radiative energy transport
/ RAY-dee-uh-tiv /
Energy is born in a star's core and must somehow reach the surface to escape as starlight. One of the two ways it travels is by radiation — that is, as light itself, photons handing energy outward. But the inside of a star is anything but clear: it is a dense fog of charged particles, so a photon cannot fly straight out. Instead it gets absorbed and re-emitted, again and again, drifting outward in a slow, stumbling random walk. Radiative energy transport is this leakage of energy carried by light through the stellar fog.
Because the gas is so opaque, the journey is staggeringly slow. A photon born in the Sun's core travels only a fraction of a centimeter on average before being absorbed, then re-emitted in a random new direction. The net result is that energy takes tens of thousands of years, by many estimates on the order of 10^4 to 10^5 years, to seep from the core to the surface — even though once it finally escapes, the light crosses the 150 million km to Earth in just over eight minutes. Heat flows outward by radiation whenever the temperature falls gently enough with depth that this photon diffusion can carry the load.
Radiative transport dominates wherever the gas is relatively transparent and the temperature gradient is shallow — for the Sun, that is the broad middle 'radiative zone' between the fusing core and the boiling outer layers. How effectively radiation carries energy depends critically on the opacity of the gas: the foggier the gas, the harder it is for radiation to do the job, and at some point the star gives up on radiation and starts to boil instead, switching to convection.
The light warming your face this afternoon was, in a sense, created in the Sun's core tens of thousands of years ago — back when mammoths still roamed — and only just finished its slow stumble to the surface before sprinting to Earth in 8 minutes. The long part of the trip was radiative transport.
Energy crawls out of the Sun by radiation over millennia, then races to Earth in minutes.
The 'thousands of years' figure is the photon-diffusion time and is uncertain by a wide margin; it is a slow random walk, not a fixed delay. And the photons that come out are not the same photons that started — energy is passed along, not delivered intact.