Observations of interacting galaxies. (a) A pair of galaxies, NGC 4038 and 4039, which have an appearance similar to the simulation. Because of their appearance, these are called 'The Antennae'. (b) HST image of the nucleus of the elliptical galaxy NGC l3l6.There is an unusually large number of bright young clusters in this image, suggesting that they were formed in some encounter with another galaxy. [STScl/NASA]

where v is the speed of the galaxy, d is the distance, and H0 is a constant, called the Hubble constant. (The subscript zero on the H indicates that this is the current value. As we will see in Chapter 20, H is constant in the sense that it is the same at every place, but can change with time.) The relationship given by equation (18.2) is called Hubble's law. Results of more recent studies are shown in Fig. 18.9.

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