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Originally Posted by kutulu
Convection has nothing to do with it. Convection is a means of heat transfer. Pressure and temperature dictate the volume of a gas. Gasses disperse in order to equilize pressure and temperature gradients.
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Close, but not quite (or just overly simple)... Yes, pressure and temperature gradients do tend to be equalized in free gases, but the underlying reason is molecular motion. However, due to gravity we have a kind of equilibrium between the molecular motion which disperses the gas (equalizing the pressure) and the force of gravity pulling the molecules down. This is why the lower atmosphere is more dense than the upper atmosphere - to an extent, the air is pulled down and it is staying down. It just happens that the more "down" you go, the more air is there, so the air above it is more likely to bounce back up as there is less room for a particle to fall without hitting another particle.
Oh, and the stars emit different wavelengths of light (thus different colors) based on their material composition and how hot they are. Although part of this is our atmosphere, they still have color outside of our atmosphere. The only colors that I've heard of stars being are white, yellow, blue, red, orange, and brown (rare). You'd think that with the wavelegnth for green being between yellow and blue, which are common colors in stars, that there would be green stars. I'd assume this does not happen because the materials which comprise stars do not have energy levels which, when reducing from one to another, would correspond to an energy release equal to the amount of energy required to produce a photon in the wavelength ranges corresponding to the colors green or purple.