Shortcomings
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). The different classification criteria can also be at odds with each other: for example, a more dominant bulge component does not always go hand-in-hand with more loosely-wound spiral arms. 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 looks very different when viewed edge-on, as opposed to face-on), because visual classifications are less reliable for faint or distant galaxies, and because the appearance of galaxies changes depending on the wavelength of light in which they are observed. Nevertheless, 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.
Read more about this topic: Hubble Sequence
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