Hubble sequence
Another criticism of the Hubble classification scheme is that, being based on the appearance of a galaxy in a two-dimensional image, the classes are only indirectly related to the true physical properties of galaxies. In particular, problems arise because of orientation effects. The same galaxy would look very different, if viewed edge-on, as opposed to a face-on or ‘broadside’ viewpoint. As such, the early-type sequence is poorly represented: the ES galaxies are missing from the Hubble sequence, and the E5–E7 galaxies are actually S0 galaxies. Furthermore, the barred ES and barred S0 galaxies are also absent.