One can write a biography of any one. For example, you could today write a biography of some historical figure. It is only an autobiography when it it written by the subject herself/ himself.

One needs go back to the greek and latin roots of the words

bio = life (greek bios)
graphy = writing (greek graphia)
auto = self (latin autos)

therefore a biography is a life-writing and an autobiography is a self-life-writing

both are the same - just depends on who wrote 'em

Amy