I am looking for a book for my son to read that I read when I was about his age. I cannot remember the name of it but I think it was a trilogy (or at least a book in three acts). I read it in the late 70's. My most vivid memory is that the last part of the book was about some kids that were all that remained of a generation ship that had been launched many years before at a distant star. There was still one creepy old man who couldn't leave the hub of the ship because the gravity would kill him. There were packs of rats that the kids had to avoid. In the closing chapter, the kids are deposited on their new world and look back to see the old ship passing overhead on its way to plunge into the sun.
Can anybody help me?