Considering Galaxy spectra as multidimensional vectors, we use the k-means algorithm and the minimum spanning tree to show that the data in the Sloan Digital Sky Survey do not fill the entire space but are confined to a manifold with very low dimensionality. More precisely, all the spectra can be described in terms of only two degrees of freedom: one discrete parameter labelling the Galaxy as "normal" or "active," and one continuous affine parameter describing its position along the corresponding sequence.