A barred spiral galaxy is a
spiral galaxy with a central bar-shaped structure comprised of
stars. Bars are found in approximately half of all spiral galaxies. Bars generally affect both the motions of stars and
interstellar gas within spiral galaxies and can affect
spiral arms as well.
Edwin Hubble classified these types of spiral galaxies as "SB" ("Spiral", "Barred") in his
Hubble sequence, and arranged them into three sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb type galaxies lie in between. A fourth type, SBm, was subsequently created to describe somewhat
irregular barred spirals, such as the
Magellanic Cloud galaxies, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures.
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