beautiful but what is it?

I believe this is a Sedum (Autumn Joy). According to Miracle Gro encyclopedia of Plant Care: There are two reasons why it would start to open up in the middle as you described. (1)These plants must be divided every 3 years to maintain form and vigor(do you know how long yours has been growing in the place it's in?); if it flops to the ground in spring it could need dividing but a lack of sun also causes it to flop (or spread out in the middle(Is it planted in too much shade?). The scientific name for this plant is Hylotelephium x 'herbstfreude'. For more information you might Google it. I hope this at least gives you a place to start.

Sorry to disagree but this is Sedum spectabile in one of its many forms, possibly 'Brilliant'.
They almost always open up in the middle no matter what you do. You can stake them or some folks cut the whole plant back around the time of the Chelsea Flower Show. This forces the plant to flower a little later and at a lower height, so the weight of the flowers does not pull the stems down.

If my plants start to flop, I reckon to dig them up the following spring and chop them into small pieces, replanting just one (the rest are potted up for plant sales). You can be really vicious with them; they are pretty tough

I have lots of these, I say they are too easy to not have around.. They divide well, and even cuttings put straight into the ground do well...
When you divide this try putting some of it out of the wind, I find the ones in the windier areas of my yard open in the middle like you have had a problem with.