Photosphere as a Function of Depth
Depth (km) % Light from this Depth Temperature (K) Pressure (bars)
0 99.5 4465 6.8 x 10-3
100 97 4780 1.7 x 10-2
200 89 5180 3.9 x 10-2
300 64 5840 8.3 x 10-2
350 37 6420 1.2 x 10-1
400 4 7610 1.6 x 10-1

The Photosphere

The Sun is a ball of gas, so it does not have a well-defined surface. When we speak of the surface of the Sun, we normally mean the photosphere.
Definition of the Photosphere
As we look down into the atmosphere at the surface of the Sun, the view becomes more and more opaque. The radius where it appears to become completely opaque defines an imaginary sphere called the photosphere, which may be thought of as the surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun or any other star usually refers to the diameter of the photosphere.

The photosphere may be viewed as the region of the Sun that is thick enough to produce substantial light, but thin enough to let it escape rapidly. The layers below the photosphere produce a lot of light but they are too thick to allow it to escape rapidly, and the layers above are thin enough to allow light to escape, but too thin to produce much light.

Negative Hydrogen Ions

Negative hydrogen ions (H-) are the primary reason for the highly opaque nature of the photosphere. These ions, which are hydrogen atoms that have gained an electron, are very efficient at interacting with photons. They are found in sufficient abundance in the lower and middle photosphere to cause strong absorption of light and thus prevent us seeing very deeply into the photosphere.

Properties of the Photosphere
The above table shows some properties of the photosphere. This table indicates that the solar atmosphere changes from being almost completely transparent to being almost opaque over a distance of about 400 km. Notice also that in this region the temperature drops rapidly as we near the surface, and that the pressure (measured in bars, where one bar is the average atmospheric pressure at the surface of the Earth) is very low--generally one percent or less of Earth surface atmospheric pressure.