the displacement of the progenitor WR star from its geometric centre (0'.6). This hypothesis is further strengthened from the observed values of vi Ηα/ [ΝΙΙ] and Ha/[SII] intensity ratios which, in the area of the inner ring, deviate from those of a typical HII region. Chapter 4: The Dumbbell (NGC 6853) Planetary Nebula. The core of the Dumbbell planetary nebula has been studied by the analysis of observations taken in the [OUI] 5007 A 0 line by a 169-fibre array converting the shapes of the line-emission from the various parts of the core imaged in the focal plane of the 4.2-m William Herschel telescope (WHT), into three long entrance slits for the Manchester échelle spectrometer (MES). The construction and performance of the array with 169 fibres feeding three parallel slits, is described. The analysis of the observational data reveals the presence of four separate velocity components in the [OUI] 5007 A 0 profiles. This leads to the detection of an inner, highly ionized shell, expanding radially at 11 km/sec and contained within the known outer one which expands at 30 km/sec. Chapter 5: Active Galaxies Various types of active galactic nuclei (AGN) are described. Particular emphasis is given on the Seyfert galaxies. Their morphology, physical parameters, narrow line region (NLR), broad line region (BLR) and variability are presented in detail. Relevant empirical models partially explaining their behaviour are also included. Chapter 6: Seyfert Galaxy NGC 4151. The Seyfert galaxy NGC 4151 is one of the nearest Seyfert galaxies. Its active nucleus is kwown to exhibit variability over a range of timescales and across the entire wavelength domain. The morphology of the galaxy and its nucleus, its kinematical structure and its line and continuum variability based on all known observations by various workers are analytically vii presented. Chapter 7: Photometric observations of NGC 4151.
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