The sun is a giant ball of super-hot plasma, made almost entirely of two elements: hydrogen (73.4% by mass) and helium (25%). The remaining 1.6% is a mix of heavier elements, mostly oxygen and carbon. But what makes the sun fascinating isn’t just what it’s made of. It’s the layered structure inside, where extreme heat and pressure drive nuclear fusion and create the energy that lights up our solar system.
What the Sun Is Made Of
Everything inside the sun exists as plasma, a state of matter where atoms are so hot that electrons are stripped away from their nuclei. This isn’t a gas in the everyday sense. Plasma is electrically charged, which means it interacts with magnetic fields in ways that ordinary gas cannot. Those magnetic interactions drive much of the sun’s behavior, from sunspots to solar flares.
Hydrogen dominates the sun’s composition and serves as its fuel. Helium is the second most abundant element, and it’s actually a byproduct of the fusion reactions happening in the core. The trace amounts of heavier elements like oxygen (0.8%) and carbon (0.3%) were present when the sun formed from a cloud of gas and dust about 4.6 billion years ago.
The Core: Where Fusion Happens
At the very center of the sun sits the core, where temperatures reach about 15 million degrees Kelvin and the pressure is unlike anything on Earth. The material here is packed so tightly that it’s denser than lead, even though it’s still plasma rather than solid metal. These extreme conditions are what allow nuclear fusion to occur.
Fusion in the sun follows a process called the proton-proton chain. It works in steps. First, two hydrogen nuclei (protons) slam together with enough force to fuse. Through a series of intermediate reactions, including the formation of a heavier form of hydrogen and then a light form of helium, the process eventually produces a stable helium nucleus. Each complete cycle releases about 25 million electron volts of energy. That energy takes two forms: light (photons) and neutrinos, tiny particles that barely interact with matter.
The core extends roughly a quarter of the way from the sun’s center to its surface, but it generates virtually all of the sun’s energy. Every second, the sun converts about 600 million tons of hydrogen into helium. A small fraction of that mass is converted directly into energy, following the famous relationship between mass and energy that Einstein described.
The Radiative Zone: A Million-Year Traffic Jam
Surrounding the core is the radiative zone, a thick shell of incredibly dense plasma where energy moves outward in the form of photons. But “moves” is generous. The material here is so dense that photons can’t travel in a straight line. They bounce off charged particles, get absorbed, and get re-emitted in random directions, over and over. According to NASA, an individual photon takes about a million years to work its way through this zone.
To put that in perspective: the light and heat you feel from the sun today was generated in the core roughly 100,000 to a million years ago. It spent most of that time pinballing through the radiative zone before finally escaping. The radiative zone makes up a huge portion of the sun’s interior, stretching from the outer edge of the core to about 70% of the way to the surface.
The Convection Zone: Rising and Sinking Plasma
Above the radiative zone, the sun’s interior switches to a completely different method of energy transport. In the convection zone, the outermost 30% of the sun’s radius, hot plasma rises toward the surface, releases its energy, cools off, and sinks back down. It works like a pot of boiling water. This churning motion is visible on the sun’s surface as a pattern of granules, bright cells surrounded by darker boundaries where cooler plasma is sinking.
The boundary between the radiative and convection zones is a thin layer called the tachocline. This region is important because the two zones rotate at different speeds, creating a shearing effect. Many solar scientists believe this shearing action is a key driver of the sun’s magnetic field. The tachocline stretches and twists magnetic field lines, amplifying them and helping generate the 11-year solar cycle that governs sunspot activity and solar storms.
How Scientists Know What’s Inside
No one has sent a probe into the sun, so everything we know about its interior comes from indirect methods. One of the most powerful is neutrino detection. Neutrinos are produced during fusion in the core, and because they interact so weakly with matter, they escape the sun almost immediately. While photons take up to a million years to reach the surface, neutrinos fly straight out of the core and arrive at Earth in about eight minutes.
This property makes neutrinos a direct window into the sun’s core. The Borexino experiment in Italy measured solar neutrinos and confirmed that the sun is releasing the same amount of energy today as it did 100,000 years ago. That’s a remarkable finding: the neutrinos tell us what the core is doing right now, while the light we see reflects conditions from the distant past. The agreement between the two confirms that the sun’s energy output has been stable over that timescale.
Scientists also study the sun’s interior through helioseismology, which analyzes sound waves that travel through the sun’s body. Different layers bend and reflect these waves in characteristic ways, letting researchers map the density, temperature, and rotation speed at various depths, much like geologists use earthquake waves to map Earth’s interior.
How the Sun’s Interior Changes Over Time
The sun isn’t static. Over billions of years, its core slowly accumulates helium “ash” from all that hydrogen fusion. As the hydrogen fuel in the core is consumed, the core gradually contracts under its own gravity. This contraction increases the temperature and density, which actually makes the remaining hydrogen fuse faster. The result is that the sun is slowly getting brighter and hotter over time, roughly 10% brighter every billion years.
Eventually, billions of years from now, the core will run out of hydrogen fuel entirely. Without fusion to push back against gravity, the helium core will contract further, heating up until the surrounding layers of hydrogen ignite in a shell around the core. This will cause the sun’s outer layers to expand dramatically, turning it into a red giant. If the core gets hot enough, the helium itself will begin fusing into heavier elements like carbon and oxygen. But that chapter of the sun’s life is still about five billion years away.