Celebrities are people you know even though you don’t know them.
We know our family, friends, and colleagues. We know of the officers of movements we belong to, and performers in the arts and sports we enjoy. The people everyone knows are celebrities. You know, say, your sister, your favorite writers on the Huffington Post, and the bullpen of the Yankees. A small group will know your sister, a larger one will know the writers and pitchers. Everyone knows Obama and A-Rod. They are celebrities.