How the Sun Works

Every second, the sun converts about 600 million tonnes of hydrogen into helium. This is not burning in the chemical sense, where atoms are rearranged. It is nuclear fusion: hydrogen nuclei are squeezed together so hard that they fuse into helium, releasing energy in the process. The energy generated in the core takes hundreds of thousands of years to fight its way to the surface, then travels at the speed of light to reach Earth in about 8 minutes.

This is the engine that powers almost everything on Earth. Photosynthesis in plants, the water cycle, wind, food chains, and the warmth that makes life possible all trace back to the sun. Understanding how it works helps explain why it will eventually die, why it behaves the way it does, and why disruptions to the stream of particles it sends toward Earth can knock out power grids and satellites.

The short answer

The sun is a G-type main-sequence star, a ball of hot gas held together by its own gravity. At its core, hydrogen nuclei fuse into helium under pressures and temperatures extreme enough to sustain nuclear fusion. This releases energy that gradually works its way outward through layers of the sun, eventually radiating into space as sunlight. The sun also constantly emits a stream of charged particles called the solar wind. Its activity follows an approximately 11-year cycle of sunspot rise and fall. The sun is about 4.6 billion years old and has enough hydrogen fuel remaining to keep shining for roughly 5 billion more years.

The full picture

The structure of the sun

The sun is not a uniform ball of fire. It has distinct layers, each with different properties.

At the center is the core, where the fusion reactions happen. The pressure from the sun’s own gravity compresses hydrogen nuclei to densities about 150 times that of water. At 15 million degrees Celsius, these nuclei have enough energy to overcome their mutual electrical repulsion and fuse together. The dominant reaction is the proton-proton chain: four hydrogen nuclei (protons) ultimately combine to form one helium nucleus, releasing two positrons, two neutrinos, and energy.

The energy released in the core as gamma rays begins a long journey outward. In the radiative zone, which extends from about 25 percent to 70 percent of the sun’s radius, the energy is transported outward by radiation, bouncing off densely packed ions in a process that takes, on average, about 170,000 years. In the convective zone, from 70 percent of the radius to the visible surface, energy is transported by convection, like a pot of boiling water. Hot gas rises, cools at the surface, and sinks back down.

The visible surface of the sun is the photosphere, a thin layer about 100 kilometers thick where the gas becomes transparent enough for light to escape into space. This is what we see as the solar disc. Above the photosphere are two more layers that are normally invisible: the chromosphere, a thin pinkish layer a few thousand kilometers thick, and the corona, the sun’s outer atmosphere, which extends millions of kilometers into space and is visible as a white halo during total eclipses.

Solar activity: sunspots and the solar cycle

The sun is not calm. Its surface is a churning landscape of plasma, with dark sunspots appearing and disappearing, solar flares erupting, and vast loops of magnetized gas arcing above the surface.

Sunspots are dark patches on the photosphere caused by magnetic fields that inhibit convection. They are cooler than the surrounding plasma, about 3,800 degrees Celsius compared to 5,500 for the photosphere, which makes them appear dark against the bright surface. They vary in size from small specks to regions larger than Earth.

The number of sunspots follows an approximately 11-year cycle, as tracked by NASA’s solar cycle prediction panel. During solar maximum, the sun has many sunspots, frequent solar flares, and coronal mass ejections. During solar minimum, sunspots are rare. The cycle is driven by the sun’s differential rotation: the equator rotates faster than the poles, gradually twisting and tangling the sun’s magnetic field. Every 11 years or so, the field becomes so tangled that it reorganizes, flipping polarity. The 11-year cycle is actually half of a 22-year Hale cycle, after which the magnetic polarity returns to its original state.

The current solar cycle, Solar Cycle 25, began around December 2019 and is expected to peak around 2025. Solar Cycle 24, which peaked in 2014, was notably weak. Solar Cycle 25 is turning out to be stronger than predicted, with significant sunspot activity in 2024 and 2025.

Solar wind and space weather

The sun does not just emit light. It also emits a constant stream of charged particles, mostly protons and electrons, that flow outward through the solar system at 400 to 800 kilometers per second. This is the solar wind.

The solar wind inflates a bubble in interstellar space called the heliosphere, which extends past the orbit of Pluto. When the solar wind encounters a planet’s magnetic field, it is deflected, forming a bow shock. Earth’s magnetosphere does this, protecting the surface from most of the solar wind. But some particles leak through and get funneled toward the poles, where they collide with atmospheric gases to produce the aurora borealis and aurora australis.

Sometimes, the sun releases a coronal mass ejection, a burst of magnetized plasma that can travel at millions of kilometers per hour. When this hits Earth’s magnetosphere, it can cause geomagnetic storms. In 1989, a geomagnetic storm caused by a coronal mass ejection knocked out Quebec’s power grid for 9 hours, affecting 6 million people. In 1859, the Carrington Event, the largest geomagnetic storm on record, induced currents in telegraph wires that caused fires in some offices and disabled the entire telegraph system across Europe and North America.

Modern society is far more dependent on electronics and satellites than 1859. A Carrington-level event today could cause widespread satellite failures, GPS disruption, and power grid collapses. The 2025 solar maximum increases the probability of significant space weather events.

Nuclear fusion and why it matters on Earth

The sun fuses about 600 million tonnes of hydrogen every second. Only a tiny fraction of this energy hits Earth, but it is enough to power virtually all life and most of human civilization.

The fusion reaction in the sun requires extreme temperatures and pressures that are hard to replicate on Earth. Scientists have been working on controlled nuclear fusion for decades. As of the 2020s, experimental reactors like ITER in France, the largest fusion project in the world, are still being built and are not expected to produce net energy until the 2030s. Private companies like Commonwealth Fusion Systems and TAE Technologies are pursuing alternative approaches and have attracted billions in investment.

The appeal of fusion is clear: hydrogen is abundant, the reactions produce no carbon emissions, the radioactive waste is far less than fission, and the energy density is enormous. A fusion power plant would not be like a fission nuclear plant. If the plasma containment failed, the reaction would simply stop. No meltdown is possible.

Why it matters

The sun is not just the source of daylight. It is the reason complex life exists on Earth.

Solar energy drives photosynthesis, which is the foundation of most food chains. The energy captured by plants becomes energy available to herbivores, then carnivores, then everything else. Nearly all biological energy on Earth traces back to the sun.

The sun also drives the water cycle, which moves fresh water from mountains to rivers to oceans and back through evaporation and precipitation. It drives wind, as differential heating of land and ocean creates pressure differences that air flows to equalize. Weather, ocean currents, and climate are all ultimately powered by solar energy.

Human civilization has organized itself around predictable access to sunlight. Agriculture depends on seasonal sunlight patterns. Solar power is one of the fastest-growing sources of electricity, and rooftop solar panels now provide cost-effective power to millions of homes in sunny regions. The cost of solar panels has fallen by more than 99 percent since 1976, making solar the cheapest source of new electricity generation in most of the world.

Understanding the sun also matters for protection. Solar flares and coronal mass ejections are not science fiction. They are recurring events that can and do disrupt the technologies modern society depends on. The 2022 geomagnetic storm caused by a coronal mass ejection, one of the largest in years, disrupted GPS signals for aviation and caused voltage anomalies in power grids across North America. As solar maximum approaches in 2025, the risk increases.

Common misconceptions

“The sun is a giant ball of fire.” Fire is a chemical reaction between oxygen and a fuel. The sun has no oxygen in its core and cannot burn. What happens in the sun is nuclear fusion, a nuclear reaction that has nothing to do with combustion. Calling it fire is a misnomer that obscures how the sun actually works.

“The sun is unusually large.” The sun is average by stellar standards. About 75 percent of all stars are red dwarfs, smaller and cooler than the sun. The sun is more massive than 95 percent of stars, but in terms of luminosity and radius, it is unremarkable. The term “yellow dwarf” that is sometimes applied to the sun is misleading, since the sun actually appears white when viewed from space, not yellow.

“Solar flares are what cause auroras.” Solar flares are bursts of electromagnetic radiation that travel at the speed of light and arrive at Earth in about 8 minutes. They can disrupt radio communications but do not directly cause auroras. Auroras are caused by coronal mass ejections, which are bursts of magnetized plasma that take one to three days to reach Earth. Both are associated with sunspot activity, but they are distinct phenomena.

Key terms

Photosphere: The visible surface of the sun, a thin layer about 100 kilometers thick where the gas becomes transparent enough for light to escape. Average temperature about 5,500 degrees Celsius.

Core: The innermost region of the sun, where hydrogen nuclei fuse into helium. Temperature about 15 million degrees Celsius, density about 150 times that of water.

Solar wind: A stream of charged particles, mostly protons and electrons, flowing outward from the sun at 400 to 800 kilometers per second. Creates the heliosphere that extends past Pluto.

Coronal mass ejection: A massive burst of magnetized plasma from the sun’s corona that travels through the solar system at up to millions of kilometers per hour. Can cause geomagnetic storms on Earth.

Sunspot: A dark patch on the photosphere caused by magnetic fields that inhibit convection. Cooler and therefore darker than the surrounding plasma. Used as an indicator of solar activity levels.

Solar flare: A sudden eruption of electromagnetic radiation from the sun’s surface, caused by the release of magnetic energy. Travels at the speed of light and reaches Earth in about 8 minutes.

Radiative zone: The layer of the sun between the core and the convective zone, where energy is transported outward by radiation. Energy from the core takes about 170,000 years on average to cross this zone.

Convective zone: The outer layer of the sun, from about 70 percent of the solar radius to the surface, where energy is transported by convection currents of hot plasma.

Aurora: Light emitted when charged particles from the solar wind collide with gases in Earth’s upper atmosphere. Occurs near the magnetic poles, called aurora borealis in the north and aurora australis in the south.

Nuclear fusion: The process of combining light atomic nuclei, typically hydrogen, to form heavier nuclei, releasing energy. Powers the sun and is the goal of fusion energy research.