Anatomy of the Sun: How Our Star Works

A Star Held Together by Its Own Weight

The Sun looks placid from 150 million kilometers away, but it is really a slow-motion explosion held in check by its own gravity. It is an ordinary G-type star, about 4.6 billion years old, spanning roughly 1.39 million kilometers across — about 109 Earths lined up edge to edge — and it accounts for 99.8 percent of all the mass in the solar system. Every layer of it is locked in a standoff called hydrostatic equilibrium: gravity crushing inward, pressure from fusion and heat pushing outward, balanced closely enough that the Sun has burned steadily for billions of years without collapsing or blowing itself apart.

That balance is built in layers, and the Sun is not the same substance all the way through. Energy is made in a fusing core, carried outward first by radiation and then by boiling convection, and finally released through an atmosphere with three layers of its own — the visible photosphere, the chromosphere, and the corona. Each was mapped by different means, from sound waves rippling through the Sun's interior to spacecraft that have flown directly through its outer atmosphere, and each explains a different piece of solar behavior, from sunspots to the aurora.

The Fusion Furnace at the Core

At the Sun's center, temperatures reach about 15 million degrees Celsius and pressure crushes hydrogen plasma to roughly 150 times the density of water. The core extends only about a quarter of the way to the surface, yet because it is squeezed so tightly, it holds close to half of the Sun's entire mass. Under those conditions, hydrogen nuclei collide hard and often enough to fuse through a sequence called the proton-proton chain, combining four hydrogen nuclei into one helium nucleus. That helium nucleus weighs very slightly less than the four hydrogen nuclei that built it, and the missing mass is released as energy, following Einstein's E = mc².

The scale of that reaction is hard to picture: the core converts roughly 600 million tons of hydrogen into helium every second, with about 4 million tons of mass vanishing into pure energy in that same second. It sounds catastrophic, but the Sun is so vast that billions of years of this steady conversion have used up only a small fraction of its hydrogen fuel — which is why it can keep shining, at a nearly constant rate, for as long as it has.

The Long Climb to the Surface

Energy made in the core does not travel straight out. It first crosses the radiative zone, a region of dense plasma where photons are absorbed and re-emitted over and over in random directions, a bottleneck so severe that a photon takes on the order of 100,000 years to diffuse its way from the core to the outer edge of this zone. Along the way, temperature drops steeply, from about 7 million degrees Celsius near the core to roughly 2 million degrees at the outer boundary.

Beyond that point, the plasma becomes opaque enough that radiation alone can no longer move heat efficiently, so the Sun switches to convection — the process that stirs a pot of boiling water. In this outer convective zone, roughly 200,000 kilometers thick, hot plasma rises toward the surface, cools, and sinks back down in continuous churning cells. That churning shows up at the surface as granulation: a shifting pattern of granules, each about 1,000 kilometers across, that individually last only about ten minutes before dissolving and reforming.

The Photosphere: A Surface That Isn't Solid

What we actually see as the Sun's disk is the photosphere, a thin layer only a few hundred kilometers deep where the plasma finally thins out enough for light to escape freely into space. It runs about 5,500 degrees Celsius and is peppered with granulation as well as sunspots — darker, cooler patches where tangled magnetic field lines suppress the convection beneath them, dropping the local temperature by roughly 1,500 degrees. Some sunspots are individually larger than Earth.

This is also the layer people mean by "looking at the Sun," and it comes with a hard rule: never view the Sun directly, whether with the naked eye, binoculars, or a telescope, without a certified solar filter or ISO 12312-2 eclipse glasses. Ordinary sunglasses, smoked glass, or exposed film offer nowhere near enough protection — a few seconds of unfiltered sunlight focused onto the retina can cause permanent eye damage, often without any pain to warn you. Safe viewing uses purpose-built filters, projection methods, or imagery from missions like NASA's Solar Dynamics Observatory.

Chromosphere and the Corona's Baffling Heat

Above the photosphere sits the chromosphere, a thin, reddish layer normally drowned out by the glare below it but briefly visible as a pink flash rimming the Moon during a total solar eclipse. Its temperature does something odd: instead of cooling with distance from the Sun, it climbs back up, from about 6,000 degrees Celsius near the photosphere to around 20,000 degrees Celsius at its outer edge, while jets of plasma called spicules shoot upward through it like flames.

Farther out lies the corona, the Sun's sprawling outer atmosphere, which reaches temperatures of one to a few million degrees Celsius — hundreds of times hotter than the visible surface it surrounds. That jump, known as the coronal heating problem, has puzzled solar physicists for decades; leading explanations involve magnetic reconnection and waves of energy rippling up from below, and NASA's Parker Solar Probe has been flying directly through the corona since 2021 to gather data to settle it. Under normal conditions the corona is only visible from Earth during a total solar eclipse or through instruments called coronagraphs that artificially block out the Sun's disk.

The 11-Year Heartbeat: Sunspots, Flares, and Space Weather

The Sun's magnetic field is generated deep inside it by churning plasma and by differential rotation — the equator spins faster than the poles, since the Sun has no solid surface to rotate as one piece — and that mismatch winds the field tighter over time. The result is the roughly 11-year solar cycle, during which sunspot numbers rise to a solar maximum, fall to a quiet solar minimum, and the Sun's global magnetic field flips polarity, so a full magnetic cycle spans about 22 years. The current cycle, Solar Cycle 25, reached its maximum phase around 2024.

Near solar maximum, twisted magnetic loops above sunspot groups can snap and reconnect violently, unleashing solar flares — sudden bursts of radiation across the electromagnetic spectrum, ranked by strength from A-class up to the most powerful X-class flares. These often accompany coronal mass ejections, in which billions of tons of magnetized plasma are hurled outward from the corona at speeds that can exceed a million miles per hour, sometimes reaching Earth in well under a day.

Even without a storm, the corona's heat drives a constant outflow called the solar wind, charged particles moving at 300 to 800 kilometers per second, first predicted by physicist Eugene Parker in 1958. This wind inflates a vast magnetic bubble around the solar system called the heliosphere. When a CME's plasma slams into Earth's magnetic field, particles funnel toward the poles and collide with atmospheric gases, lighting up the sky as the aurora — most dramatically during severe geomagnetic storms, like the extreme May 2024 event that pushed visible auroras as far south as Mexico and Florida while straining satellites, GPS signals, and power grids.

The Sun's Distant Future

The Sun is roughly halfway through a main-sequence lifetime expected to last about 10 billion years, meaning it has around 5 billion years left of fusing hydrogen much as it does today. As that hydrogen supply eventually runs low, the core will contract and heat further while the outer layers swell dramatically, and the Sun will balloon into a red giant, likely engulfing Mercury and Venus and possibly reaching, or exceeding, Earth's current orbit as it grows to well over a hundred times its present diameter.

Once the core becomes hot and dense enough, helium itself will ignite in a sudden runaway event called the helium flash, after which the Sun contracts somewhat and settles into a quieter phase burning helium for roughly 100 million years. Unable to sustain fusion indefinitely, it will finally shed its outer layers into a glowing shell known as a planetary nebula, leaving behind its exposed core as a white dwarf — an Earth-sized ember that will cool for a very long time to come. Astronomers already study this fate playing out in other aging stars across the galaxy, a preview of the Sun's own ending billions of years before it arrives.