Universe [information over pictures]

Most astronomers [scientists who study stars, planets and galaxies] think that the Universe was formed during an event called the Big Bang. This was a giant explosion which occurred between 10 and 20 billion years ago.	This giant explosion hurled matter in all directions and caused space itself to expand. As the Universe cooled, the material in it combined to form spinning, swirling disks of material we call galaxies.	At the centre, gravity [the force by which a planet or other body draws objects toward its centre] pulled more material in. As these bodies smashed into one another they formed larger and larger galaxies.
Earth is located in an outer spiral arm of one of the galaxies called the Milky Way. The Milky Way is one of perhaps 100 billion galaxies in the universe.	So we live on a planet in our own Solar System with a star at its centre. Most of the hundreds of billions of stars in our galaxy are thought to have planets of their own.

Look at all of the pictures in turn. As you hover over each one a blue screen appears with an explanation of what the picture shows. Important words are highlighted. If you are certain that you know about the picture click on it and it will disappear. As you go through all of the pictures you will get rid of all those which you know about and be only left with those you may be unsure of. In some cases you may get rid of all of the pictures in which case you are ready to move on to other activities which will test your knowledge on that particular topic.