gravity to make the first stars and galaxies.

• After 5 billion years, these first stars ran out of fuel and died. They exploded an forced new, heavier elements into space. These gathered together and formed new stars, planets, planetary systems and galaxies

• Energy called cosmic background radiation remains everywhere in space. Scientists predicted that this would be present if the Big Bang had taken place. It was detected in 1964 and this strongly supported the theory.

• Edwin Hubble and Vesto Slipher discovered that the universe is continuously expanding. They noticed that distant galaxies are moving away faster than nearby ones. This would suggest that all galaxies started from one point. You can compare it to a race; everybody starts at the same point, but the faster runners will move further away from the starting position. This is the same for galaxies.

• The Big Bang theory predicts how much of certain elements should have been made in the early universe. What we see in very distant galaxies and old stars firs in with this prediction.

• When we look at stars in the night sky, the light we see is from distant galaxies that has taken millions of years to get here! Looking at distant galaxies with telescopes is like looking back in time to what our galaxy was like a long time ago. Scientists are able to look at the light coming from galaxies forming that are less than a billion years old. This gives us information about what happened in our galaxy billions of years ago.