Substances which transfer heat energy easily are called thermal conductors -usually metals. Substances which resist heat transfer are called thermal insulators. As air is heated it expands, its density decreases, and it is buoyed up and displaced by cooler air. Hot air balloons depend upon this principle. Unless heat is being absorbed faster than it is radiated away, all objects will cool through heat energy lost by radiation.
Radiation is the only mechanism by which heat energy can be transferred through empty space.
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