Types of Galaxy

Galaxies are classified on the basis of their shape. There are two main types of normal galaxy. These are the spiral and elliptical galaxies and are distinguished according to their shape. Seyfert Galaxies are also a type of spiral galaxy which is much more luminous than normal galaxies. Other galaxies belong to the irregular type. Images of all these galaxy types can be viewed at http://rst.gsfc.nasa.gov/Sect20/A2.html and at http://www.aao.gov.au/images.html/general/galaxy_frames.html.

Radiogalaxies form a separate group of galaxies identified by their emission of thermal and radio radiation.

Quasars (quasi-stellar objects) are very bright objects which only exist at immense distances. They are thought to be the cores of extremely distant, young galaxies, that emit huge amounts of energy, but which now are extinct. They therefore represent a process which took place early in the history of the Universe but, which has now ceased. Further details of quasars and their images may be found at http://apod.gsfc.nasa.gov/apod/ap971206.html.

There is thought to be an evolutionary sequence to galaxies which has the pattern:

Quasars (oldest) → Radiogalaxies → Normal galaxies (youngest)

(i.e. decreasing redshift from oldest to youngest)