I know the sun is a star, but is a star always a sun?

Answer

Dear Ad,

The sun is a star. There is no such thing as a sun, for there is only one. A star is a type of celestial object (namely an object maintained by its own gravity that emits self-produced radiation), and the sun is the object closest to Earth that meets this definition. In all the universe there are many billions of stars, but only one of them is called the sun. In (among other things) English this is all a bit clearer, since ‘Sun’ is capitalized there, and ‘star’ is not.