World-Building: Choosing a Star

This is the first post in a series of posts in which I will describe my method to creating a new world for science fiction.  Back in November, I described how to colonise a world, but this is different.  While that is more about the process of a world being selected and colonised, this is about how to create the world.

Choosing a Star

I’m not even going to look at the planet to begin with.  What we need for a plausible habitable world is a star that can host it.  I’ll look at the various types of stars and their suitability to host a habitable world.

First of all, we can safely say that giant and supergiant stars are not suitable.  They are very short-lived, and while they used to be smaller stars that were stable, their giant size has swallowed up any planets that would have been habitable.  More distant planets may have become habitable, but with a giant star’s unstable atmosphere and impending destruction, it’s not a good idea to use these stars, unless your story has some kind of outpost on an outer planet.  But I want to discuss habitable planets that you can breathe the atmosphere.

Neutron stars are also out.  The radiation is incredibly strong and would kill everything.  But planets do exist around neutron stars.

The Hertzsprung-Russell Diagram showing the main sequence stars down the middle of the chart. This image is used under the Creative Commons license, and was created by Richard Powell.
The Hertzsprung-Russell Diagram showing the main sequence stars down the middle of the chart. This image was found at Wikipedia, and is used under the Creative Commons license. It was created by Richard Powell.

White dwarf stars are the dead remnant of regular main sequence stars.  Our sun will become one in a few billion years.  After a giant star has expelled its atmosphere and collapses into a white dwarf, it can form new planets close in, as there’s a lot of debris from the former star.  However, the habitable zone is so close to the star that it would experience very strong tides and make the orbit potentially unstable.  This doesn’t mean that a white dwarf can’t support a habitable planet.  I would suggest that it’s unlikely, though.  It is possible, so go ahead and make your planet’s star a white dwarf if you like.

The best kinds of stars to use are main sequence stars.  There are several stellar types, and they’re nice stable stars undergoing hydrogen fusion in the core.  They range from very hot to very cool, with spectral types O B A F G K M.

O, B, and A type stars are very hot.  They’re also large.  They have a habitable zone farther from the star, but the problem with a habitable planet developing is that it doesn’t have much time.  These stars are quite short-lived.  O stars are the most massive, and they burn through their hydrogen very quickly.  A planet would have no time to develop a habitable atmosphere.  These stars are only around for a few million years before going supernova.  B type stars aren’t much better.  They are also extremely hot and massive, and they’ll go supernova in a few million years, as well.  A type stars are around for a few hundred million years.  They’re a safer bet, but unlikely to develop much in the way of life.  There have been planets found around these stars, but as it takes time for life to evolve, and for a breathable atmosphere to develop, it would not be a good place for intelligent alien life or a human colony to easily be established.  O, B, and A type stars represent less than 1% of the stars, so they are not common.

The best types of stars for more Earth-like planets would be F, G, and K type stars.  The Sun is a G2 star, which means it’s slightly cooler and smaller than an F type star, but quite close to that type.  F type stars have a shorter life than G type stars, but should have enough time for life to develop and to have a habitable planet.  These are good stars.  Of course, G type stars are suitable.  K type stars are smaller and cooler than our sun, so have a habitable zone closer to the star.  However, they are around for a lot longer than G stars.  The year may be shorter, but the lifespan of a planet is far longer.  Excellent candidate for a planet.

Finally, we have the M type star.  These are red dwarf stars, and make up 76% of the stars in our galaxy.  As they are the most common type of star, they are also the most likely to find planets.  The habitable zone is very close to the star.  This poses a problem.  As it’s so close to the star, the planet could be tidally locked.  One side is always facing the star, while the other is facing away.  The sunlit side is always hot, while the dark side is always incredibly cold.  Atmospheric circulation could lessen this a bit, but expect both extremes to be uninhabitable.  The fringe around the terminator (the area where you have a permanent sunset/sunrise) could be habitable.  This would give a very interesting environment for life.  No day and night cycle.  The life that would evolve there would be quite different.  For a human colony, it’s quite feasible.  Just don’t venture too far into the sunlit side, and be prepared for deadly cold in the night side.

F, G, and K main sequence stars would be the best, but M type red dwarfs would make a very interesting setting for a science fiction story.  Any other kinds of stars are risky or even suicidal (neutron stars).

What I did

In the case of Ariadne, I chose a G type star that was relatively close to us, Beta Comae Berenices.  It’s 29.78 light years away, and is a G0 spectral type star.  It’s slightly hotter and larger than the Sun, but it has a similar habitable zone as the Sun.  It is about 1.5 billion years younger than the Sun, though.  We don’t know enough about how evolution happens on other planets, so it’s quite possible it could be faster or slower.  At an age of 3 billion years, our own planet Earth only had simple multicellular organisms and an atmosphere that wasn’t breathable, only 1% of today’s level of oxygen.  But who’s to say that it can’t happen faster on other planets?  On Ariadne, it was faster.  The scientists who studied the planet found that oxygen levels were comparable to the Earth’s so it was deemed a very good candidate for colonisation.  A space probe confirmed that.  Complex life was there, but this is for a later discussion on world-building.