Slide 19 of 49

The size of the parent star is crucial for the ability of that star to support life. Large stars undergo rapid and unstable burning (extreme temperature variations over millions of years), which cannot support life. Stars increase in luminosity as they age. For example, our star, the Sun, has increased its energy output by 35% since its beginning. Fortunately for the Earth, this change has been compensated by a decrease in the levels of greenhouse gases (another design parameter), which lowered the ability of the Earth to hold onto the increased heat produced by our more luminous Sun. For stars larger than our Sun, the increase in luminosity is much greater, which could not be compensated. In addition, large stars have very short lifespans (as short as a few million years).

Stars smaller than our Sun are not suitable to support life on planets. Although these stars are able to undergo quite stable burning for billions of years, their small mass requires that life-containing planets be much closer to the star. Planets within the life zone have to be so close to the star that the gravitational interaction (which increases with the fourth power as the distance decreases) causes the planet's rotational period to be increased significantly. For example, both Mercury and Venus have rotational periods that are as long or longer than their revolutionary period. These 88 and 243 Earth-day rotational periods (for Mercury and Venus, respectively) result in extremes of temperatures on the surface of these planets, which prohibits the survival of lifeforms.