Is a straight back, as it is the case of ballet and also gymnasts, the result of stronger muscles or just a case of learning the "Correct Stance", that is, how to keep your back straight?
I'm not entirely sure what you mean by straight back without pictures (do you mean generally, or when they're holding specific poses?), but I know in some gymnastic based programs, they recommend training a position called the Hollow Body position.
It involves flattening the back against the ground with resistance provided by the rest of your body.
My understanding is that it leads to a "straighter" body when performing a handstand (and presumably other positions as well)