galaxy The distribution of stars in the cosmos is not random. A large number of stars (~ 10¹⁰) form an aggregate which are widely separated from one another. Such an aggregate is called a galaxy. Our galaxy in which the solar system is situated is called Galaxy (with capital g) or the milky way system. According to their shapes, galaxies have been classified into three types- irregular, elliptical and spiral. Our galaxy falls into third category. The shape our galaxy is shown in fig. g1, along with the location of solar system. It is disc shaped with central bulge called halo. The halo contains the oldest stars.

Only three galaxies are visible by the unaided eye from Earth. The nearest two galaxies are at ~ 75000 light years away. These are irregular galaxies called the large and small Magellanic Clouds. These galaxies are faintly visible. The third galaxy is bright, called the Andromeda Nebula M13, which is ~ 2´ 10⁶ light years away.

The galaxy themselves form clusters. Our galaxy belongs to a cluster of 19 members known as Local Group. Further the clusters group themselves into superclusters. It was previously believed that galaxies, clusters and superclusters are uniformly distributed in space. However new picture evolving since 1980’s is that clusters and galaxies are distributed on the surfaces of large empty bubbles. These have diameter of 20 to 50 Mparsec, one or two orders of magnitude larger than the thickness of the surface of the bubbles.