Most spiral galaxies can be categorised into of one two basic forms: simple spirals, in which the spiral arms extend directly outward from the nucleus, and barred spirals, in which a linear bar emerges from the galaxy'score, and the arms extend from the ends of that bar. There is also a third classification of spiral galaxies that lies between these two main types, the intermediatespiral galaxy, sometimes described as a 'weakly barred' galaxy.
Intermediate spirals show some structure in their core regions, and so are more complex in this sense than classical spirals, but that structure is not so extended as to constitute a true bar. An example of such an intermediate galactic structure is M66 or NGC 3627 in Leo, a galaxy whose nucleus is distinctly elongated along one axis, with a spiral arm extending from each end of the elongated core. The galaxy'sarms are thus offset from the centre of the spiral structure, but the galaxy does not possess a distinct bar. Other examples of intermediate galaxies from the Messier Catalogue include M65 (a near neighbour to M66 in the sky) and M106 in Canes Venatici.
On the Hubble classification system, true spirals are coded as 'S', and barred spirals as 'SB'. As part of an extension to this model devised by Gérard de Vaucouleurs, intermediate spirals are listed as 'SAB'-type galaxies. This type can be further refined from SABa with tightly wound arms to SABd with a loose and somewhat irregularspiral pattern. A rare additional type is SAB0, a lenticular galaxy showing the characteristics of an intermediate spiral. Finally, the 'SABm' type is a 'Magellanic' intermediate, in which the galaxy has become distorted, but retains sufficient structure that its original intermediate spiral nature is nonetheless detectable.
