The Sun, the central star of our solar system, is a massive and complex celestial body that provides the energy necessary for life on Earth. Without the sun we would not exist. As a G-type main sequence star, the Sun is made up of several different layers, each with unique properties and characteristics.
We start from the inside out. We start with the heart of the sun. It is the innermost layer, a high-pressure, high-temperature environment where nuclear fusion occurs. The density in the core reaches about 150 g/cm³ and the temperature is about 15 million Kelvin. In the nucleus, hydrogen nuclei (protons) collide and combine to form helium through a process known as the proton-proton chain. This fusion process releases an immense amount of energy in the form of photons, neutrinos and kinetic energy, which powers the sun and provides the energy that supports life on Earth.
The radiation zone
We travel a little further and find a sea of photons. This is exactly what we find above the core: it is a layer in which the energy generated in the core is transported to the outside by radiation. In this zone, photons (light particles) undergo a series of interactions with matter, being repeatedly absorbed and re-emitted. This process, known as radiative diffusion, causes energy to slowly travel outward. How far does this area extend? The radiation zone extends from the outer edge of the core to about 70% of the Sun’s radius, with temperatures ranging from about 7 million Kelvin at the edge of the core to about 2 million Kelvin at the outer edge.
The convection zone
We move to the next layer of the sun and encounter the convection zone, which extends to the visible surface of the sun, the photosphere. In this layer, temperatures have fallen so low that convection, rather than radiation, becomes the dominant mechanism for transferring energy to the outside. Convection occurs when hot, floating plasma rises into the cooler, less dense regions above, loses its heat, and then sinks again, creating a continuous flow of plasma called convection currents. Curiously, at the beginning of the 20th century, the Danish astrophysicist Ejnar Hertzsprung was one of the first scientists to study the role of convection inside the sun. His work laid the foundation for our understanding of energy transport in the Sun’s layers outside the stars.
We enter the visible surface of our star. It is the layer of the sun that we see when we observe it from Earth; It is relatively thin, about 500 kilometers thick, and has numerous features such as sunspots, cooler, darker regions caused by the Sun’s magnetic field, which obstructs the upward flow of heat. The temperature in the photosphere averages about 5,800 Kelvin, but varies depending on location and the presence of sunspots. To find the first studies of sunspots, we have to go back to the beginning of the 17th century, when astronomers like Christoph Scheiner began observing and documenting these features on the sun’s surface. Specifically, Scheiner discovered them in 1611 and his work officially, still published by a friend under a pseudonym, established a long tradition of solar observation and research that continues to this day.
The chromosphere and the corona
We reach the last stations of the sun, the outer atmosphere of the sun, which consists of two layers: the chromosphere and the corona. The chromosphere is a thin layer of plasma that emits a reddish glow that is visible during solar eclipses. The corona is the outermost layer of the Sun, an extremely hot and thin plasma that extends millions of kilometers into space and is visible as an ethereal halo during a total solar eclipse. The temperature in the chromosphere varies from about 4,500 Kelvin at the base to about 25,000 Kelvin at the top, while the corona reaches temperatures of 1 to 3 million Kelvin, with temperatures exceeding 10 million Kelvin in some regions.