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, Alpha Centauri is 4.3 light years away, this is approximately 41,000,000,000,000 km away!
In astronomy we measure the brightness of stars in different ways.
Stars come in lots of different sizes and colours, which can tell us a lot about what type of star they are.
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 during most of that time. However, amazing things can happen when they are born or when they run out of nuclear fuel and die. Explore this more in the life cycle section.