A ball of hot gas so heavy it crushes its own core into a nuclear furnace, then pours the energy out as the light that grows nearly everything alive. A picture for every idea.
mostly hydrogen, held by gravity
The Sun is not solid and not on fire in the way a campfire is. It is an enormous ball of hot gas, about three quarters hydrogen and most of the rest helium, the two lightest elements. It is so big that more than a million Earths could fit inside it. With that much material, the Sun's own gravity pulls every part of it inward toward the center, packing the gas tighter and tighter the deeper you go.
pressure builds, the center gets extreme
All that inward pull has to go somewhere. The weight of the entire Sun bears down on its center, so the core is squeezed to a pressure hundreds of billions of times what you feel at sea level, and heated to roughly 15 million degrees Celsius. Under those conditions, gas is torn into a soup of bare nuclei and loose electrons, all slamming into each other at tremendous speed. This crushing heat and pressure is the setup for everything else: it is what makes the core hot enough to fuse atoms together.
the opposite of splitting atoms
In the core, hydrogen nuclei are crammed so close and moving so fast that they stick together, building up into helium. This is nuclear fusion, and it is the exact opposite of the splitting in our nuclear energy guide: there, heavy atoms break apart; here, light atoms join. The clever part is the bookkeeping. A helium nucleus weighs slightly less than the hydrogen that went into it, and that missing scrap of mass is converted into energy. The Sun runs through about 600 million tonnes of hydrogen every second, and the tiny mass shortfall each time is where sunlight comes from.
a tug of war that stays balanced
You might expect a giant nuclear furnace to either blow itself apart or cave in. The Sun does neither, because two forces are locked in a near perfect standoff. Gravity pulls everything inward. Fusion in the core pushes outward, like the pressure of a hot gas trying to expand. These balance almost exactly, which makes the Sun remarkably self-correcting: if the core squeezed in a little, it would heat up, fusion would speed up, and the extra push would gently expand it back out. If it puffed out, it would cool, fusion would slow, and gravity would pull it back. That quiet thermostat is why the Sun has shone steadily for billions of years.
a slow crawl, then a fast sprint
Here is the surprise. The energy made in the core does not rush straight out. The Sun is so dense that a packet of light gets absorbed and re-emitted countless times, staggering through the layers in a random walk. By most estimates this trip from core to surface takes on the order of tens of thousands of years (the exact figure is debated, but it is certainly very long). Once that energy finally escapes the surface as sunlight, the rest is easy: across the near vacuum of space it travels at the speed of light and reaches Earth in just about 8 minutes. So the sunshine warming you today was forged in the core long before humans existed.
from the surface to your backyard
Working outward, the Sun has a core where fusion happens, layers that carry the energy out, and the photosphere, the bright visible surface, wrapped in a thin outer atmosphere. The Sun also has powerful, ever-shifting magnetic fields. Where they tangle and bottle up heat, you get sunspots: patches that are cooler and so look darker. When that pent-up magnetic energy snaps loose, it can fire off a solar flare, a sudden burst of radiation and particles that can disturb satellites and radio on Earth.
For all that machinery, the payoff is simple. Nearly every living thing depends on the Sun. Plants turn sunlight into food, and almost the entire food chain builds on that. Sunlight also drives the weather: it heats the air and oceans unevenly, which stirs the wind and lifts water into the sky to fall again as rain, the engine behind the water cycle and weather. Even fossil fuels are ancient sunlight, stored in plants long ago.
A ball of gas. The Sun is mostly hydrogen and helium, held together by its own gravity.
Gravity crushes the core, heating it to about 15 million degrees under enormous pressure.
Fusion runs. Hydrogen joins into helium, and the tiny lost mass becomes energy.
It stays stable. Inward gravity and outward fusion pressure balance and self-correct.
Light escapes slowly, taking ages to reach the surface, then 8 minutes to reach Earth.
It powers us. Sunspots and flares ripple across it; its light drives life and weather.