A type of disk galaxy with spiral arms extending from an almost rectangular or cigar-shaped bar of stars across its central regions, which can account for up to one-third of the galaxy’s total light output; Hubble type SB. The central bars are typically between 2.5 and 5 times as long as they are wide. The masses of barred spirals range from about 10⁹ to 5 × 10¹¹ solar masses, and their diameters from about 10 000 to over 300 000 l.y. Almost half of all disk galaxies, including our own, contain a noticeable bar; similar but much less prominent structures may exist undetected in most disk galaxies. The bars often show sharp, straight dust lanes on the edge leading the rotation. There is often a cluster of bright nebulae (H II regions) at the outer ends of the bar, where the spiral arms usually start. Many barred galaxies show a narrow ring of stars near the outer end of the bar. The bars probably differ in width and thickness as well as length, and there may be considerable streaming motion or flow of interstellar gas along them. It is not yet clear whether bars are a permanent structure in a galaxy, or represent a transient and perhaps recurrent feature. Bars are generally thought to be permanent structures, although they might be destroyed in interactions between galaxies.

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