The Sun is the nearest star to the Earth and is responsible for the fact that life dominates the surface of our planet. Although the Sun is so brilliant that we cannot look at it directly, it is an ordinary star in terms of its mass, size and temperature. There are countless stars like the Sun in our galaxy alone. The Sun is a ball of hot gas with a diameter of 1.4 million km and it consists mainly of Hydrogen and Helium. The temperature at its core is so high that nuclear fusion is possible. As a result Hydrogen is constantly transformed into Helium with the simultaneous production of energy. This energy is carried to the "surface" of the Sun which is called photosphere and is emitted as visible and infrared light.
At visible wavelengths we are not able to see below the photosphere. However there are many interesting features that can be observed in the photosphere. The most interesting are called sunspots and the first person who noticed them was the Italian astronomer Galileo. We can also observe granulation which is a result of convection and other formations like faculae and pores. Finally, an interesting phenomenon that can be observed in the photosphere is called limb darkening. The photosphere is 500 km thick and is actually the lowest of the three layers that constitute the Sun's atmosphere.
The next layer is the chromosphere which is 2000 km thick and its spectrum is dominated by emission lines. The stronger line is the H-alpha which is responsible for the pinkish color of the chromosphere. If we use a H-alpha filter we will be able to see many interesting features in the chromosphere. Prominences are formed above active sunspot regions and are spectacular. Flares are the most violent eruptive events on the Sun and like prominences they are associated with active regions. Other formations in the chromosphere include filaments, plages, fibrils and spicules.
The outermost layer of the atmosphere of the Sun is the corona. This layer is large (millions of km thick), extremely rarefied, and extremely hot. It can be observed during a total solar eclipse or by using a coronagraph. Corona is very bright in X-rays because of its high temperature and an interesting feature that is observed there is coronal holes which are regions devoid of coronal gas.
The Sun is the only star available for detailed observation and by studying it we can learn about the physical procedures taking place in other stars.
Text and sun photo by the author