I shall skip straight through the historical part in two leaps:964 A.D.: Abd-al-rahman al Sufi is the first to record on a map the dim light which, a millenium later, will open up to us the extragalactic universe.1667: An engraving appearing in a paper by Bouillaud relates what was then known about that same Andromeda nebula after the introduction of the first telescopes (Fig. 1).1923: The extragalactic world is reached: Hubble recognizes the true nature of Messier 31.