The Sun, a massive sphere of incandescent material, does not possess a solid core like the Earth. This is a common misconception, as the immense gravity and density at the star’s center might suggest a solid structure. Instead, the Sun is composed entirely of super-hot, electrically charged material that exists in a state far beyond the familiar liquid, solid, or gas.
The Core is Not Solid: Defining Plasma
The material that makes up the Sun’s core exists as a plasma, often called the fourth state of matter. Plasma is an ionized gas created when gas atoms become so hot that their electrons are stripped away from the atomic nuclei. This process leaves behind a churning mix of free-moving ions and electrons, all carrying an electrical charge. The particles within this core region are moving far too rapidly due to the extreme heat to form any stable molecular bonds. The core is primarily composed of hydrogen and helium nuclei existing in this dense, electrically conductive plasma state.
Extreme Conditions: Temperature and Pressure
The plasma state of matter in the Sun’s core is a direct result of the immense physical conditions present there. The temperature at the Sun’s center reaches approximately 15 million degrees Celsius, making it the hottest region in the entire solar system. Accompanying the high temperature is immense pressure, estimated to be around 250 billion times the atmospheric pressure at Earth’s surface. This pressure is generated by the weight of the Sun’s overlying layers, compressing the core material to a density 150 times greater than that of water. The Sun maintains a stable size through a balance where the outward thermal pressure from the core counteracts the inward gravitational pressure of the star’s mass, a state known as hydrostatic equilibrium.
The Engine of the Sun: Nuclear Fusion
The primary function of the Sun’s core, which extends out to about 20% to 25% of the star’s radius, is to act as a colossal energy generator. This power is created through nuclear fusion, a process where light atomic nuclei are forced together to form heavier ones. The dominant reaction is the proton-proton chain, responsible for nearly all of the Sun’s energy output. In the proton-proton chain, hydrogen nuclei combine under the extreme pressure and heat of the core. This multi-step process ultimately results in the formation of a single helium nucleus from four hydrogen nuclei. During this conversion, a small amount of mass is lost and transformed into a vast quantity of energy, following Einstein’s famous equation, E=mc². The energy is released mainly in the form of gamma-ray photons and neutrinos, with the neutrinos escaping the Sun almost instantly. The ongoing fusion reactions convert approximately 600 million tons of hydrogen into helium every second.
Sun’s Internal Structure Beyond the Core
Immediately surrounding the energy-producing core are two distinct layers that govern how the newly generated energy moves toward the star’s surface. These two zones function as a complex transport system, moving the power generated in the core out to the star’s atmosphere.
The Radiative Zone
The layer closest to the core is the Radiative Zone, which extends outward to about 70% of the Sun’s radius. In this zone, the plasma is still incredibly dense, and energy moves outward slowly, primarily through the absorption and re-emission of photons. The photons produced by fusion bounce around in the dense radiative zone, taking a million years or more to travel through this layer.
The Convective Zone
Above the radiative zone lies the Convective Zone, the outermost layer of the Sun’s interior. Here, the plasma is less dense and cooler, allowing for a more efficient method of energy transport through circulating currents. Hot plasma rises toward the surface, cools, and then sinks back down in a continuous boiling motion, similar to currents in a pot of water. This convection process carries the energy the rest of the way to the visible surface of the Sun.