ON THE NATURE OF VARIABILITY OF EXTRAGALACTIC RADIO-SOURCES - FUNDAMENTAL IDEAS BY SHKLOVSKY,I.S

The majority (maybe all) of proposed relativistic and non-relativistic models for variable extragalactic radio sources are based on the same general ideas proposed in the early model by Shklovsky (1960, 1965) - in particular, that the variability is caused by the synchrotron emis sion of magnetized clouds during their expansion. This model was a ''s tandard'' or ''canonical'' model for more than 20 years. One of the mo dels that uses its ideas and approach is the ''hedgehog'' model, in wh ich clouds or jets emit and expand in the strong external radial magne tic field of the nucleus of the source. This model has been suggested for a long time, but recent new results comparing it's predictions wit h observations of multifrequency spectra - especially, instantaneous s pectra of many variable sources - and with the typical VLBI structure of compact objects have given strong new arguments in it's favour. The nature of the variability can be satisfactory explained by the hedgeh og jet model for many variable and compact extragalactic radio sources . Thus, following up the ideas of I.S. Shklovsky, we conclude that the observed spectra and structure of compact sources can be caused by th e synchrotron emission of a quasi-stationary jet in a longitudinal mag netic field, and the nature of variability can be explained by the var iable ejection of the continous jet from the nucleus.