I'm considering writing a story in which we Earthlings have a few years to prepare for a cosmological catastrophe in which our planet will lose its energy source and be adrift in space. This would mean the end of life on Earth - but not right away. We could insulate giant underground chambers and tap existing energy sources in order to perpetuate human life for a number of generations into the future. Fun and complications would ensue. So my first problem is figuring out how to create such a disaster in a believable way. I assume that long before our sun dies, it would go through a contraction phase to become a red dwarf or something and fry us for dinner. Do we believe that some stars of the size of the sun are capable of simply becoming dimmer and finally winking out? Didn't think so. How about a scenario in which we become aware of an approaching mass - a planet-sized rock or black hole or something that we knew would hit our solar system and might somehow knock the Earth into a more distant orbit so that we would no longer receive enough sunshine to sustain life? I'd appreciate any suggestions from you brainiacs.