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A typical simple spiral galaxy has a central bulge at the core, with galactic arms spiralling outward from that bulge. Many galaxies, however (in fact, up to two thirds of all spiral galaxies) have a more complex form. Tidal gravitational forces around the central bulge cause the orbits of stars in that region to become synchronised, so that a distinct bar forms, extending outward laterally from the core on either side. These galaxies still have spiral arms, but those arms extend from the outer ends of the bar, and not directly from the central core. The barred structure also has the effect of concentrating material in the regions of the galaxy's nucleus.
On the Hubble Classification, barred spirals are categorised as 'SB' (rather than just 'S' for simple spirals). There are three classical subcategories, SBa, SBb and SBc, describing the structure in more detail. At one end of this scale, SBa galaxies have tightly wound arms around a relatively small and often indistinct bar, while SBc galaxies have a quite distinct elongated bar extending out across much of their diameter.
A fourth specialised category, SBm, describes irregular galaxies that nonetheless show some barred structure (galaxies of this kind are thought to have typically originated as full barred spirals and been distorted by external forces). The variant class SAB is used for barred galaxies with an especially indistinct bar, known as intermediate or weakly barred spirals.
Some examples of barred spiral galaxies include M91 in Coma Berenices, M109 in Ursa Major, and NGC 1300 in Eridanus (an SBc galaxy with a highly distinct bar structure). The Milky Way Galaxy itself is also a barred spiral (probably of the classification SBc) as is one of its attendant galaxies, the Large Magellanic Cloud, which is a distorted irregular galaxy of the SBm classification.
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