What is a star?

On a clear, dark night, you can see thousands of stars. Each one is a sun, like ours, but so far away that it looks like a tiny dot. Stars make their own light and heat by squeezing hydrogen together into helium in their cores. Planets and moons do not; they only reflect starlight.

Distances to stars are so big that we measure them in light-years, the distance light travels in one year, which is about 9.5 trillion kilometres. After the Sun, the nearest star is Proxima Centauri, about 4.2 light-years away. When you look at a star, you see light that left it years ago.

Stars come in different sizes and colours. The colour shows how hot a star is: red stars are cooler, yellow ones like our Sun are in the middle, and blue-white stars are the hottest. Some stars are much bigger than the Sun, and others are smaller.

Every star has a life. A star begins in a cloud of gas and dust called a nebula. Gravity pulls it together until it ignites. A star like the Sun shines steadily for billions of years. Then it swells into a red giant and finally shrinks to a small white dwarf. Much bigger stars end their lives in a huge explosion called a supernova, which can leave behind a black hole. You can read about the star we know best in the Sun.

Stars gather in huge groups called galaxies. Our galaxy, the Milky Way, holds hundreds of billions of stars. People joined bright stars into patterns called constellations, such as Orion or the Big Dipper, to tell stories and find their way. Stars appear to twinkle because their light passes through moving air in our atmosphere. See the solar system for our local neighbourhood.