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 of masses and sizes of protogalaxies is very uncertain: guided by hindsight, we shall start with a 1011 M0 cloud with a radius of a few kiloparsecs. At the epoch we are now considering, the protogalaxies were still expanding, but theorists presume that some of them did so slightly more slowly than did the rest of the Universe. These ‘lagging’ clouds eventually stopped expanding and contracted by their own gravity. On top of this large-scale motion, it seems probable that the protogalaxies had some chaotic internal motions on smaller scales, but the properties of this turbulence are a major uncertainty.

At this stage, protogalaxies were extremely near neighbours: their average separation was little more than their average size. Consequently, the clouds must have interacted fairly strongly. The gravitational torques between protogalaxies passing close to each other may have set them rotating, over and above the spin they probably had as a consequence of their inner turbulence. Some clouds were so close together that they could not escape each other’s gravitational pull: they remained to form BINARY
GALAXIES.

The protogalaxy expands more and more slowly, until after some 20 million years its expansion stops and reverses to gravitational collapse. At this stage, our sample cloud measures about 80kpc across. Possibly some stars have already formed in the turbulent interior of the cloud, but since the average density has been going down to about 10-22kgm-3 during this stage, there probably were not many. As soon as the protogalaxy starts to collapse, however, the increasing density makes it easier for stars to form . During the initial stages of collapse, the cloud is so tenuous that radiation can escape freely from it. Thus it is possible for smaller condensations to form and radiate away their excess energy into the space between the protogalaxies. These subcondensations are supposed to have formed the first generation of stars in the galaxy. Some of them were probably very massive (more than a hundred solar masses) and rapidly evolved to explode as super-novae. Others somehow divided the available mass among many small stars, thereby forming globular dusters.

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