During a total eclipse of the Sun, when the photosphere is covered by the moon, the chromosphere becomes briefly visible as a thin red rim of light. Thin and faint as the chromosphere is, in one way it is the most important layer of the Sun or any other star. As light from the photosphere shines through the chromosphere, the relatively cool gases of the chromosphere absorb some of the light. The absorption of light by atoms in the chromosphere makes spectroscopy possible and enables astronomers to determine the chemical composition of the Sun and other stars. Although very thin, the coronal gases are also very hot, over 1,000,000 degrees.
Nobody is quite sure yet why the corona is so hot.
Acceleration of particles by the Sun's magnetic field, and compression of the coronal gases by shock waves from disturbances in the photosphere may play a role in heating the corona. The Sun's visible surface shows a variety of features. Most conspicuous are the sunspots , great dark disturbances which can be many times larger than the earth. Observations of sunspots from day to day show that the Sun rotates. The Sun is gaseous, not solid, and does not rotate uniformly.