The green sun, easily five times the diameter of the moon of old Terra, dropped slowly toward the horizon.  Wine-colored shadows stretched away from Jim’s feet, and the gentle warmth of the alien sun on his face was a welcome change from the blandness of the ship. He wasn’t supposed to leave the ship unprotected, yet – not until he’d watched for a full local year.  But that was three Earth years, and he’d finally decided the hell with it, he couldn’t stand the claustrophobia . . .


This is supposed to be a habitable planet?

There are certain basic principles of astrophysics, optics, elemental abundances and visual perception that have to be taken into account when building a planet suitable for human habitation.  The brightness, color, apparent size, warmth and stability of a star are all closely related to the mass and age of the star and to the distance between the planet and its primary.  So is the year length.  If you change any one parameter, the others will change as well.  And the combination in the opening paragraph is simply not possible.

Let’s begin at the beginning, with a cloud of gas and particles slowly collapsing under the gravitational force of its own mass.  Most of the mass is going to be composed of hydrogen, because the original makeup of the universe was mostly hydrogen and a little helium, and these two elements still make up some 98% of the mass (and considerably more than 98% of the atoms) of the universe.   As the mass falls together and is compressed, gravitational potential energy is transformed into kinetic energy of the atoms and molecules making up the cloud – into heat.  This is a very fast process in geological terms.  A sun-sized mass of collapsing interstellar matter would be a hundred times brighter than the sun – and much redder and larger – in only about a thousand years.  Obviously this is not a suitable primary for a habitable planet.  It is not even a safe area for hands-on investigation.

If hydrogen is subjected to enough heat and pressure, four hydrogen nuclei can be pushed together to make a helium nucleus.  In this process, less than one percent of the mass is transformed into a very large amount of energy. This is basically what occurs in a hydrogen bomb.  The process is often called hydrogen burning, though it has nothing to do with the chemical oxidation process we call burning on Earth.  A collapsing protostar continues to collapse until its core temperature is high enough to allow this fusion of hydrogen atoms into helium to occur – a few million degrees Kelvin.  If the collapsing mass is too small – less that 8% of the solar mass – the temperature never becomes high enough for hydrogen burning to occur.  Even so, a body the size of Jupiter (about 0.1% the mass of the sun) radiates more energy from gravitational collapse than it receives from the sun.  If the mass of the protostar  is more than 80 solar masses, it becomes unstable as gravitational collapse continues, and blows itself apart.

The exact course followed by a collapsing protostar depends on its mass.  If the mass is several times that of the sun, the protostar becomes hotter, but at the same time shrinks to a smaller size.  The two balance each other, and the total energy output of the protostar, its luminosity, stays nearly constant.  The temperature changes very rapidly – a protostar fifteen times the mass of the sun will evolve to the point that hydrogen fusion begins in only about ten thousand years.  If the mass is small, a fraction of that of the sun, it still becomes smaller, but its temperature does not change much.  If its mass is half that of the sun, it will take more than ten million years to start hydrogen burning, and its luminosity will decrease substantially during the process.  Regardless, the collapsing mass eventually reaches a temperature and pressure at which hydrogen fusion begins, and heating from that fusion stops further gravitational collapse.  The process is not peaceful – the protostar may blow away a substantial portion of its mass during the collapse, and the initiation of hydrogen burning may occasion a few hiccups.  Not a good neighborhood for a planet.

Next time (weekend of Aug 22)—the part of a star’s lifespan that is suitable for it to have habitable planets.