Corporate Security vs. Startup Risk

I hear Captain Kirk’s voice ringing in my head nearly everyday ...”to boldly go where no man has gone before...” It seems everyday I ask myself the same question - to continue working at my secure, corporate job? Or explore a new, unknown territory and join a startup that is actively recruiting me? Is remaining unhappy the “price” of a steady income and good benefits? How to know if joining a startup is a smart risk instead of a stupid risk? When is the right time to leave your job?