Worldbuilding For Writers Number One: Selecting A Star

Every week in the Worldbuilding for Writers series, I present a step-by-step guide to creating a realistic, Earth-like planet for use as a core setting for your storyworld.

For the purposes of Worldbuilding for Writers, "Earth-like" means the following:

Close enough in size and density to the Earth to allow for a similar gravityOrbiting in the "habitable zone" of it's star and having appropriate orbital characteristics to allow for a climate similar to that of EarthHaving an active, self-replenishing crust to allow for geologic processes similar to EarthHaving an atmosphere similar in composition and density to the EarthBeing an age sufficient to allow for the development of complex organisms, including at least one sentient species

The first step in creating an imaginary Earth-like planet is to assign an appropriate star to orbit.

Selecting An Appropriate Star For Your World

Creators of science fiction and fantasy worlds in books, movies, television and comics love to present an exotic setting with plenty of unusual and stunning details to convey a sense of the alien or fantastic. It's not uncommon to start with the sun… or suns… up in the sky. Nothing says "another world" like a lurid, balefully glowing red ball smoldering in the sky, or perhaps a pair of blistering white stars hovering above the horizon.

Unfortunately, a creator's choices when thinking about their milieu's star very often simply don't make good sense when you take into account the features of the world orbiting that star. When it comes to building a planetary system that includes an Earth-like planet (including a diverse ecosystem that features sentient native life) our choices are apparently rather limited.

Too Hot, Too Cold, Too Quick, Too Old

Stars, generally, are classified from hot and bright to cool and dim. While all the details of stellar classification are beyond the scope of this series, it's enough to understand that spectral classes are designated O, B, A, F, G, K and M, with each letter being further divided by numbers from zero to nine and the entire sequence ranging from hot and bright to dim and cool.

Hot, bright stars — O2 through F4, perhaps — burn themselves out in less than two billion years, which, if we consider the timescale of life's development here on Earth to be typical, is probably too short a time for complex life to develop. Even though stars from the upper K range through M-class exist for billions of years, they may not produce enough energy at a distance appropriate to place a planet and still consider it Earth-like.

The Best Star For Your World

The most likely stars for your Earth-like planet? Stellar classes F5 through K5. These include stars roughly +/- 20% the temperature of Earth's Sun that exist in a stable state for three to fifteen billion years or longer. Scientists like Jill Tarter and Margaret Turnbull think these stars are excellent candidates for Earth-like planets.

Our own Earth orbits Sol, a G2 star with a surface temperature of about 5,770 Kelvin and a predicted lifespan of approximately ten billion years. My creation, The Shaper's World, orbits Tah, a G3 star with a temperature of about 5,690 K and an even longer predicted stable lifespan than Sol.

What about your world? Here are some stars and their approximate temperatures to get you started:

Spectral ClassTemperature in KelvinsSpectral ClassTemperature In KelvinsSpectral ClassTemperature In KelvinsF56767G25770G95300F66600G35703K05200F76433G45636K15056F86267G55569K24911F96100G65501K34767G06000G75434K44622G15885G85367K54478

Next:

Your star's class and temperature directly affect its habitable zone, where conditions are best suited for an Earthlike planet. That's what we'll examine next week!

Matthew Wayne Selznick - Telling stories with words, music, pictures and people.