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 3'00 billion stars. There are three main types of galaxy, classified
according to their shape: elliptical, which are oval shaped; spiral, which have arms
spiraling outward from a central bulge; and irregular, which have no obvious shape.
Sometimes, the shape of a galaxy is changed by the collision with another galaxy. Quasars
(quasi-stellar objects) 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 farthest known "ordinary" galaxies are about 10
billion lightyears away, the farthest known quasar is about 15 billion lightyears away.
Active galaxies, such as Seyfert galaxies and radio galaxies, emit intense radiation. In a
Seyfert galaxy, this radiation comes from the galactic nucleus; in a radio galaxy, it also
comes from huge lobes on either side of the galaxy. The radiation from active galaxies and
quasars is thought to be caused by Black holes.