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 stars form, some of which pick up matter that has already been processed by the first supernovae and mixed into the rest of the gas by the ubiquitous turbulence. As soon as a star has been formed, it becomes separated from the rough and tumble. Virtually un¬hampered by the gas, extremely unlikely to collide with another star, it follows a path dictated by the average gravitational field of the protogalaxy. Since this field changes rapidly in the course of time because the collapse is still proceeding, the star experiences a rapidly changing gravitational pull. This affects the star as if it were undergoing a huge collision: roughly speaking, one could say that the star collides with the whole protogalaxy! Consequently, this ‘collision’ flings the star into an orbit which mimics the average state of motion of the cloud. This process of VIOLENT RELAXATION smoothes the distribution of the stars over the system and causes the symmetrical shape of elliptical galaxies and the nuclear bulges of spiral galaxies.

Not all the gas in a protogalaxy forms stars: a certain amount is left behind, partly because some gas moves too chaotically to contract smoothly, partly because the radiation from the stars that happen to form at an early stage heats up the gas surrounding them. This heating, which increases the gas pressure and hampers contraction, occurs especially in the later stages of collapse, when the protogalaxy has become so dense that some radiation is inter¬cepted on its way out. Let us consider two extreme cases.

If the star formation is very efficient and almost complete before the protogalaxy enters its later stages of collapse, violent relaxation serves to give the galaxy a smooth shape: an elliptical galaxy is formed. If the galaxy happens to rotate, contraction is first stop¬ped hi the equatorial plane by the centrifugal acceleration. The contraction at right angles to the plane comes to a halt as soon as the random motions of the stars reach equilibrium with the average gravitational pull of the galaxy. The laws of mechanics show that such equilibrium is almost always reached before the thickness of the galaxy is less than a third its diameter, thereby explaining that no ellipticals are flatter than E7.

If, on the other hand, the stars do not form very efficiently, some gas will be left. This, as we have seen, behaves rather differently from the stars. Being collision dominated, it settles as a thin disc hi the equatorial plane of the galaxy. There, it is slowly used up in the formation of subsequent generations of stars, which form fitfully hi irregular galaxies and more or less regularly along the arms of spirals. It is not known what causes the differences in efficiency of the formation of the first generations of stars. Possibly the rotation of a galaxy plays a decisive role: it is to be expected that a rapidly rotating protogalaxy has a more violent internal turbulence than does a slow rotator. This interferes with star formation because centrifugal acceleration counteracts gravitation.

These two extreme cases illustrate the way in which the populations of a galaxy are established. The first generations of stars and the globular clusters form very early, so that they do not usually contain many elements heavier than helium, which are added to the interstellar gas by exploding stars. Also, violent relaxation freezes their orbits to mimic the shape the galaxy had when they were formed. Therefore, the extreme Population II objects are old, poor in heavy elements, and occupy the halo. At the other extreme, the population of the galactic plane is young and rich in heavy elements. Depending on their collapse history, galaxies contain various proportions of these populations and their intermediates.

A few hundred million years after the beginning of its contraction, a protogalaxy has been shaped by the forces of the collapse into a young galaxy. Probably faint wisps of infalling gas, the straggling remains of the outermost layers of the initial cloud, still continue to drizzle into the galaxy. Otherwise, a very quiet period begins: ten billion years without any action even remotely resembling the dramatic events during the first one per cent of that time.

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