So how on earth (or nu-earth!) does Captain Jack turn from his current human-like form to a big head in a jar? Maybe this will be covered in the next series?

the last time we see Face of Boe so far is around the year five billion. we know that old peoples ears and noses grow as they get older whilst their height shrinks. Maybe FOB is the ultimate extension of that for when you are 5 billion years old!

my theory is that he cant die but he can and does age gradualy or he did what casandra did and had work done or u could google it