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The more energy being released, the hotter the star is. This helps determine the colour of the star, along with material composition, they aren't just hydrogen and helium you know.
As for the pond thing, the cold water is actually more dense than the warmer water, so it sinks, up until a certain point near the freezing temperature, where the water actually begins to expand. So the colder water at the bottom actually begins to rise to the top at this point where it then freezes, forming a layer of ice, now the ice acts as an insulator for the rest of the body of water, and the thicker the ice, the more it insulates the water underneath, so the colder it gets, the thicker the ice layer gets, and the harder it becomes to freeze the water underneath. And the ground acts as an insulator also, even in some of the more polar regions, the ground is only frozen for a few feet down.
As for the atmosphere, there are many reasons that the air does not just settle. Convection as the air is heated and cooled, molecular motion, and the rotation of the earth are the ones that I can think of right now. Convection causes the gases to heat and sink as they cool and heat, molecular motion causes the molecules to bounce around one another, never becoming still, and the rotation of the earth just mixes up the whole mess, kind of like the blades in a blender.
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Apathy: The best outlook this side of I don't give a damn.
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