Stars are giant, hot spinning balls of gas. Our Sun is a star. Stars give off massive amounts of energy, including light and heat. They vary enormously in size and by the amount of light they give off. Stars vary in colour according to their temperature. Blue stars are hot; red stars are cool. Orange and yellow stars (like our Sun) are moderately hot.

Giants and dwarfs

Stars come in many colours and sizes. The largest star in this scale illustration is the red supergiant Betelgeuse in the constellation of Orion. Next are the blue giants Rigel and Polaris (the Pole Star). Our Sun, known as a "yellow dwarf" is the small yellow dot. Betelgeuse's diameter is nearly 900 times that of the Sun.

Stars are born when a giant molecular cloud, huge regions of dust and gas, collapses to become dense blobs. At this stage, they are known as protostars. A protostar becomes so hot that it starts to produce its own energy in its core. A spinning disc of gas and dust surrounds it. This may eventually become the birthplace of new planets.

Double stars

Many pairs of stars are seen close together in the sky. Some of these double stars are known as binary stars. They stay in pairs because of the force of each other’s gravity. This causes them to orbit around one another.

Variable stars

A variable star is one whose brightness, as seen from Earth, varies. For some variables, this is because something is partly blocking our view of their light. Other variables naturally swell up and shrink, or are simply unpredictable in the way they give off light. A well known variable star is Eta Carinae, one of the most massive and brightest stars in the Galaxy.