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.