Interactions Between Galaxies (The Nature of Normal Galaxies)

It we were to be really pedantic we could assert that a normal galaxy is an abstraction: all galaxies, when inspected sufficiently closely, deviate from the norm. Most of the peculiarities are minor, but there are quite a few puzzling ones. An important class of PECULIAR GALAXIES are those which were originally normal but have […]

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A Galaxy’s First Hundred Million Years (The Nature of Normal Galaxies)

The Initial stages of the collapse of protogalaxies were taking place when the Universe was a few hundred million years old. The Cosmological redshift corresponding to this epoch is about 10.Considering that the largest redshift observed to date is about four, it doubtful that galaxy formation will be amenable to direct observation in the near […]

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Galaxy Formation (The Nature of Normal Galaxies)

The collapse now proceeds faster and faster. The central parts of the protogalaxy contract more rapidly than do the outskirts; the formation of a nuclear region is thus started. In many cases the nuclear region may have started to contract even before the outer parts of the cloud have expanded to their maximal extent. More […]

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Protogalaxies (The Nature of Normal Galaxies)

It is not known how the Universe came to develop regions of above average density. or perturbations, which eventually formed these PROTOGALAXIES, but the end product of the initial million years of cosmic evolution seems reasonably certain: a hydrogen-helium cloud with a temperature of a few thousand kelvin, and a density around 10-19kgm-3. The range […]

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Young Galaxies (The Nature of Normal Galaxies)

Possibly the most fascinating aspect of astronomy is its intimate connection with origins. As we have seen, the mass distribution due to the stars in a galaxy has remained virtually unchanged since the time the galaxy formed. Hence, the structure of galaxies should give us some clues about their origin. The formation of our Galaxy […]

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Interpretation of Spiral Galaxies (The Nature of Normal Galaxies)

Since most of the mass of a spiral galaxy is in the form of old (Population II) stars, let us consider the stellar component first. As in the case of ellipticals, the stars form a collisionless system, so .that we expect the mass distribution to change extremely slowly. The evolution time turns out to be […]

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Dynamics of Spiral Galaxies (The Nature of Normal Galaxies)

Spiral and irregular galaxies look so much more complicated than do ellipticals, that it is reasonable to expect their dynamics to be vastly more complex too. Consider firstly the motions of the stars Since spirals are so highly flattened, the method of velocity dispersion can fruitfully be used only in the nuclei and central bulges […]

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Interpretation of Elliptical Galaxies (The Nature of Normal Galaxies)

We have outlined the contents and dynamics of elliptical galaxies, and we now interpret these observations by a sequence of theoretical models. We have to explain how a cloud of stars can be made and maintained so that: (i) it has a density which is highest in the centre and which smoothly decreases outwards; (ii) […]

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The Large-Scale Dynamics of Galaxies (The Nature of Normal Galaxies)

So far, we have limited ourselves to a consideration of the outward appearance of galaxies, somewhat like a taxonomist classifying the shapes of plants and animals. A taxonomist knows that his subjects are part of a large chain of evolution, allowing the recognition of order in the living diversity; also, he probably recognizes the function […]

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Lenticular, Irregular And Barred Spiral Galaxies (The Nature of Normal Galaxies)

Since there is a correlation between the gas content of a galaxy and its spiral type, it is natural to ask whether there are any spiral galaxies with either no gas at all, or at any rate less than the few per cent contained in Sa types, or else with more gas than the Sd […]

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