A GALAXY IS A HUGE MASS OF STARS, nebulae,and interstellar material. The smallest galaxies contain about 100,000 stars, while the largest contain up to 3,000 billion stars. There are three main types of galaxy, classified according to their shape: elliptical which are oval shaped; spiral, which have arms spiralling outwards from a central bulge(those whose arms spiral from a bar shaped bulge are called spirals); and irregular, which have no obvious shape.
Sometimes, the shape of a galaxy is distorted by a collision with another galaxy. Quasars (quasi-stellar object) are thought to be galactic nuclei but are so far away that their exact nature is still uncertain. They are compact, highly luminous objects in the outer reaches of the known Universe: while the furthest known "ordinary" galaxies are about 12 billion light years away. Active galaxies, such as Seyfert galaxies and radio galaxies, emit intense radiation.
In a Seyfert galaxy, this radiation comes from the galactic nuclear; in a radio galaxy, it also comes from huge lobes on either side of the galaxies and quasars is thought to be caused by material falling into central black holes
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