Astronomers study one of physics' most fascinating subjects - outer space. Astronomy is the study of all that is found in space. It includes everything from amateur star-watching with an ordinary pair of binoculars to the very precise measurements taken by astronomers when they analyse the radio waves given off by stars in distant galaxies.

The word astronomy comes from a Greek word that means the "arrangement of the stars". Astronomy is a very old science; people have always been fascinated with the stars. For example, Aristotle, who lived around 400 BC, described the motion of the planets in his work On the Heavens. He believed that the earth was at the centre of the universe and was surrounded by the moon, the sun, the planets, and a sphere of stars, all moving in perfect circles. Aristotle's ideas were accepted until the 1500's when Copernicus suggested that the sun, and not the earth, was at the centre of the universe.

Astronomers study many interesting things in space, like planets, galaxies, meteors, comets, and asteroids. Of course, astronomers also study stars. The life of a star starts in huge clouds of dust and gas. The clouds swirl around and form clumps which shrink together and become very hot. A fully-formed star is made up of many tiny nuclear reactions. When you put the energy from all those reactions together, they form a star that gives off heat and light. You can tell how big and how hot a star is by looking at its colour and brightness. Red stars are the coolest, yellow stars are in the middle and blue stars are the hottest. Often blue stars are very young stars.

There are many different kinds of stars. Our sun is a star - the nearest star to the earth. A white dwarf is a dying star that has collapsed to something around the size of a planet. A supernova is a star that has exploded for the last time. It may release as much energy in 24 hours as our sun would give off in a billion years.

Like all scientists, astronomers use tools to make measurements about the things they study. Astronomers use very powerful telescopes that work just like the smaller telescopes that amateur astronomers use, to look at the objects they study in space. They also use huge emitters and receivers to send and receive radio signals from space. These radio signals can tell the astronomers how far away a planet or a star is, how big it is, and, if it's moving, how fast it's moving. Radio waves and telescopes are only a couple of examples of the many instruments astronomers use to observe and study space.

Space holds many unknowns. It is a place humans can visit, but we are not yet able to live there. The study of astronomy can help us learn about the many mysteries of the universe.