A star with no surface to stand on
In the earlier guides of this rung you followed energy from the Sun's fusion core outward — through the radiative zone where light crawls, and then the convective zone where great cells of hot gas boil upward like a pot just below the boil. Now we arrive at the part you can actually *see*. But here is the first surprise: there is nothing solid to arrive at. The Sun is gas all the way down, getting denser and hotter inward with no real boundary. What we call its 'surface' is not a surface at all but the depth at which the gas finally thins out enough to let light escape freely to space.
That escape depth is the photosphere — literally the 'sphere of light' — and it is the photosphere we mean whenever we speak of the Sun's surface. Below it, the gas is so thick that a photon can travel only a tiny distance before being absorbed and re-emitted; the Sun is opaque, like fog. Above it, the gas is thin enough that light streams straight out; the Sun is transparent, like clear air. The photosphere is simply the thin shell where fog turns to clear air. It looks knife-edge sharp from Earth, but that crispness is an illusion of scale: this glowing skin is only a few hundred kilometres thick, a hair's breadth on a star nearly 1.4 million kilometres across.
The boiling skin: granulation and limb darkening
Point a good solar telescope at the photosphere and it is not smooth at all — it is a seething mosaic of bright cells edged by darker lanes, like the top of a pot of simmering porridge. This is granulation, and it is the convection from below breaking the surface. Each bright cell, a *granule*, is the top of a column of hot gas welling up; it glows brighter because it is hotter. At the granule's edge, the gas has cooled, given up its light, and sinks back down in the darker lanes. So granulation is literally a live photograph of the Sun's boiling — convection caught in the act, made visible the instant the gas becomes transparent.
Each granule is roughly a thousand kilometres across — about the width of France — and lives only five or ten minutes before its upwelling fades and a new pattern takes its place. So the entire face of the Sun is reshuffling itself every few minutes, a restless boil covering the whole star at once. It is a useful jolt to remember the scale: each of those modest-looking 'grains' could swallow a small country, and there are millions of them on the disk at any moment.
There is a second clue written across the disk: the Sun looks brighter in the middle and visibly dimmer toward its edge, or *limb*. This is limb darkening, and once you see why, it is beautifully simple. When you look at the centre of the disk, your line of sight plunges straight down into the photosphere and you see the deepest, hottest, brightest layers. When you look near the limb, your sightline slices in at a shallow, grazing angle and can only reach the higher, cooler, fainter layers before the gas turns opaque. Same skin, viewed at a slant — so limb darkening is really a depth gauge in disguise, telling us the photosphere gets cooler as you move outward through it.
Up into the thin air: the chromosphere
Climb just above the photosphere and you enter the chromosphere — the 'colour sphere' — a layer a few thousand kilometres deep and thousands of times thinner than the air you are breathing. It is normally invisible, drowned out by the photosphere's overwhelming glare. But for a few precious seconds during a total solar eclipse, when the Moon hides the photosphere, the chromosphere flashes into view as a thin rim of vivid rose-red light hugging the dark disk. That red is the H-alpha line of hydrogen you met in the spectroscopy rung — the chromosphere wears its name on its sleeve.
Through an H-alpha filter, the chromosphere is anything but calm. It bristles with *spicules* — countless slender jets of gas, each shooting upward at tens of kilometres a second and falling back within minutes, so the whole layer looks like a field of grass thrashing in the wind. And here is the quiet hint of the puzzle to come: as you rise through the chromosphere, the temperature stops falling and starts to *climb* again, from a few thousand kelvin at the bottom to tens of thousands near the top. After dropping steadily from the core all the way out to the top of the photosphere, the temperature has, oddly, turned around. Hold on to that reversal — it is about to become genuinely strange.
The corona, and a temperature that makes no sense
Above the chromosphere stretches the corona — the Sun's outermost atmosphere, a pearly-white halo of wispy streamers that reaches out for millions of kilometres. Like the chromosphere, the corona is normally invisible against the photosphere's glare; a total eclipse is the great unveiling, when the corona blooms around the black disk of the Moon as one of the most beautiful sights in nature. It is fantastically tenuous — far emptier than the best vacuum we can make in a laboratory — which is exactly why so faint a thing only shows when its brilliant backdrop is blocked.
Now for the fact that stops physicists in their tracks. The photosphere is about 5,800 kelvin. The corona, hovering just above it, is *one to three million* kelvin — hundreds of times hotter than the surface beneath it. Sit with how wrong that should feel. Heat flows from hot to cold; move away from a fire and you get cooler, never hotter. The Sun's own furnace is its core, far below, and the temperature dutifully falls all the way out to the photosphere — and then, in the thin air above, it leaps to millions of degrees. It is as if you walked away from a campfire and, several steps out, the air abruptly became hundreds of times hotter than the flames. This is the coronal heating problem, and it is one of the great open questions in solar physics.
Temperature on the way out of the Sun: core ........... ~15,000,000 K (fusion furnace) radiative zone . falling convective zone falling PHOTOSPHERE .... ~5,800 K <- the visible 'surface', coolest point chromosphere ... ~10,000 K and rising CORONA ......... ~1,000,000-3,000,000 K <- the puzzle: hotter again! heat 'flowing uphill' from a 5,800 K surface to a 1,000,000 K corona => the energy is NOT carried as ordinary heat. Something else does it.
The likely culprit: magnetism, not heat
The way out of the paradox is to notice the hidden assumption. The corona is *not* heated by ordinary heat flowing up from the surface — that genuinely would be impossible. The energy arrives by a completely different channel: the Sun's magnetic field. The churning convection below drags and tangles magnetic field lines that thread up through the surface, storing energy in them like a coiled spring. Up in the thin corona, where there is almost no gas to dilute it, even a modest trickle of magnetic energy can heat the sparse particles to ferocious temperatures. So the coronal heating problem is not really 'how does heat go uphill' — it is 'exactly how does tangled magnetism dump its stored energy into the corona?'
Researchers have two leading ideas, and the honest answer is that both probably matter. One is *nanoflares*: countless tiny bursts as tangled field lines snap and reconnect, each releasing a little heat, together adding up to a steady roast. The other is *waves*: magnetic waves launched by the boiling below that travel up the field lines and break in the corona, dumping their energy like surf on a beach. Which dominates, and where, is still actively debated — spacecraft such as the Parker Solar Probe were sent flying through the corona itself precisely to settle it. This is a live frontier, not a closed case, and that is exactly what makes it worth knowing about.
What is solid, and what is still open
Let us separate the bedrock from the frontier. Rock-solid: the Sun has no surface, only the photosphere where it turns transparent; granulation is convection seen from above; limb darkening is the slant-view depth gauge; and the layers really do go cool photosphere, warmer chromosphere, scorching corona. The temperatures themselves are measured, not guessed — we read them straight from the spectra of each layer. None of that is in doubt.
Genuinely open: the precise mechanism, or mix of mechanisms, that heats the corona to millions of degrees. We are confident the energy is magnetic; we are not yet sure of the exact recipe of nanoflares and waves, nor of how it varies across the Sun. That honest gap is not a failure — it is what an active science looks like. A two-hundred-year-old question, studied with eclipses and now with spacecraft diving through the corona, is still teaching us how stars wrap themselves in magnetism. You now have the map of the solar atmosphere; the next guide follows that magnetism off the Sun entirely.