Stars are huge globes of searing-hot gas, which pour out enormous energy as light, heat and other forms of radiation. The Sun is our local star. It is much like all the other stars we see in the night sky, but appears very much bigger and brighter because it is very much nearer to us. The Sun is a very average star, not particularly big and not particularly bright. There are some stars, called red giants, which are tens of times bigger, and stars called supergiants that are tens of times bigger still. These supergiants have diameters measured in hundreds of millions of kilometres. On the other hand there are also stars much smaller than the Sun. White dwarfs are only about the size of the Earth. And neutron stars may be no more than 10-20 km! The stars we see in the night sky looked almost exactly the same to our earliest ancestors 2-3 million years ago. They pour out as much energy today as they did then. Where does a star get such enormous energy come from? It comes from nuclear reactions going on inside its core. They are nuclear fusion reactions in which nuclei (centres) of light atoms fuse (combine) together to form heavier ones. In the most common reaction, nuclei of hydrogen atoms fuse to form helium. Hydrogen and helium are the two most plentiful elements in the universe. A star is born in a nebula, a cloud of gas and dust. Something, maybe a star explosion, or supernova, makes the cloud start to contract under gravity. It gets denser and denser and forms into a spinning globe, which gets hotter and hotter as it gets smaller and smaller. Eventually the globe becomes so hot inside - tens of millions of degrees - that nuclear fusion reactions begin. Vast amounts of energy are released, and the globe starts to shine, as a star.
