The sun is a massive, layered star composed of superheated plasma, and its temperature changes dramatically across its different regions. Temperature distribution within the sun does not follow a simple pattern of cooling the farther one gets from the center. Instead, the sun’s structure is divided into an interior, where energy is created, and an atmosphere, where that energy is eventually radiated into space.
The Sun’s Interior: The Absolute Hottest Layer
The sun’s innermost region, the Core, is the absolute hottest point in the entire star. The Core is the site of nuclear fusion, where the immense pressure and temperature force hydrogen nuclei to combine and form helium, a process that releases vast amounts of energy. The temperature in this powerhouse reaches approximately 15.7 million Kelvin (K).
Surrounding the Core is the Radiative Zone, which extends outward to about 70% of the sun’s radius. Energy is transported through this dense plasma as photons are repeatedly absorbed and re-emitted, slowly making their way through the layer. The temperature drops significantly across this zone, decreasing from about 7 million K at the boundary with the Core to around 2 million K at its outer edge.
The outermost internal layer is the Convective Zone, where the plasma is no longer dense enough for radiative transfer to be efficient. Here, large-scale convection currents—similar to boiling water—carry heat toward the surface. Hot plasma rises, cools, and sinks, creating a turbulent churning motion that transports the remaining energy. By the time the energy reaches the boundary of the visible surface, the temperature has fallen to about 5,700 K.
The Photosphere and Chromosphere: The Temperature Minimum
The Photosphere is the sun’s visible surface, which represents the layer where the plasma becomes transparent enough for photons to escape into space as sunlight. This layer has an average temperature of approximately 5,800 K.
Just above the Photosphere lies the Chromosphere, which is a thin layer extending for thousands of kilometers. Within the lowest part of the Chromosphere, a region called the temperature minimum exists where temperatures can fall to around 4,000 K. Moving upward through the Chromosphere, the temperature begins to rise slightly, reaching about 8,000 K at its outer boundary.
This atmospheric heating is thought to be caused by mechanical energy, such as waves and turbulence, propagating up from the Convective Zone below. The Chromosphere is only visible to the naked eye during a total solar eclipse, appearing as a reddish glow.
The Solar Corona: The Paradoxical Temperature Spike
The outermost layer of the sun’s atmosphere, the Solar Corona, presents one of the greatest mysteries in astrophysics due to its extreme heat. This tenuous, wispy layer extends millions of kilometers into space, yet its temperature soars to between 1 million and 2 million K. This makes the Corona dramatically hotter than the Photosphere below it, a counter-intuitive phenomenon known as the coronal heating problem.
This superheating is not caused by the simple conduction of heat from the sun’s interior, as the temperature should naturally decrease with distance from the energy source. The density of the Corona is also extremely low, meaning that while the particles are moving very fast (high temperature), the total heat content is low.
Current leading theories suggest that the energy is supplied by the sun’s complex and powerful magnetic field. One prominent theory involves magnetic reconnection, where tangled magnetic field lines break and explosively snap back together, releasing energy in events called nanoflares. These tiny, frequent explosions are thought to dump thermal energy into the coronal plasma.
Another major theory proposes that the heating is caused by magnetohydrodynamic waves, specifically Alfvén waves, which are generated by the turbulent motion in the Convective Zone. These magnetic waves travel upward along the field lines and dissipate their energy as heat in the Corona. Observations suggest that both magnetic reconnection and wave heating likely contribute to the extreme temperature of the Corona.