The Sun is the most important star from our point of view. It is the only star close enough to be studied in considerable detail. The grandeur of the Sun is partly because of its size. Its diameter is almost 110 times that of the Earth. We can best visualise the volume of the Sun if we realise that more than a million earths can be dropped into the space it occupies. As we have said above, the mass of the Sun is about 2 X 1033 grams, which makes it more than 300,000 times the mass of the Earth. The average density of the Sun, i.e.. its mass per unit volume, is about 1.4 g/cm3.
Although the Sun appears to be unchanging, it rotates about its axis once every 25 days. From time to time dark patches appear on the surface of the Sun, usually in pairsor in groups. These dark patches are called sunspots. Their movement is an indication of the Sun's rotation. This fact was recognised for the first tiine by Galileo. Actually, a sunspatis a regiim cn the surface of the S& that consists of gases almkt 10000c cooler than those surrounding the area. The number of sunspots increases and decreases in a cyclc every 11 years. In the long term, there are periods of low number of sunspots and High'number of sunspots.