期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1977
卷号：74
期号：5
页码：1767-1774
DOI：10.1073/pnas.74.5.1767
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：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 [lsim] 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 {gtrsim} 103 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.
关键词：cosmology ; history of astronomy ; quasars ; space
