The size of the Sun is important for the life on Earth. If we have a small Sun, the habitable zone will be contracted and the distance from Sun to the habitable zone will become much closer than the current one. If the planet is too close to star, tidal lock between the planet and star can occur and habitable zone can’t be exist in such a system because only one side of the planet is always facing to the star and becomes extremely hot and the other side is always facing away from the star and becomes freezing cold. The distance of the Mercury is about 1/3 of that between the Sun and Earth. Because of this proximity, the Mercury is tidally locked with the Sun.
If we have a large Sun, we will have an advantage of having large habitable zone, but will also have another disadvantage. The lifetime of the star is inversely proportional to its size. If the Sun is much larger than the current one, Sun will consume its hydrogen gas in relatively short period of time. If it ran out of hydrogen fuel, it will gradually expand and its envelope will engulf the Earth. If that happens, we have to migrate to other habitable planet.
Fig. 1.17 shows mass distribution of stars. Most of stars are smaller than the Sun and only a few percent of them have a size of the Sun. Another condition needed for us to stay habitable zone for a long period time is that Sun has to be a single star. Unlike human, more than 50% of stars are binary systems (see Fig. 1.18). If we have two Suns on the sky, we will have strange weather pattern and smaller habitable zone than the current one. Fortunately, we have only one Sun having a right size for us to stay comfortablly for a very long period of time.
Fig. 1.17 – Mass distribution of stars. Most of the stars are smaller than the Sun.
Fig 1.18 – Close binary system (top) and wide binary system (bottom). Yellow and green objects represent Sun and planet respectively.