56 - Observed dynamics of galaxies

Summary

examine this region of short distances and definite places

W. H. Auden

History has shown that the Universe is complex. The more closely we examine astronomical systems the more branches of physical theory we need to understand their behavior and origin, and the more we must specialize the general theory of the last three parts to apply to particular systems. Passing through this book we have successively examined systems of decreasing symmetry. In Part II we dropped the homogeneity that dominated Part I. In Part III we dropped the infinite boundaries that dominated Part II. Now, in Part IV, we are about to drop the spherical symmetry that dominated Part III. As a result there is less to say of a general nature. To compensate, there arises a richness of detail which can often be compared with observations. Although such comparisons are not within the scope of this book, a sketch of some basic observed dynamical properties of non-spherical systems helps give a useful perspective.

In forming impressions of galactic structure we must be especially careful of selection effects. These influence what we observe in subtle ways. Rates of extragalactic information flow are slow, and the natural desire to get interesting results from long nights spent observing often subconsciously affects the choice of objects and places to examine. Versatile and sensitive detectors continue to improve this situation.

On a more fundamental level the brightness of the night sky limits our vision.