This was a painful movie to watch, but also incredibly rewarding. It takes an unromanticized, deeply honest look at dementia and dying from both sides of a relationship.
It's one of the best movies I have ever seen and one of a handful that I can see myself watching over and over.
And, the supernova imagery is explained by Tusker when he's stargazing with his niece; we are the stars. We are made of bits of each other. Everyone who touches us becomes a part of who we are.