The barred spiral NGC 4725. Image courtesy Richard Crisp.
A barred spiral galaxy is a type of spiral galaxy with a rectangular or cigar-shaped nucleus, from the ends of which spiral arms extend. Bars are large bodies of gas, dust, and stars that rotate as if they were solid objects. Typically they are 2½ to 5 times longer than they are wide and may contribute up to one-third of a galaxy's luminosity. By channeling gas and dust to the center of the galaxies, they may trigger bursts of star formation, or, alternatively, feed material to a supermassive black hole in the galactic core.
Bars are believed to represent a temporary stage in the life of some spiral systems. According to one theory, they form spontaneously through global disturbances in disk galaxies. Another theory suggests they come about from interactions with nearby galaxies.
Barred galaxies are grouped by their appearance using three criteria: the central bulge and light distribution; the tightness with which the spiral arms are wound; and the degree to which the spiral arms are resolved into stars and nebulae. The three main types are SBa, SBb, and SBc.