Thanks, Bob, for the added info; that helps nail the situation. This is what I had suspected, and the same as what I've seen in the past.
Those 3 2X6's that are side by side are a "flush beam." Your ceiling joists are end nailed to this beam, and extend outward from both sides of the flush joist.
Several things can cause this type of crack, but the most likely is that as the home has aged and "settled in" (not meaning settling problems,just "settled in") a little, the joists have slightly loosened from that beam, and pulled at the drywall from both sides, until eventually it had to give somewhere, and it's usually right near that flush beam. This is usually a combination of several factors: a little shrinkage in the framing, which is normal, a little desire for those joist to be pulled outward and away from the beam from load on the roof- also normal, and that those joists are quite often end and/or toe nailed into the beams without any joist hangers or cross-nailing to prevent them pulling away.
Normally, this is the extent of the problem, but it's always a good idea to check your foundation or basement walls for any cracks or signs of motion whenever you see cracking drywall.
As far as repair, it sounds like there's open attic above this room, which makes things much easier. First, I would ensure that the flush beam was not coming apart; that the 3- 2X6's were still tightly together. If they aren't, use long lag bolts to lock them back into a bundle. I would then use metal mending plates in the attic which lay on top of each joist, spanning over top of the beam, and across to the joist on the opposite side of the beam. Screw them down into each joist and the beam. I might also use corner mending plates up the sides of the joists and beam if there are no joist hangers. This will tie these members together and prevent further spread.
The crack can then be repaired by conventional mud and taping, with the real difficulty depending on what your ceiling finish is. If it's blown texture, it will probably need to be redone throughout the entire room, unless you can find a really good drywall man and the ceiling's aren't discolored from smoke or age.
Bob, that's about all I think of at the moment; good luck!