#scifi Now that we’ve discussed the role of water in affecting albedo, as a greenhouse gas and in moving energy around a planet, let’s go back to that alien planet we’re building. What will its sun look like?

In the first place, we do not need to consider all stars, or even all main-sequence stars. Massive stars have rather short lifetimes. Our sun has a projected lifetime of some 10 billion years. A star three times its mass would stay on the main sequence only half a billion years. Further, such massive stars are much rarer than sun-sized stars, and put out a very large fraction of their energy in ultraviolet (dangerous) wavelengths.


Apparent size of a star in the sky of one of its planets as a function of its color. The sun has a size of 1.



(In fact, named stars are almost all poor candidates for having livable planets. If they are named, they are relatively bright. Almost all bright stars are either very massive or are in the later, helium-burning stage of their existence.)

What about smaller stars? Lifetime is not a problem, nor is finding one—less massive stars are far more numerous than those of sun size. They are harder to find in the night sky because unless they are extremely close they are too dim to see. However, a planet would have to be quite close to a very small star to keep warm, and smaller stars are dangerous at that distance.

If we want an earth-like planet, with temperatures that allow water in liquid, solid and gaseous phases, we could take as a first approximation that the energy received from the star at the planet’s distance would be equal to that received by the earth from the sun—the solar constant. Let’s assume this in determining how the star’s color will affect its apparent size in the sky and the year length of the planet.


The length of a planet's year, in earth-years, as a function of the color of its sun.


First, we need to define color. Any star will look white, because our eyes automatically adjust to the color of any light containing a continuum of wavelengths in the visible part of the spectrum. But different stars have their light peaking in different parts of the spectrum. Our sun, for instance, peaks at a wavelength of about .5, which is green. Red is .7 and blue-violet is about .4, so the left side of each chart is ultraviolet and the right side is red—but all of the stars will look nearly white.

The difference is in the non-visible light. A star toward the left—blue—end of the chart will be putting out a lot of energy in the ultraviolet. We don’t see that part of the spectrum, though some other organisms can see in the near ultraviolet. But aside from a small amount needed to make vitamin D, ultraviolet is generally not good for living things.

Stars at the right—red—end of the spectrum again will look white to our eyes—they are in fact less red than an incandescent light bulb. But they will put out much more infrared radiation, and less ultraviolet. They would feel warmer than they looked, but human beings living under such a sun might well need to take supplemental vitamin D, and would probably evolve toward fair skin, just as Europeans have. Plant growth might also be slow.

If we assume a solar constant matching Earth’s, we can predict the apparent size of the sun in the sky and the length of the planetary year as functions of the star’s color. The charts show this, with the apparent size being scaled to that of the sun and the year lengths being scaled to ours. From this it is apparent that a redder star than our sun will appear larger in the sky and be associated with a shorter year, while a bluer star will appear smaller in the sky and the planet will have a longer year.

Next time: the sky color and the effect of the atmosphere on how things look.