It is indeed a ridiculous concept for one thing the sun can't die out the way they portrayed it. As I recall from my astronomy class the sun will eventually turn into a red giant so the sun losing its luminosity is hardly believable but I had no problem with that. I had no problem with suspending my disbelief until they found Pinbacker on board their ship. That's when I kind of started saying WTF? That's when IMO the movie went from being really cool and somewhat original to being something of a standard horror movie. But again that is just my opinion. I still like the movie its just one thing I didn't like about it. The ending was fantastic I just didn't like how they lead up to it.
Yes quite. If you had read my entire post above you would find that I said IMO it would have been a better story if they had actually shown the process of a crew member losing it and turn into the Pinbacker character. Instead of introducing him the way they did I personally would have preferred if they showed why and how such a thing could happen to someone originally so committed to someone so willing to sacrifice the earth for his twisted belief system.