In the absence of any official explanation, here's my theory about the "head characters"/angels in Battlestar Galactica, including Starbuck. I think this explanation is internally consistent and explains just about everything that the creators of the show did not themselves explain about the subject.

( Cut for spoilerage )

I just watched the latest episode of BSG last night. ( Battlestar Galactica speculation (including spoilers if you haven't seen the most recent episode )