Stars are massive, luminous balls of hot gas (plasma), which are held together by gravity.
Although they look small, they are actually large bright objects like the Sun. They look so small because they are very far away - the nearest star is 41,000,000,000,000 km away!
All the energy in a star is produced in its centre, or core, by nuclear fusion. This energy emerges from the star as heat and light so that the star appears to glow, i.e. it is luminous.
Stars come in lots of different sizes and colours.
Stars exist because of a balance between gravity trying to make the star shrink and all the heat from the middle trying to make it grow.
Stars "live" for many millions of years, but they do not change much. However, amazing things can happen when they are "born" or when they run out of nuclear fuel and "die".