Outstanding problems and instrumental prospects for the coming decade (A Review)

After a review of the discovery of external galaxies and the early classification of these enormous aggregates of stars into visually recognizable types, a new classification scheme is suggested based on a measurable physical quantity, the luminosity of the spheroidal component. It is argued that the new one-parameter scheme may correlate well both with existing descriptive labels and with underlying physical reality.Two particular problems in extragalactic research are isolated as currently most fundamental. (i) A significant fraction of the energy emitted by active galaxies (approximately 1% of all galaxies) is emitted by very small central regions largely in parts of the spectrum (microwave, infrared, ultraviolet and x-ray wavelengths) that were previously inaccessible to observation. The physical processes by which regions with less, similar 10(-9) the volume of the luminous stellar parts of galaxies produce such enormous quantities of energy are currently the subject of much speculative debate. (ii) It appears that most of the mass of ordinary galaxies resides far from the central luminous region, with the volume containing most of this mass greater, similar 10(3) times the volume containing most of the light-emitting stars; the nature, amount, and extent of this mass are quite unknown.New instruments that will be operating in the next decade and that may be helpful in solving these two problems are briefly mentioned with particular emphasis on the advances expected in angular resolution at wavelengths for which picture-taking ability has historically been poor or nonexistent.