How hot is the Sun?

The Sun is a star: a huge ball of hot gas, mostly hydrogen and helium, held together by its own gravity. Its heat starts at the very centre. There, gravity squeezes the gas so hard that hydrogen fuses into helium. Each fusion turns a tiny amount of mass into energy.

The core is about 15.7 million degrees Celsius. Nobody can measure it directly. The number comes from careful models of the Sun's interior, which agree closely with what we can measure from outside.

From the core, the energy works its way slowly outward. First it travels as light, bouncing from particle to particle. Farther out, great bubbles of hot gas carry it upward, the way water rolls in a boiling pot. At last it reaches the surface and escapes into space.

The Sun has no solid surface. What we see is the photosphere, the layer where its light finally escapes into space. Its temperature is 5,499 °C. NASA notes that this is hot enough to make carbon, the stuff of diamonds and graphite, not just melt but boil.

That number is the Sun's effective temperature: the temperature that matches the light it gives off. The layer itself is hotter at the bottom than at the top, so it is the honest single number for the surface.

Sunspots are patches where strong magnetic fields push up through the surface. They are slightly cooler than the gas around them, so they look dark beside it.

Above the surface lies the corona, the Sun's thin outer atmosphere. We see it with our own eyes only during a total eclipse, as a white crown around the black Moon. It is far hotter than the surface beneath it: between 1 million and 3 million degrees, depending on where and when we measure.

That should not happen. The Sun's energy comes from its core, and things usually get cooler the farther they are from the fire. It is as if the air grew hotter as you walked away from a campfire. Scientists call it the coronal heating problem, and it has puzzled them for generations.

The leading ideas both involve the Sun's tangled magnetic field. Waves running up the field may carry energy into the corona. Or countless small explosions, called nanoflares, may heat it as field lines snap and reconnect. Many scientists think both play a part. NASA's Parker Solar Probe now flies through the corona itself. With ESA's Solar Orbiter, it has found strong signs that turbulence in the field does much of the work. The question is closing, but it is not closed.