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 […]
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 […]
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 […]
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 […]
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) […]
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 […]
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 […]
The tightness of winding is the feature that is most commonly used to characterize the overall shape of a spiral galaxy. Because the pitch angle is not always the same at every distance from the nucleus, spiral types are not simply distinguished by a number giving that angle. Instead they are indicated by S followed […]
Galaxies display a great variety of appearances and in studying this, diversity, it is difficult to distinguish the typical from the incidental The contents of a galaxy do not readily change, because of the slowness of evolution of the low-mass dwarf stars that make up most of a galaxy’s mass. But what about the evolution […]
Short-exposure photographs of the central regions of galaxies usually show a bright spot which is star-like in appearance, or just resolved (one second of arc diameter which, although frequently absent in some small galaxies, is a characteristic feature of the larger ones. The angular diameter of one second corresponds to a linear size of 3.3pc […]