The Sun, the star at the center of our Solar System, is a massive sphere of hot plasma and a powerful energy source that sustains life on Earth. While it appears as a uniform disk, its structure is complex and multi-layered. To understand how the Sun generates and transmits its energy, scientists divide its structure into six distinct layers, organized sequentially from its center outward. These layers are categorized into three internal zones where energy is created and moved, and three external layers that form the solar atmosphere.
The Sun’s Interior: Energy Generation and Transfer
The innermost region of the Sun is the Core, extending from the center to about 25% of the solar radius. Immense gravitational pressure and extreme temperature, reaching approximately 15 million Kelvin, create the conditions necessary for nuclear fusion. This process, primarily the proton-proton chain, converts hydrogen nuclei into helium nuclei, releasing energy as gamma-rays. The Core is the densest part of the Sun, with a central density up to 150 grams per cubic centimeter.
Surrounding the Core is the Radiative Zone, stretching to about 70% of the solar radius. Energy moves outward primarily through radiative diffusion, where photons travel a short distance before being absorbed and re-emitted by the dense plasma. This energy transfer is incredibly slow, as photons perform a “random walk” that can take an estimated 170,000 years to traverse the zone. Temperature within this layer drops from around 7 million Kelvin to about 2 million Kelvin at its outer boundary.
The outermost internal layer is the Convective Zone, occupying the final 30% of the Sun’s radius just beneath the visible surface. Here, the temperature and density are low enough that the plasma becomes opaque, blocking the efficient outward flow of radiation. This forces the energy transfer mechanism to switch from radiation to convection, similar to boiling water. Hot plasma rises towards the surface, cools, and then sinks back down in massive convection currents, carrying the remaining energy to the Sun’s exterior.
The Sun’s Atmosphere: Visible Light and Beyond
The energy finally reaches the Photosphere, the visible “surface” of the Sun and the first layer of the solar atmosphere. This thin layer, about 500 kilometers thick, is where the plasma becomes transparent enough for photons to escape into space as sunlight. The temperature here averages around 5,772 Kelvin. The visible surface is characterized by a mottled appearance of bright granules, which are the tops of the convection cells from the layer below.
Just above the Photosphere lies the Chromosphere, a layer extending for thousands of kilometers. It can be seen as a reddish ring during a total solar eclipse, earning its name, meaning “sphere of color,” from this glow caused by hydrogen light emission. This dynamic layer features finger-like jets of plasma called spicules that shoot up from the surface. It is a transitional zone where the temperature begins to increase, rising from about 4,000 Kelvin at its base to roughly 20,000 Kelvin at its top.
The outermost and most expansive layer is the Corona, a vast halo of extremely hot, tenuous plasma that extends millions of kilometers into space. Although its density is millions of times lower than the Photosphere, the Corona’s temperature soars to between 1 and 2 million Kelvin. This phenomenon, known as the coronal heating problem, is thought to be related to the Sun’s complex magnetic fields. The Corona is the source of the solar wind, a continuous stream of charged particles that flows outward through the solar system.
Comparative Properties of the Layers
The six layers exhibit a wide range of physical properties, particularly density and temperature. The Core represents the extreme of both metrics, with a central density of 150 g/cm³ and a temperature of 15 million K. Moving outward, the Radiative Zone maintains high temperatures, dropping to around 2 million K, while its density falls sharply to about 20 g/cm³ at its outer edge.
The Convective Zone sees a further temperature decrease, reaching approximately 5,700 K at the bottom of the visible surface. The Photosphere marks a significant boundary, possessing the lowest temperature (around 5,772 K) and a density millions of times less than the Core. In the atmospheric layers, a peculiar inversion occurs: the Chromosphere and Corona become progressively hotter despite being farther from the energy source. The Corona is the least dense layer, yet its temperature rises up to 1–2 million Kelvin, demonstrating a shift in energy dynamics.