the Doctor says "every sun will supernova". There is no reason why that should be (from what we have seen the cracks are more sinister, less dramatic), and I cannot help thinking that the only reason why it is in the episode is because "supernova" connotes big, bad bang.

Then again, the next episode is called Big Bang, so perhaps I should not get too annoyed yet.

Edit: I just realised (brain works a little slow sometimes). That might be what is happening in the final shots. The "fireworks". They are every sun supernovaing.