<P> A common criticism of the Hubble scheme is that the criteria for assigning galaxies to classes are subjective, leading to different observers assigning galaxies to different classes (although experienced observers usually agree to within less than a single Hubble type). Although not really a short - coming, since the 1961 Hubble Atlas of Galaxies, the primary criteria used to assign the morphological type (a, b, c, etc .) has been the nature of the spiral arms, rather than the bulge - to - disk flux ratio, and thus a range of flux ratios exist for each morphological type, as with the lenticular galaxies . </P> <P> 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 . </P> <P> Visual classifications are also less reliable for faint or distant galaxies, and the appearance of galaxies can change depending on the wavelength of light in which they are observed . </P> <P> Nonetheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates . </P>

Where does the milky way fit on the hubble diagram