What is the difference between a megastar and superstar?

Superstars are greatly known artist or person for his/her work field. As in Superstars are known in a worldwide basis. Superstars in music maybe in the likes of Rihanna, Justin Bieber, etc. etc. Angelina Brad, are like superstars.

As for a megastar, Megastar are legends! they are much higher in degree compared to superstars. Megastars are greatly known worldwide. In Music MJ, Elvis, Beatles, Madonna, Mariah, Whitney, (Britney??), Maybe Obama, Oprah are megastars? What do you think? :)