I am writing a screenplay that hinges around the imminent destruction of the earth. Can somebody describe a scientifically plausible cosmic event that could obliterate the earth. The only criteria I need to fill is that it can be identified four years before it reaches us. My first thinking was an exploding star, but could that be identified as a threat four years before it actually effects us? I am not keen on the idea of an asteroid. I am looking for something that would totally obliterate us into dust. Any ideas? (It would be great if you could describe the event, but also how it could be discovered on earth by scientists).