Why do stars shine? How long do they shine? This lecture describes the physics of stars on the main sequence, describes the mass-luminosity relation of main sequence stars, introduces nuclear fusion power and the nuclear fusion lifetimes of stars. From this we gain an important insight into one of the criteria we might apply to the search for life around other stars: we want planets around low-mass main sequence stars that can shine more or less steadily for more that 500 Myr to 1 billion years - maybe longer if our goal is to find intelligent life. Recorded live on 2009 Nov 10 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.