Galaxies

A Galaxy is a huge mass of stars ,nebulae,and interstellar material.The smallest galaxy contain about 100,000 stars,While a largest contain about 3,000 billion of stars ..There are three Main Type of galaxy,classified according to their shape:Ellipticat which are oval shaped;Spiral Which have arms spiralling outwards from a central buldge(those whose arms are spiral from a bar shaped bulge are called spiral);and irregular,which has no obivious shape.Some times the shape of the galaxy is distorted by the collision with another galaxies.Quasars(Quasistellar objects)Are thought to be galactic neuclei ,but they are too far away that thier exact natue is still uncertain.They are compact,highly luminiuos objects in the outer reaches of know universe:While the furthest known "odinary" galaxies are about 12 billion light years away,the furthest known quasar is about 13 billion light years away.Active galaxies,such as seyfert galaxies and radio galaxies ,Emit untense radiation.In Seyfert galaxy,this radiation come from the galactical neucleus ; In a radio galaxy,it comes from huge lobes on either side of the galaxy.The radiation from active galaxies and quasars is thought to be caused by material falling into central black holes.