From the core of the Sun to a leaf
The light on your face left the surface of the Sun eight minutes ago. Getting to the surface took far longer. Follow it out from the centre, one step at a time.
Inside the Sun
Seen from outside, the Sun is a ball of glowing gas 695,700 kilometres in radius. Inside, it has layers.
The core, the inner quarter of the radius, is where almost all of its energy is made. Around it is the radiative zone, where that energy moves outward as light. In the outer 29 percent, the convection zone, the gas itself boils and carries the heat up to the surface.
The centre
At the centre the temperature is about 15.7 million kelvin, and the gas is squeezed to about eight times the density of gold.
Here hydrogen nuclei fuse into helium. Each fusion releases energy, much of it as gamma rays, the most energetic kind of light.
Zoom in: a few millimetres
Follow one gamma ray. It sets off at the speed of light, but the core is so crowded with charged particles that it gets about 0.09 centimetres before it hits one. That takes three trillionths of a second.
The particle absorbs it, or knocks it aside. Either way, light carries on in a new direction, chosen at random.
16 steps
Every step points somewhere new. Nothing pulls the light outward, and nothing remembers which way it came from.
After 16 steps it is typically about four steps away from where it started, and just as likely to be heading back towards the centre as away from it.
Zoom out
This is a random walk. The distance it covers grows only with the square root of the number of steps: to get ten times farther, it needs a hundred times as many steps. To get a thousand times farther, a million times as many.
At this size the tangle already holds thousands of steps, and it is still only a few centimetres across.
A thousand kilometres
With steps this short, getting a thousand kilometres from the starting point would take about a billion billion of them.
The steps lengthen farther out, where the gas thins. But the Sun is 695,700 kilometres in radius, and the light has barely begun.
Farther out
Strictly, nothing makes this whole journey. Each time light is absorbed, the gas sends out new light, and the energy is handed on.
Farther from the centre the gas is cooler, so the light it gives off has less energy: gamma rays become X-rays, and later ultraviolet. The thread on the screen follows the light as it is handed on, not a single particle.
The edge of the radiative zone
The most quoted estimate for the trip from the core to the top of the radiative zone, 71 percent of the way to the surface, is about 170,000 years. It comes from a 1992 calculation with a solar model. Other models give anything from tens of thousands to a couple of hundred thousand years. No one can time it directly.
Moving at the speed of light the whole time, the light has zigzagged through about 170,000 light-years to get half a million kilometres from where it started.
Light is not the slowest thing in the Sun. Counting the heat stored in the gas, the energy itself takes about 30 million years to work its way out, as a 2003 paper points out, because most of it is held in the hot gas rather than in the light.
The convection zone
At the base of the convection zone the temperature is down to about 2 million kelvin, and the gas blocks light so well that radiating the heat away is no longer the fastest route.
Instead, the gas boils. Hot plumes rise, cool and sink again, and they carry the energy with them. The whole climb of about 200,000 kilometres takes around three months.
A real photograph · 36,500 km across
This is the real surface of the Sun, photographed by the Inouye Solar Telescope in Hawaii. The picture is 36,500 kilometres across, nearly three times the width of the Earth, and it is drawn here at that size.
Each cell, about the size of Texas in the observatory's words, is the top of a rising plume. Hot gas comes up in the bright centres and sinks again in the dark lanes. At about 5,772 kelvin the gas is finally thin enough for light to get out.
Free
Now nothing stands in the way. The light that leaves the surface is mostly visible light, the kind your eyes are made for, and it goes in a straight line.
The speed of light
The Earth is on average 149.6 million kilometres away. Light covers that in 8 minutes 19 seconds.
So the Sun you see is the Sun of eight minutes ago. The energy in that light was made long before.
Arrival
A tiny share of the Sun's light hits the Earth at all. Some is reflected back to space by clouds, ice and sea, some warms the ground and the air, and a little lands on a leaf.
Trillionths of a second again
In the leaf, chlorophyll absorbs the light. On average the energy reaches a reaction centre in around 50 trillionths of a second, and there, in about 3 trillionths more, it moves an electron. That is the first step of photosynthesis.
A journey of hundreds of thousands of years ends in a few trillionths of a second, as sugar in a leaf.
With the Sun high in a clear sky, about 120 million billion photons of visible light land on every square centimetre of your skin each second.
Every one of them is the end of a walk that began in the core, on the usual estimate, about 170,000 years ago, before the last ice age began. Around then, Neanderthals deep in a cave in southern France were breaking off stalagmites and laying them out in rings.
Every number on this page comes from one of these. The walk on screen is drawn, not computed: its steps are the real length in the core, its tangle is only an illustration of how a random walk spreads.
