This was just great! I totally didn't expect the trailer so it made the shock of finding it so much bigger.

THANKS BETHESDA!!
So an Elder Scroll can close the sun? And if the sun were closed, would magic be unusable?
No, I believe you can still use magic afterwards. The sun is a large tear through fabric of the world of Nirn and goes straight into Aetherius from where magicka flows to Nirn. It was left when Magnus found out about Lorkhan's trick and escaped the trap. The stars, although smaller, are the exact same things made by smaller Aedra. So Magicka would still flow into Nirn.
Or atleast I believe it was something like this.