The Sun, a massive sphere of hot, luminous plasma, is the star at the center of our solar system and the primary source of energy that sustains life on Earth. Its power output results from physical layers, each playing a specific role in generating and transporting energy. Understanding the Sun’s structure requires mapping these internal and atmospheric layers, from the dense core where energy is born to the tenuous corona that streams out into space.
The Internal Engine: Core and Energy Generation
The core is the innermost region of the Sun, extending to about 20 to 25 percent of the solar radius. This layer features extreme conditions, with temperatures reaching 15 million Kelvin and pressures over 265 billion times that of Earth’s atmosphere. These conditions overcome the natural repulsion between positively charged atomic nuclei, allowing nuclear fusion to occur.
Within the core, the Sun converts hydrogen into helium primarily through the proton-proton chain reaction. In this process, four hydrogen nuclei combine to form one helium nucleus, converting mass into an immense burst of energy, predominantly gamma-ray photons. The core generates virtually all of the Sun’s energy output, fusing approximately 600 million tons of hydrogen into helium every second.
Energy Transport Zones
The energy generated in the core travels through two subsequent internal layers before reaching the visible surface. The Radiative Zone extends from the core’s edge to about 70 percent of the Sun’s radius. Here, the plasma is so hot and dense that energy is transported almost exclusively by radiation.
Energy moves through this zone as photons are repeatedly absorbed and re-emitted by the surrounding matter, a process called radiative diffusion. Due to continuous scattering and high plasma density, a single packet of energy can take over a hundred thousand years to traverse the Radiative Zone.
Above the Radiative Zone is the Convective Zone, the outermost layer of the solar interior. In this region, the temperature drops, allowing ions to hold onto their electrons, which increases the plasma’s opacity and traps heat. This instability causes the plasma to move in a circulation pattern.
Hot, less dense plasma rises toward the surface, cools, and then sinks back down, similar to boiling water. This physical movement, known as convection, transfers energy much more rapidly than the radiative process below it. The turbulent cells in the Convective Zone directly influence the appearance and magnetic activity of the Sun’s surface.
The Visible Surface: Photosphere
The Photosphere is the Sun’s visible surface, a thin boundary only about 500 kilometers thick. It marks the point where the plasma becomes cool and tenuous enough for light to escape freely into space. The bulk of the light energy that eventually reaches Earth is emitted from this layer.
The temperature of the Photosphere averages around 5,772 Kelvin. Granulation, a mottled pattern resembling bubbling cells, is a visible feature caused by the rising and falling convection cells beneath it. Darker areas called sunspots are also observed here, which are regions where intense magnetic fields suppress the flow of hot plasma, making them cooler and less bright.
The Solar Atmosphere
The Sun’s atmosphere extends outward from the Photosphere, beginning with the Chromosphere. This thin shell above the visible surface often appears as a fleeting reddish glow during a total solar eclipse. The reddish color is caused by the emission of hydrogen atoms as the temperature begins to rise, reaching about 8,000 Kelvin.
The outermost and most expansive layer is the Corona, a vast, faint halo of plasma that stretches millions of miles into space. The Corona is hotter than the Photosphere below it, reaching temperatures between one and two million Kelvin. The mechanism responsible for this extreme heating remains an unresolved question in solar physics.
This superheated plasma flows away from the Sun as the solar wind, continuously filling the solar system with charged particles. The Sun’s magnetic fields shape the Corona, leading to features like streamers and coronal loops, and directing the solar wind into the wider cosmos.