I8 Galaxies

The galaxies are the fundamental building blocks of the

Universe. Some of them are very simple in structure, containing only normal stars and showing no particular individual features. There are also galaxies that are almost entirely made of neutral gas. On the other hand, others are complex systems, built up from many separate components — stars, neutral and ionized gas, dust, molecular clouds, magnetic fields, cosmic rays____The galaxies may form small groups or large clusters in space. At the centre of many galaxies, there is a compact nucleus that may sometimes be so bright that it overwhelms all the normal radiation of the galaxy.

The luminosity of the brightest normal galaxies may correspond to 1012 solar luminosities, but most of them are much fainter — the smallest ones that have been discovered are about 105 L®. Since galaxies do not have a sharp outer edge, to some extent their masses and radii depend on how these quantities are defined. If only the bright central parts are included, a giant galaxy may typically have a mass of about 1013 M®, and a radius of 30 kpc, and a dwarf, correspondingly, 107 M®, and 0.5 kpc. In addition, it seems that the outer parts of most galaxies contain large quantities of non-luminous matter that might increase galaxy masses by an order of magnitude.

The density of matter may be very different in different galaxies and in different parts of the same galaxy. Thus the evolution of a galaxy will be the result of processes occurring on vastly different time and energy scales, and no generally accepted comprehensive picture of it exists as yet. In the following, the most important observed properties of galaxies will be presented. Many of them still await an explanation in current theories of galaxy evolution.

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