Galaxies structural units of the cosmos

So we are not even at the hub of our own star system The disk itself is only a few hundred light-years thick, if we exclude the central bulge and halo, a spherical region filled with ageing stars. 

The mean distance between any two stars is of the order of a few light-years. Even the nearest stars are so far away that we perceive them as mere points of light. Amongst all the stars, the Sun located just 8 light-minutes away is the only one close enough to appear as a disk. It is an incandescent sphere of gas 

Immense clouds of cold and rarefied gases float between and around the stars. These clouds are mainly composed of hydrogen, the most widespread element in nature. Stars form there and contribute their ashes to them when they reach the end of their evolution (Fig. 6.1). 

The flattening of the Galaxy suggests that it is rotating about an axis perpendicular to the disk, and this is indeed confirmed by direct observation of the large-scale motions of the stars. In fact, they orbit about the galactic centre, making a complete revolution every 200 million years. 

Apart from rotating about their axes, the galaxies display systematic motions relative to one another. In fact, they are moving apart at speeds proportional to the distance between them. The recession speed amounts to some 100 km s for every 3 million light-years of separation. This overall motion is the clearest evidence we have for the expansion of the Universe. 

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