Sunlight is way older than you think Sten Odenwald
You may know that it takes lighta zippy eight minutes to reach us from the surface of the Sun, so how long do you think it takes light to travel from the Sun's coreto its surfaceé A few seconds or a minute at mosté Well, oddly enough, the answeris many thousands of years. Here's why. Photons are produced by the nuclear reactions deep in the core of our Sun.
As the photons flow out of the core,they interact with matter and lose energy, becoming longer wavelength forms of light. They start out as gamma rays in the core, but end up as xrays, ultravioletor visible light as they near the surface. However, that journeyis neither simple nor direct. Upon being born, each photon travels ata speed of 300,000 kilometers per second until it collides with a protonand is diverted in another direction, acting like a bullet ricocheting offof every charged particle it strikes.
The question of how far this photon getsfrom the center of the Sun after each collision is known as the random walk problem. The answer is given by this formula: distance equals step size times the squareroot of the number of steps. So if you were taking a random walk from your front door with a one meter stride each second, it would take you a million stepsand eleven days
just to travel one kilometer. So then how long does it take for a photongenerated in the center of the sun to reach youé We know the mass of the Sun and can use that to calculate the numberof protons within it. Let's assume for a second that all the Sun's protons are evenly spread out, making the average distance between themabout 1.0 x 10^10 meters. To random walk the 690,000 kilometersfrom the core to the solar surface
would then require 3.9 x 10^37 steps, giving a total travel time of 400 billion years. Hmm, that can't be right. The Sun is only 4.6 billion years old,so what went wrongé Two things: The Sun isn't actually of uniform density and photons will miss quite a few protonsbetween every collision. In actuality, a photon's energy,
which changes over the course of its journey, determines how likely it is to interact with a proton. On the density question, our models show that the Sunhas a hot core, where the fusion reactions occur. Surrounding that is the radiative zone, followed by the convective zone,which extends all the way to the surface. The material in the core is much denser than lead,