I suggest a logarithmic scale for celebrity, starting at 1 for people who are not celebrities at all, and therefore know and are known to the 150 people that anthropologists tell us we can comfortably hold in our heads. People who are known to 1500 people would be at 2, those who are known to 15,000 would be 3, and so on. So, a local hero would be a 2, a typical author would be 3 or so, Neil Gaiman would be, what, a five? Barack Obama would be a seven? People ambitious to be eights will have to wait for the population to make up the numbers, or for the saucer people to arrive.
I don't know what use this scale is, but I'm sure that someone will think of something.