Editing Hubble sequence (section)

==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).<ref name=dressler_et_al>{{cite journal |last1=Dressler |first1=A. |last2=Oemler, Jr. |first2=A. |last3=Butcher |first3=H.R. |last4=Gunn |first4=J.E. |date=July 1994 |title=The morphology of distant cluster galaxies. 1: HST observations of CL&nbsp;0939+4713 |journal=[[The Astrophysical Journal]] |volume=430 |issue=1 |pages=107–120 |bibcode=1994ApJ...430..107D | doi = 10.1086/174386}}</ref><ref>{{Cite journal |last1=Lahav |first1=O. |last2=Naim |first2=A. |last3=Buta |first3=R. J. |last4=Corwin |first4=H. G. |last5=de Vaucouleurs |first5=G. |last6=Dressler |first6=A. |last7=Huchra |first7=J. P. |last8=van den Bergh |first8=S. |last9=Raychaudhury |first9=S. |last10=Sodré |first10=L. |last11=Storrie-Lombardi |first11=M. C. |date=1995-02-10 |title=Galaxies, Human Eyes, and Artificial Neural Networks |journal=Science |language=en |volume=267 |issue=5199 |pages=859–862 |doi=10.1126/science.267.5199.859 |pmid=17813914 |arxiv=astro-ph/9412027 |bibcode=1995Sci...267..859L |issn=0036-8075}}</ref> Although not really a shortcoming, since the 1961 ''Hubble Atlas of Galaxies'',<ref name=Sandage-1961/> 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,<ref name=Sandage-1961>{{cite book |first=Allan |last=Sandage |author-link=Allan Sandage |year=1961 |title=The Hubble Atlas of Galaxies |publisher=Carnegie Institution of Washington |location=Washington |oclc=537465 }}</ref><ref>{{cite journal |bibcode=1962MCG...C01....0V |title=Morphological catalogue of galaxies. Part 1 |last1=Vorontsov-Vel'Yaminov |first1=B. A. |last2=Arkhipova |first2=V. P. |journal=Morphological Catalogue of Galaxies |date=1962 |volume=C01 }}</ref><ref>{{cite journal |bibcode=1973SvA....16..824V |title=Photometric Parameters of Flat Galaxies |last1=Vorontsov-Vel'Yaminov |first1=B. A. |last2=Noskova |first2=R. I. |journal=Soviet Astronomy |date=1973 |volume=16 |page=824 }}</ref> as with the lenticular galaxies.

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 (geometry)|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.
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.

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 [[Luminosity|luminosities]], colours, masses (of stars and gas) and star formation rates.<ref name=roberts_haynes>{{cite journal |last1=Roberts |first1=M.S. |last2=Haynes |first2=M.P. |year=1994 |title=Physical Parameters along the Hubble Sequence |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=32 |issue=1 |pages=115–152 |bibcode=1994ARA&A..32..115R |doi=10.1146/annurev.aa.32.090194.000555}}</ref>

In June&nbsp;2019, [[citizen scientist]]s in the [[Galaxy Zoo]] project argued that the usual Hubble classification, particularly concerning [[spiral galaxies]], may not be supported by evidence. Consequently, the scheme may need revision.<ref name=EA-20190611>{{cite press release |publisher=[[Royal Astronomical Society]] |title=Citizen scientists re-tune Hubble's galaxy classification |date=11 June 2019 |website=[[EurekAlert!]] |url=https://www.eurekalert.org/pub_releases/2019-06/ras-csr061119.php |access-date=11 June 2019 |archive-date=11 June 2019 |archive-url=https://web.archive.org/web/20190611165824/https://www.eurekalert.org/pub_releases/2019-06/ras-csr061119.php |url-status=dead }}</ref><ref name=MNRAS-20190430>{{cite journal |author=Masters, Karen L. |display-authors=et al. |date=30 April 2019 |title=Galaxy Zoo: Unwinding the winding problem – observations of spiral bulge prominence and arm pitch angles suggest local spiral galaxies are winding |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=487 |issue=2 |pages=1808–1820 |doi=10.1093/mnras/stz1153 |doi-access=free |arxiv=1904.11436 |bibcode=2019MNRAS.487.1808M }}</ref>