Why is space cold if the Sun is so hot?

The Sun is ferociously hot, and yet the space around it sits near absolute zero. The trick is that temperature is a property of stuff — jiggling atoms — and space is almost perfectly empty. Sunlight streams through the vacuum carrying enormous energy, but it only becomes heat when it strikes something: a planet, a spacecraft, an astronaut's suit. The near-nothing in between has essentially nothing to warm, so it stays cold while the destinations bake.

Heat needs a road

Heat travels three ways: by touch, by moving air or water, and by glow — energy carried as light. The first two need matter, and space has almost none, so they are dead ends. Only the glow crosses a vacuum. That is why a spacewalker faces both extremes at once: NASA notes the sunlit side of a suit can reach about 250°F while the shaded side plunges toward minus 250°F. Same suit, same moment — one side is being struck by sunlight, the other is radiating its warmth into the dark.

Cold is not a thing

Nothing in space "seeps cold" into you. Cold is simply absent heat, and a vacuum is a terrible thief — with nothing touching you, your body can only lose warmth as a slow glow, far more slowly than it would in icy water. A thermos flask uses exactly this loophole: the gap between its walls is a tiny slice of vacuum that heat can barely cross, in or out.

The universe's idle temperature

Even the emptiest deep space is not quite at absolute zero. It is bathed in the cosmic microwave background — the Big Bang's leftover glow — which NASA's COBE mission measured at about 2.7 kelvin, roughly minus 455°F. That faint whisper of ancient warmth is the floor. Everything above it is borrowed from a star.