<P> A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes . Lenticular galaxies are disc galaxies (like spiral galaxies) that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation . They may, however, retain significant dust in their disks . As a result, they consist mainly of aging stars (like elliptical galaxies). Because of their ill - defined spiral arms, if they are inclined face - on it is often difficult to distinguish between them and elliptical galaxies . Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations . Both can be considered early - type galaxies that are passively evolving, at least in the local part of the Universe . </P> <P> 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 galaxies, yet may exhibit a central bar . 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 . 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 . </P>

Where do lenticular galaxies fit into the geometry of galaxies