The Sun's Atmosphere Is Hotter Than Its Surface, and No One Knows Why

The strangest temperature in the solar system is hiding right above the Sun's surface.

The Sun's visible surface burns at about 5,500 degrees Celsius. But its outer atmosphere, the corona, blazes at more than a million degrees.

That makes no sense. As you move away from a heat source, things should get cooler, not hotter. Yet the corona is hundreds of times hotter than the surface beneath it. This puzzle, called the coronal heating problem, has stumped scientists for decades. The leading suspects are the Sun's powerful magnetic fields. They snap and reconnect like cosmic rubber bands, and tiny explosions called nanoflares may dump enormous energy into the corona. NASA's Parker Solar Probe is flying through the corona right now to catch the process in the act.

So the surface of the Sun is hundreds of times cooler than the faint halo floating above it. It's one of the strangest puzzles in physics, hiding in the one star we can study up close.

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