Suitable Stars Despite the fact that hotter, more massive stars have wider habitable zones, astronomers are focusing their search on main sequence stars with masses of 0.5 to 1.4 solar masses. Why are these types of stars more likely to have intelligent life evolve on planets around them? Let's assume that it takes 3 billion years for intelligence to evolve on a planet. You will need to include main sequence lifetime and the distance and width of the star's habitable zone in our considerations. First consider the lifetime of a star. The star must last at least 3 billion years! Use lifetime = (mass/luminosity) × 10 billion years = 1/ M 3 × 10 billion years if the star's mass is in units of solar masses. The most massive star's (1.4 solar masses) lifetime = 3.6 billion years (a 1.5-solar mass star with a lifetime = 3.0 billion years would just barely work too). The less massive stars have longer lifetimes but the habitable zones get narrower and closer to the star as you consider less and less massive stars. At the outer boundary of the habitable zone
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