Editing Lenticular galaxy (section)

===Classification===
[[File:NGC 2787.jpg|thumb|left|[[NGC 2787]] is an example of a lenticular galaxy with visible dust absorption.  While this galaxy has been classified as an S0 galaxy, one can see the difficulty in differentiating among spirals, ellipticals, and lenticulars.  Credit: [[Hubble Space Telescope|HST]]]]
[[File:NGC1387 - hst 10217R850GB475.png|thumb|left|[[NGC 1387]] has a large nuclear ring. This galaxy is a member of the [[Fornax Cluster]].]]
[[File:Hubble-Grid.jpg|thumb|300px|Grid showing the location of early-type galaxies (including the lenticular S0 galaxies) relative to the late-type spiral galaxies. The horizontal axis shows the morphological type, primarily dictated by the nature of the spiral arms.]]
[[File:wikifigure.png|thumb|300px|The percentage of galaxies with a particular axis ratio (minor/major) for a sample of lenticular and spiral galaxies. The inset is a visual representation of the profile of either at the specified minor (b) to major (a) axis ratios.<ref name=bmbook />]]
Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component. They have much higher bulge-to-disk ratios than typical spirals and do not have the canonical spiral arm structure of late-type<ref group=note name=latevsearly>Galaxies to the left side of the Hubble classification scheme are sometimes referred to as "early-type", while those to the right are "late-type".</ref> galaxies, yet may exhibit a central bar.<ref name=bmbook /> This bulge dominance can be seen in the axis ratio (i.e. the ratio between the observed minor and major axial of a disk galaxy) distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises steadily in the range 0.25 to 0.85 whereas the distribution for spirals is essentially flat in that same range.<ref name="Shapes">{{cite journal|last=Lambas|first=D.G.|author2=S.J.Maddox and J. Loveday |journal=MNRAS|date=1992|volume=258|issue=2|pages=404–414|bibcode = 1992MNRAS.258..404L|title=On the true shapes of galaxies|doi=10.1093/mnras/258.2.404|doi-access=free}}</ref> Larger axial ratios can be explained by observing face-on disk galaxies ''or'' by having a sample of spheroidal (bulge-dominated) galaxies. Imagine looking at two disk galaxies edge-on, one with a bulge and one without a bulge. The galaxy with a prominent bulge will have a larger edge-on axial ratio compared to the galaxy without a bulge based on the definition of axial ratio. Thus a sample of disk galaxies with prominent spheroidal components will have more galaxies at larger axial ratios. The fact that the lenticular galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component.<ref name=bmbook />

Lenticular galaxies are often considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their intermediate placement on the [[Hubble sequence]]. This results from lenticulars having both prominent disk and bulge components. The disk component is usually featureless, which precludes a classification system similar to spiral galaxies. As the bulge component is usually spherical, elliptical galaxy classifications are also unsuitable. Lenticular galaxies are thus divided into subclasses based upon either the amount of dust present or the prominence of a central bar. The classes of lenticular galaxies with no bar are S0<sub>1</sub>, S0<sub>2</sub>, and S0<sub>3</sub> where the subscripted numbers indicate the amount of dust absorption in the disk component; the corresponding classes for lenticulars with a central bar are  SB0<sub>1</sub>, SB0<sub>2</sub>, and SB0<sub>3</sub>.<ref name=bmbook />