4.5 The Sun's Energy

1. The photosphere.

Though the Sun is made up largely of hydrogen and helium, it also contains many of the same elements found on Earth. The extremely high temperatures on the Sun mean that all those elements are in gaseous form. Because of this, there is not really a solid surface on the Sun, but an atmosphere of varying density. As you move from the outer regions of the atmosphere toward the core, the layers of gas get more and more dense. The layer at which we can no longer see through the gases is known as the visible surface.

Though the photosphere is not actually a solid boundary, we often think of it as one. When we talk about the size of the sun, we generally refer to the region surrounded by the photosphere.

The temperature range on the photosphere is 4200–5800°C.

There are two transparent layers outside the photosphere known as the chromosphere and the corona.

The chromosphere is made up of the gases just outside the photosphere. It is about 2500 km thick. As you move away from the photosphere, the density of the chromosphere decreases; but, strangely enough, the temperature increases. The temperature of the chromosphere ranges from 4200°C near the photosphere, to 9700°C at the outer regions.

3. The corona

The outer regions of the chromosphere merge into the outermost layer of the Sun, called the corona. The corona extends millions of kilometres into space, and is extremely hot (millions of degrees). Most of the radiation from this region of the sun is in the form of x-rays and ultraviolet rays. The magnetic fields of the Sun play a role in the high temperatures of the corona, but the exact details remain a mystery.

The brightness of the photosphere usually blocks our view of the corona. One exception is during a solar eclipse, when the Moon blocks out the photosphere and allows us a peek at the magnificent corona.

For viewing at other times, there are special instruments that use a filter to block light from the photosphere so that the corona can be studied.