Science & history · Inside and above
Why is the corona hotter than the surface?
Moving away from a fire usually means getting cooler. Above the Sun's surface the opposite happens.
The puzzle
The corona reaches up to 2 million degrees Celsius, hundreds of times hotter than the photosphere below it. NASA calls this one of the Sun's biggest mysteries.
Spacecraft that fly close to the Sun and instruments that watch the corona in extreme-ultraviolet light are both attempts to answer it. The colours in those images stand for temperatures and wavelengths; they are converted from light we cannot see.
Sources
- NASA Sun: Facts (opens in a new tab) for age, size, distance, temperatures, layers, rotation, cycle, type, future · checked 2 October 2026
- NASA Scientific Visualization Studio Why Does NASA Observe the Sun in Different Colors? (opens in a new tab) for the Sun appears white because it emits all visible colours; SDO's colours stand for wavelengths, temperatures and regions · checked 2 October 2026