Anatomy of a Star
The Sun is the nearest star, and a star is simply a giant ball of hot gas held together by its own gravity and lit from within by nuclear reactions. What makes the Sun easy to study is only that it is close — 150 million kilometres away, against light-years for the next one. In every other way it is utterly ordinary: a middle-weight, middle-aged yellow star, the kind astronomers label a G-type star (the letter is just a slot in the colour-and-temperature scheme we meet in §3.2). Hundreds of billions like it fill our galaxy alone.
Its bulk numbers set the scale for everything that follows. The Sun's radius is about 696,000 kilometres — roughly 109 Earths set side by side. Its mass is about two thousand trillion trillion kilograms (a 2 followed by 30 zeros), some 333,000 times the mass of the Earth, and that mass is what its gravity uses to hold the whole thing together. The visible surface sits at about 5,800 K — kelvin, a temperature scale that starts at the coldest possible cold, so 5,800 K is about 5,500°C. Deep in the centre the temperature climbs to roughly 15 million K. And it pours out energy at a steady 3.8 × 10²⁶ watts — a 38 followed by 25 zeros in watts, the power of trillions of trillions of light bulbs, every second.
That energy keeps the Sun from collapsing. Gravity pulls every part of the star inward, and left alone it would crush the Sun to a point. What holds the line is the outward push of hot gas — pressure — fed by the heat of nuclear reactions in the centre. The inward pull and the outward push settle into a near-perfect standoff, and that balance is what makes a star stable for billions of years.
Cut the Sun open and you find it built in layers, each one a different way of moving energy outward. Click through them in the cutaway below, or step with the arrow keys, from the centre out. The innermost is the core, the fusion furnace, where the 15-million-degree heat is generated. Around it is the radiative zone, so dense that energy can only seep through as light, bouncing from particle to particle. Outside that is the convective zone, cool enough that the gas itself churns — hot plasma rises, sheds its heat, and sinks back to be reheated, like a pot at a boil. The churning surfaces at the photosphere, the thin visible skin where light finally escapes. Above it lie two faint atmospheric layers: the reddish chromosphere and, far beyond, the wispy corona — both normally lost in the glare, but briefly visible during a total solar eclipse.
The Furnace
What is actually happening in that core? The Sun runs on nuclear fusion — forcing the nuclei of light atoms to merge into heavier ones. The fuel is hydrogen, the simplest atom, whose nucleus is a single proton. Under the crushing heat and pressure of the core, protons slam together hard enough to stick, and through a few steps four of them are welded into a single nucleus of helium, the next-lightest element. The dominant route is called the proton–proton chain. Step through it with the arrow keys in the figure below.
Along the way the chain throws off two positrons and two neutrinos — the antimatter twins of the electron and the ghostly, near-weightless particles we first met in §1.2 — plus a burst of light. The key is the bookkeeping of mass. One helium nucleus weighs about 0.7% less than the four protons that made it. That missing scrap of mass is not lost; it leaves as energy, following Einstein's E = mc² from §0.1 — mass and energy are two forms of one thing, and a tiny mass converts to an enormous amount of energy. (This is the same fusion that forged the first helium moments after the Big Bang, in §1.3 — only now it runs steadily inside a star instead of in a one-time cosmic flash.)
The numbers are staggering. Every second, the Sun fuses about 600 million tonnes of hydrogen, and of that, roughly 4 million tonnes vanish as pure energy. The light born in the core does not escape quickly: a single packet of light random-walks through the dense radiative zone, taking tens of thousands of years to reach the surface — and then crosses the empty 150 million kilometres to your eyes in just about eight minutes. So the sunlight on your skin began its journey before human history started.
Burning four million tonnes a second sounds like it should empty the tank fast, but the Sun is so vast that it can keep this up for roughly 10 billion years — its main hydrogen-burning lifetime. It formed about 4.6 billion years ago, which puts it a little past halfway. What happens when the core hydrogen finally runs low — and how the Sun will end — is the story of §3.3.
An ordinary yellow star, built in layers around a furnace that turns four million tonnes of itself into light every second — and has done so for 4.6 billion years, with as long again to go. Next we ask where the Sun sits among all the other stars.