Stars are comprised mainly of large masses of hydrogen and helium which exist in a gaseous form. They could otherwise be seen as huge nuclear power houses.
In order to compare stars and see where they were in their life cycle, Hertzsprung and Russell came together and designed a diagram which showed just this, by plotting luminosity against surface temperature a picture was formed, the line of stars travelling across the diagram is known as the main sequence stars, and these are sometimes called dwarfs, this is where our Sun is placed yet some other stars are 20 times larger and 20,000 times brighter. The super giants are in the top right hand corner, Betelgeuse is 600 million miles across. Rigel a blue super giant is 100 times the size of our Sun.
