radiative zone
The radiative zone is the thick middle layer of the Sun, sitting just above the core. Here the energy made by fusion travels outward not by the gas churning and rising, but by light itself slowly being passed along from atom to atom, like a message whispered through a vast, dense crowd.
It stretches from about one-quarter to about seven-tenths of the way out to the surface. The gas here is so hot and packed that a photon — a particle of light — can travel only a tiny distance before it is absorbed by an atom and then re-emitted in a random new direction. The photon zig-zags through this layer in what scientists call a random walk, taking a path that is mind-bendingly long. As a result, energy made in the core can take on the order of 100,000 years or more to grind its way through the radiative zone alone, even though if it could travel in a straight line it would cross the whole Sun in about two seconds.
The radiative zone matters because it is the slow bottleneck of the Sun's energy flow, and its smooth, layered structure is very different from the boiling layer above it. The boundary where radiation gives way to churning convection, called the tachocline, is thought to be where the Sun's magnetic field is generated, making this quiet zone secretly important to sunspots and solar storms.
Imagine dropping a marble into a crowded room and having every person bounce it in a random direction — it would take ages to reach the far wall. That is how a photon crosses the radiative zone.
The random walk turns a two-second trip into a journey of tens of thousands of years.
Estimates of how long energy takes to cross the radiative zone vary widely, from tens of thousands to many hundreds of thousands of years, because the answer depends on uncertain details of how opaque the gas is.