The only difference in terms of appearance among double-radio sources, quasars, and blazars may be: the angle between the line of sight and the line of the two oppositely directed jets of matter in the source. their age. Blazars are the youngest, showing no spectral line emission, quasars are of intermediate age, and double-radio sources are the oldest, the result of the dying out of the central source of the system. the mass of their central black holes. Double-radio sources have the lowest-mass black holes, quasars have the highest, and blazers have a mass in between. their distance from Earth. Double-radio sources are close enough to be resolved, quasars are sufficiently close to show some structure, whereas blazars always appear as point sources because of their extreme distance.