Have you ever seen photographs of a total eclipse of the sun?
All around the dark sun, there is an uneven glow of light, and this light is called the corona.



To understand what the corona is, we have to know some things about the sun like the earth. This surface, which is all that we can observe of the sun, is composed of gases.
In fact, the sun is surrounded by four layers of gaseous matter that hide what is underneath. The innermost of these layers is called the photosphere. The next two layer are known as the reversing layer and the chromosphere. Together they form the sun's atmosphere. The outermost layer is the corona.


Let's see what each of these layers of gas is. The photosphere (or “light sphere”) is what we see when we look toward the sun.
Most of the time, dark sunspot can be seen on this bright surface. The “reversing layer”, which is made of gaseous vapours, extends several hundred miles out from the photosphere of the sun. This layer is never seen, but it can be studied by an instrument called the spectro-graph.


Outside the reversing layer is the chromosphere, or colour shper. It is about nine thousand miles thick and is made up mostly is hydrogen and helium gases. At the time of a total eclipse, it shines out around the dark disc with a brilliant Scarlet light. From this red border, flame-coloured clouds of the same material shoot out to great heights, some-times even as much as one million miles! They are called prominences, and they look great flame of fire.


Then comes the outer layer, which is called the corona. It is composed of light, gaseous matter, and has two parts. The inner corona, lying next to the red chromosphere, is a band of pale yellow. The outer corona is white, with streamers extending out of miles from the edge of the sun.
All of this describes only the layers that surround the sun.
What is beneath them still a mystery!

Source: Xtremescience.