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.
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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. This 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 10 12 solar luminosities, but most of them are much fainter - the smallest ones found are about 105 L 0. Since galaxies do not have a sharp outer edge, 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 M0 and a radius of 30 kpc, and dwarf, correspondingly, 107 M0 and 0.5 kpc. 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 clear unified picture of it has yet been obtained. In the following, the most important observed properties of galaxies will be presented. Their explanation remains a task for a future theory of galactic evolution.
19.1 The Classification of Galaxies As a first step towards a theory of galaxies, a classification based on their various forms is useful. Although such a classification must always be to some extent subjective, it provides a framework within which the quantitative properties of galaxies can be studied in a systematic fashion. However, it should be remembered that the picture thus obtained will be limited to those galaxies that are large and bright enough to be easily visible in the sky. An idea of the consequent limitations can be obtained from Fig. 19.1, showing the radii and magnitudes of normal galaxies. One sees that only within a narrow region of this diagram can galaxies be easily found. If a galaxy has too large a radius for its magnitude (small surface brightness), it disappears in the background light from the sky. On the other hand, if its radius is too small, it looks like a star and is not noticed on a photographic plate. In the following, we shall mainly be concerned with bright galaxies that fit within these limits. If a classification is to be useful, it should at least roughly correspond to some important physical properties of the galaxies. Most classifications accord in their main features with the one put forward by Edwin Hubble in 1926. Hubble's own version of
H. Karttunen et al. (eds.), Fundamental Astronomy © Springer-Verlag Berlin Heidelberg 1994
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19. Galaxies Fig. 19.1. Magnitudes and diameters of observable extragalactic objects. Objects to the upper left
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