What Are Stars?

Stars are huge balls of gas that give out large amounts of heat and light. Our Sun is just like the other stars in the sky, but it looks very different to the stars that we see at night because it is much closer.

Sizes of StarsOur Sun is about 1.4 million km across, but its size will change throughout its lifetime. This happens to all stars. White dwarf stars can be one thousand times smaller than our Sun, whilst red giant stars can be over one hundred times larger than our Sun.

The white dwarf stars are all circled. They are much smaller than the other Sun-like stars in the picture.

Colours and Temperatures

When you look up at the night sky, the stars all look white, but if you stop and look more closely, their colours are different. Some stars look redder in colour, like the star called Betelgeuse (shown on the right). These are cooler stars. Others stars look blue, like Sirius, the Dog Star. These are hot stars. Our Sun is an average yellow star. It has a surface temperature of about 6000 C.

Twinkling

When you look up at stars, many of them will seem to twinkle as you watch. Astronauts in space do not see this because it is caused by the Earth’s atmosphere. The atmosphere is made up of many different layers which bend light from the stars in a different way. As these layers move and change, the light we see also changes and the stars seem to twinkle. This is a bit like looking at the bottom of a swimming pool through the water. The black lines on the bottom seem to move around as the water moves.

Distances to Stars

Our closest star is the Sun. The other stars are much further away. The nearest is called Alpha Centauri. Even the very closest stars are much too far away for us to explore by spacecraft. It would take a rocket about 42,000 years to reach Alpha Centurai.

Measuring the Brightness of Stars

The brightness of a star is called its magnitude. The brightest stars have a magnitude of 1, while the dimmest stars that we can see have a magnitude of 6. The lower the number, the brighter the star. We can use the magnitude to work out the distance to the star.