Science & history · Inside and above
The surface you see is not solid
The Sun's visible edge is the depth where its gas becomes transparent enough for light to escape: the photosphere.
The photosphere
NASA describes the photosphere as about 250 miles (400 km) thick, at about 5,500 °C. The IAU's nominal effective temperature for the Sun is 5772 kelvin, which is the same thing on the Kelvin scale (5772 − 273 ≈ 5,500).
Below it the energy is carried by radiation and then by churning convection; above it lie the chromosphere and the corona. Sunspots are patches of the photosphere that are cooler, and so darker, than their surroundings (NASA).
Sources
- NASA Sun: Facts (opens in a new tab) for age, size, distance, temperatures, layers, rotation, cycle, type, future · checked 2 October 2026
- International Astronomical Union IAU 2015 Resolution B3: nominal solar and planetary conversion constants (Mamajek et al.) (opens in a new tab) for nominal radius 695,700 km, irradiance 1361 W/m², luminosity 3.828×10²⁶ W, effective temperature 5772 K · checked 2 October 2026
- NASA and NOAA Sun Reaches Maximum Phase in 11-Year Solar Cycle (15 October 2024) (opens in a new tab) for solar maximum announced; the peak month is identified only in hindsight; May 2024 storm · checked 2 October 2026