From luminosity, size and temperature you know the Sun's surface is about $5800$ K and gives off $3.83 \times 10^{26}$ W. That energy is made deep inside and takes a long, slow path out. Along the way it passes through layers, each named for how energy moves through it.
| Layer | Where | Temperature | What happens |
|---|---|---|---|
| Core | center to $0.25\,R_\odot$ | about 15 million K | nuclear fusion makes the energy |
| Radiative zone | $0.25$ to $0.7\,R_\odot$ | 7 million to 2 million K | light carries energy out, absorbed and re-emitted again and again |
| Convective zone | $0.7\,R_\odot$ to the surface | 2 million K to about 6000 K | hot gas rises, cools and sinks, like boiling water |
| Photosphere | the visible "surface," a few hundred km thick | about 5800 K | light finally escapes to space |
| Chromosphere | a thin layer above it | thousands to tens of thousands of K | a reddish glow seen during eclipses |
| Corona | millions of km out | over 1 million K | very thin, very hot gas; the pearly halo of a total eclipse |
Why it works
Energy always flows from hot to cold, outward from the 15-million-kelvin core. Deep inside, the gas is so dense that light travels only a tiny distance before being absorbed and sent off in a random direction, so energy drifts outward through the radiative zone over tens of thousands of years or more. Farther out, the gas is cool enough that light gets trapped more easily, and it becomes faster to move energy by carrying hot gas upward: convection. At the photosphere the gas is thin enough for light to fly straight out. The Sun is hot plasma all the way through; the photosphere is just the depth we can see down to.
Key idea
Core (fusion), radiative zone (energy carried by light), convective zone (energy carried by moving gas), photosphere (the visible surface), chromosphere and corona (the atmosphere). The Sun is gas all the way down, with no solid surface.
Worked example
Worked example: How deep is the boiling?
The convective zone starts at $0.7$ of the Sun's radius of $696{,}000$ km. How thick is it?
- Its thickness is $1 - 0.7 = 0.3$ of the radius.
- $0.3 \times 696{,}000 = 208{,}800$ km, about 16 Earths lined up across.
Answer: About $209{,}000$ km.
Common mistake
Photos make the Sun look like a ball with a crisp surface, and people often picture something solid underneath. There is no solid surface: the Sun is gas (plasma) from the corona to the center. The edge looks sharp only because the photosphere, where the gas turns from opaque to clear, is just a few hundred kilometers thick, tiny next to the Sun's radius. Check: a spacecraft falling in would never land on anything; it would just meet hotter, denser gas.