Editing Hubble sequence (section)

===Lenticulars===
[[Image:Ngc5866 hst big rotated.jpg|thumb|The [[NGC 5866|Spindle Galaxy]] (NGC 5866), a lenticular galaxy with a prominent [[dust lane]] in the [[Draco (constellation)|constellation of Draco]].]]
{{see also|Lenticular galaxy}}
At the centre of the Hubble tuning fork, where the two spiral-galaxy branches and the elliptical branch join, lies an intermediate class of galaxies known as [[lenticular galaxy|lenticulars]] and given the symbol S0. These galaxies consist of a bright central [[bulge (astronomy)|bulge]], similar in appearance to an [[elliptical galaxy]], surrounded by an extended, [[galaxy disk|disk]]-like structure. Unlike [[spiral galaxy|spiral galaxies]], the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.

When simply looking at a galaxy's image, lenticular galaxies with relatively face-on disks are difficult to distinguish from ellipticals of type E0–E3, making the classification of many such galaxies uncertain. When viewed edge-on, the disk becomes more apparent and prominent dust-lanes are sometimes visible in [[absorption (electromagnetic radiation)|absorption]] at optical wavelengths.

At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly flattened "ellipticals" and spirals. Later [[observational astronomy|observations]] (by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by [[Allan Sandage]].<ref name="sandage">{{cite conference |first = A. |last = Sandage |author-link = Allan Sandage |year = 1975 |title = Classification and stellar content of galaxies obtained from direct photography |book-title = Galaxies and the Universe |editor1 = Sandage, A. |editor2 = Sandage, M. |editor3 = Kristian, J. |url = http://nedwww.ipac.caltech.edu/level5/Sandage/frames.html |access-date = 2007-11-20}}</ref> Missing from the Hubble sequence are the early-type galaxies with intermediate-scale disks, in between the E0 and S0 types, Martha Liller denoted them '''ES''' galaxies in 1966.

Lenticular and spiral galaxies, taken together, are often referred to as [[disk galaxy|disk galaxies]].  The bulge-to-disk flux ratio in lenticular galaxies can take on a range of values, just as it does for each of the spiral galaxy morphological types (Sa, Sb, etc.).<ref>{{cite journal |last=Graham |first=A. |author2=Worley, C. |date=August 2008 |title=Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity relations |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=388 |issue=4 |pages=1708–1728 |bibcode=2008MNRAS.388.1708G |doi=10.1111/j.1365-2966.2008.13506.x |doi-access=free |arxiv = 0805.3565 |s2cid=14406950 }}</ref>

Examples of lenticular galaxies: [[Messier 85|M85]], [[Messier 86|M86]], [[NGC 1316]], [[NGC 2787]], [[NGC 5866]], [[Centaurus A]].