From the July 2010 issue

I heard that the light from nuclear fusion reactions in the Sun’s core takes about 200,000 years to reach its surface. I thought the speed of light is constant. Why does it take so long?

Tom Foltz, Lansing, Michigan
By | Published: July 26, 2010
September 2010 nuclear reactions
Nuclear reactions in the Sun’s core produce light and other types of radiation. As light photons leave the core, they run into electrons and atomic nuclei, scattering off each one. These interactions cause photons to take, on average, 200,000 years to move from the Sun’s core to its surface.
Astronomy: Roen Kelly

The speed of light is constant, but the problem is that the light (or more generally, radiation) doesn’t travel directly from the Sun’s center to the surface. The light that originates at the Sun’s core is scattered many times — sometimes toward the surface, sometimes back toward the core, sometimes sideways — before finally arriving at the surface and escaping into space.