ABSTRACT
Stars are not found in isolation but in huge clusters called galaxies. The Sun is part of the Milky Way, which is a barred spiral galaxy and contains more than 200 billion stars. The main features of the Milky Way are the central bulge, the disk, and the halo, as shown in Figure 1.2. The disk diameter is about 30 Kpc and disk thickness approximately equal to 1 Kpc. Surrounding the disk is the galactic halo, which extends much beyond the disk. The Sun is located at about 8.5 Kpc from the center. The density of stars is much higher in the disk in comparison to the halo. It is also very high toward the center of the galaxy, which is located in the constellation Sagittarius. Away from the city lights, one can observe the disk as a white band stretching across the entire sky. Nearly all the objects that are directly visible in the night sky lie within the Milky Way. The only exception is the Andromeda galaxy, the closest bright spiral galaxy, and the Large and Small Magellanic Clouds, which are irregular dwarf galaxies and can be seen from the southern hemisphere.
