What is the Sun made up of, and why is it so hot

Every morning, the Sun rises, bringing light, warmth, and energy– something so familiar that we rarely stop to think about its source. But behind this daily event is a powerful astronomical force: a massive sphere of gas that keeps our solar system in balance.

The Sun’s heat is not only intense but also uneven across its layers. Temperatures range from relatively moderate levels at the surface to extremely high ones in both its core and outer atmosphere. Surprisingly, the outermost layer, far from the heat-producing core, is hotter than the surface. Why this happens remains one of the greatest unsolved questions in solar science. Here’s a closer look at the Sun's structure and how each layer contributes to its extreme temperatures.

At the Sun’s core, it's a massive ball of gas, mostly made up of hydrogen and helium. These gases exist in a state called plasma, where intense heat strips electrons from atoms. This plasma is full of energy and constantly moving.

The Sun’s energy comes from a process called nuclear fusion. As per a report by Space, deep in its core, hydrogen atoms are forced together under intense pressure to form helium. This reaction releases an enormous amount of energy, which slowly moves outward. It’s this fusion process that not only makes the Sun shine but also keeps its core at extremely high temperatures.

The core of the Sun is where temperatures soar to about 15 million degrees Celsius, the hottest part of the entire star. The pressure here is so immense that hydrogen atoms constantly fuse into helium, producing the energy that powers the Sun.

Above the core is the radiative zone, where energy moves very slowly. Here, light particles (photons) are absorbed by atoms in the plasma and re-emitted, a process that can take thousands of years. Although the temperatures in this zone drop gradually, they still range from 2 to 7 million degrees Celsius, according to Study(dot)com, as quoted by Space.

As energy reaches the convective zone, it starts to move more freely. Hot plasma rises toward the surface, cools slightly, and then sinks back down, creating large circulating currents. This layer, with temperatures around 2 million degrees Celsius, plays a key role in transporting the Sun's energy to its outer layers.

The photosphere is what we perceive as the Sun's surface, though it's not a solid layer. Temperatures here reach about 5,500 degrees Celsius. This is the layer from which sunlight is emitted into space, according to Study(dot)com, as quoted by Space. Sunspots, darker areas on the photosphere, occur due to lower temperatures caused by magnetic activity.

The temperature in the chromosphere varies. It starts at approximately 6,000°C near the photosphere and decreases to about 4,000°C before rising again in the upper layers. This layer becomes visible as a reddish ring during a total solar eclipse, according to a report by UCAR.

The outermost layer of the Sun, known as the corona, is the hottest part of its atmosphere, with temperatures reaching up to 2 million degrees Celsius, despite being farthest from the core. The reason for this temperature difference remains a mystery, but scientists suspect that magnetic forces may play a role in heating the corona.