In his studies Hubble realized immediately that not all spiral galaxies have the same appearance.
Furthermore, he found galaxies that do not have a spiral structure. Hubble classified the galaxies he studied according to their basic appearance. It was originally thought that the different types of galaxies represented different stages of galactic evolution. (Similarly, some astronomers thought that different spectral type stars along the main sequence were evolutionary states of the same star.) We now know that this is not the case. However, Hubble's classification scheme, depicted in Fig. 17.1, is still quite useful.
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