The Sun, the star at the center of our solar system, is not a solid object but a massive, nearly perfect sphere of superheated gas known as plasma. Its internal structure is dynamic and highly layered, driven by the continuous production and outward flow of energy. Understanding the Sun’s interior means exploring these distinct zones, where temperature and density vary dramatically. This layered organization dictates how the Sun functions as a powerful energy source, facilitating the journey of energy from the core to the visible surface.
The Solar Core: Engine of Energy
The innermost region of the Sun, the core, extends from the center to about 25% of the solar radius, acting as the star’s powerhouse. Here, temperatures reach approximately 15 million Kelvin, and the density is immense, exceeding 150 grams per cubic centimeter, roughly ten times the density of gold. These extreme conditions overcome the natural electrical repulsion between positively charged atomic nuclei, allowing them to fuse together. This fusion process converts hydrogen into helium, releasing vast amounts of energy as gamma-ray photons and neutrinos.
The specific reaction responsible for nearly all the Sun’s energy is the proton-proton chain, a multi-step sequence where four hydrogen nuclei ultimately combine to form a single helium nucleus. In the first step, two protons merge to create deuterium, a positron, and a neutrino. The sheer volume and density of the core ensure a continuous, powerful output, generating 99% of the Sun’s total power.
The Radiative Zone
Surrounding the core is the radiative zone, which extends from about 25% to 70% of the Sun’s radius. In this region, the plasma is incredibly dense, but the temperature gradually decreases from roughly 7 million Kelvin to about 2 million Kelvin at its outer boundary. Energy transport here occurs solely through thermal radiation, where photons carry the core’s energy outward.
The plasma is so compressed that photons do not travel in a straight line; instead, they are repeatedly absorbed by particles and then re-emitted in a random direction. This process is known as a “random walk,” making the journey through the radiative zone exceptionally slow. It can take an individual gamma-ray photon hundreds of thousands of years to diffuse through this thick layer before reaching the next zone.
The Convective Zone
The convective zone is the outermost layer of the solar interior, stretching from the top of the radiative zone to just beneath the visible surface. The temperature here is cool enough for certain ions to hold onto more of their electrons, which increases the plasma’s opacity. This opacity makes it difficult for photons to continue the radiative energy transport seen in the inner layer, creating a thermal instability.
Energy transfer switches to convection, a physical movement similar to boiling water, where bulk motion of the plasma carries the heat. Hot, less dense plasma rises upward toward the surface, cools as it releases its energy, and then sinks back down as cooler, denser plasma to be reheated. These enormous circulating currents, called convection cells, efficiently transport the remaining heat to the surface. The interaction between these turbulent motions and the Sun’s rotation generates the Sun’s powerful magnetic field.
The Visible Surface
The final internal layer is the photosphere, which is what we perceive as the Sun’s visible surface. This layer marks the point where the plasma becomes transparent enough for photons to escape into space. The temperature here averages about 5,800 Kelvin, a dramatic drop from the millions of degrees found in the layers beneath.
The appearance of the photosphere is not uniform but shows a mottled, granular pattern known as granulation. These features are the tops of the convection cells rising from the convective zone below. The brighter centers of the granules are the rising columns of hot gas, while the darker, cooler edges are where the plasma sinks back down. Because the photosphere is the boundary where light is emitted, it provides visual evidence of the forces operating deep within the Sun.