The Photosphere

The layer of the Sun that we see is the photosphere ("light-sphere"). It is bright and yellow. It looks solid, but it is made of hydrogen and helium gas. Here is a picture, made with ordinary white-light film.

The spots are sunspots. We will discuss them shortly.

The photosphere looks like a sharp surface, but really there the gas is just denser lower in the Sun, less dense higher up. The less dense it is, the more transparent it is. The surface we see is where the gas has become reasonably transparent. This layer is some 300 km thick.

The picture illustrates this. Imagine three of the yellow atoms emitting light. In order for a photon to get out of the Sun, it must be going up and it must avoid being absorbed by another atom. Thus the atom that emits it must be in the less dense part of the diagram.

In a closeup view, the photosphere has a mottled appearance called granulation.

The granulation provides evidence of convection on the Sun. If you heat a gas from the bottom, the warm gas expands and rises. This transports energy from the lower regions to the upper regions. In this case, the gas can radiate into space when it reaches the photosphere, This cools it and it sinks.

The convection cells on the Sun that make the granulation in the photosphere are roughly 1000 km in diameter.

Roughly the same thing happens on Earth.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA