ABSTRACT
The discussion of strong radio sources in Chapter 8 was almost entirely concerned with sources lying within the Galaxy — the clouds of hot hydrogen situated along the galactic plane, radio emission from the direction of the galactic centre, supernova remnants and the 21-centimetre line of neutral hydrogen. We must now extend the discussion to include a class of object which has provided astronomers and physicists with so many puzzling problems that we still know very little of its true nature and even the interpretation of the present observations is open to controversy. These objects are the quasistellar radio sources, often termed the quasars.
