The Sun is a G-type main-sequence star, a massive sphere of superheated gas that generates energy through nuclear fusion. Extreme pressure and temperature create distinct concentric regions that define its structure. These layers range from the dense core to the vast outer atmosphere, with each region performing a unique function. Scientists divide the Sun into internal zones, where energy is created and transported, and external atmospheric layers that can be directly observed.
The Sun’s Internal Engine
The deepest layer is the Core, the site of the Sun’s power generation. Temperatures here reach approximately 15 million Kelvin, and immense pressure forces hydrogen nuclei to fuse into helium nuclei (nuclear fusion). This reaction converts mass into enormous amounts of energy, creating the outward pressure that keeps the Sun stable against gravity.
Surrounding the core is the Radiative Zone, extending to about 70 percent of the Sun’s radius. Energy moves through this dense region primarily as photons. Transport is slow because photons are constantly absorbed and re-emitted by the plasma, often traveling only millimeters before collision. This zig-zag path means energy can take over 100,000 years to traverse the zone.
The Convective Zone is the final interior layer, where the plasma becomes opaque and energy transport switches to convection, much like boiling water. Hot plasma at the base rises rapidly toward the surface, cools as it releases energy, and then sinks to be reheated. These massive circulating currents act as the final conveyor belt for energy before it reaches the visible surface. The motion of these currents also plays a significant role in generating the Sun’s magnetic fields.
The Photosphere
The Photosphere is the visible surface of the Sun, forming the boundary between the opaque interior and the transparent atmosphere. Here, the gas density drops sufficiently, allowing photons to finally escape into space. The effective temperature of the Photosphere is around 5,772 Kelvin, which is associated with the Sun’s bright yellow light.
The Photosphere is not a solid surface, but its appearance is textured due to the underlying Convective Zone. This texture is known as granulation, consisting of bright, short-lived cells roughly 1,000 kilometers across. The bright centers of these granules are plumes of hot, rising plasma, while the darker edges are where cooled plasma is sinking back down.
Darker patches on the Photosphere are known as sunspots, which are regions where intense magnetic fields impede the flow of heat from the interior. These spots appear dark only in contrast to the surrounding plasma, as they are still extremely hot. Inner regions, known as the umbra, average around 3,400 Kelvin.
The Chromosphere
Immediately above the Photosphere lies the Chromosphere, a thin atmospheric layer a few thousand kilometers thick. Its name, meaning “color sphere,” comes from the reddish glow it exhibits in the light of the hydrogen alpha spectral line. This layer is usually invisible, overpowered by the Photosphere’s brilliant light, but is easily observed during a total solar eclipse as a bright pink or red rim.
Temperatures surprisingly begin to increase within this layer, starting at about 4,000 Kelvin near the bottom and rising sharply to over 8,000 Kelvin at its top. This unexpected temperature increase is thought to be related to the propagation of magnetic energy.
The Chromosphere is characterized by dynamic, short-lived jets of plasma called spicules, which shoot upward at high speeds. These hair-like structures can reach heights of thousands of kilometers before falling back down, giving the Chromosphere a distinctive, grassy texture. Larger, arching structures known as prominences also originate here, held aloft by strong magnetic fields.
The Corona
The outermost and most expansive layer of the Sun’s atmosphere is the Corona, a vast, tenuous halo of superheated plasma. This layer extends millions of miles into space, eventually blending into the solar wind that permeates the solar system. Like the Chromosphere, the Corona is typically only visible during a total solar eclipse, appearing as a pearly white crown of light.
The most striking characteristic of the Corona is its extreme temperature, which can reach between one and two million Kelvin. This is dramatically hotter than the 5,772 Kelvin Photosphere below it, a phenomenon that remains a major unanswered question in solar physics. Current theories suggest that the heating is caused by complex interactions of the Sun’s magnetic fields, possibly through tiny explosive events called nanoflares.
Despite its high temperature, the Corona has an extremely low density, meaning there are very few particles in any given volume of space. Specialized instruments called coronagraphs are used to artificially block the bright light of the Photosphere, allowing scientists to study the faint Corona. This constant outflow of charged particles is a direct result of the coronal gas expanding at speeds up to a million miles per hour, forming the solar wind.