The interstellar medium is filled with gas and dust. In some regions the density of gas and dust is much higher than the mean value in the medium. Stars are formed due to the collapse of such clouds or regions of high density. The temperature of these clouds is normally quite small and hence they do not emit any visible radiation. However, they are often illuminated by light due to stars in their neighborhood. Such illuminated clouds are called nebula. Due to their high density, they cause considerable extinction of star light. Hence the density of stars appears much reduced in the direction of these clouds.