[Ugly Joe]

I'd say a PVC frame would be great for this.
Electrical conduit PVC is pretty cheap ($1.50 or so for a 10 foot length).
Quick look at how much needed:
Diameter of your Death Star: 5 feet, so circumference is about 15 feet (really it's 15.7 feet, a touch over, but rounding down barely looses size).
So, 1 and half pieces of conduit per loop = $2.25 per loop, give or take.
6 vertical loops is $13.50
At least 1 loop horizontal to give stability, so add $2.25 for $15.75 for the PVC frame.
Then you can add chicken wire, cardboard, mache, monster mud, etc. on the outside.

Curving PVC is simple (time consuming, but simple) - draw your curve as a template, then use a heat gun on the PVC. Once it's softened, bend it to (roughly) match that curve. You'll have your hoops in no time.
Use duct tape to bind the joints, or bolts if you're really wanting to be certain.

 

[Ugly Joe]

GodOfThunder, I'm going to add to / change up my first response with the suggestion of a metal framework inside

Use galvanized 1/2 inch conduit instead of the PVC, as the weight of paper mache and other sculpting materials may even be a bit strong for the PVC.
Like this stuff: Galvanized conduit

The weight might be a factor, but it shouldn't be too bad, and the strength of steel will let you do a lot more with it once it's constructed.

As for weight? Well, since the steel is much stronger, you may be able to get by with 3 vertical loops, compared to 6 with PVC, and one horizontal.
The galvanized is as long as the PVC, so still 1 and half lengths per loop.
4 x 1.5 = 6 lengths of conduit.
Each length is 3 pounds (according to Home Depot), so 3 x 6 = 18 pounds.

(compared to using 1/2 inch PVC - 7 loops - 18.9 pounds)


This will still give you the hollow, unblocked interior you were aiming for, so you can set up interior illumination and poke holes in the exterior to let the light out (for that kick-*** Death Star look).