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.
Related entry galaxy classification
Related category GALAXIES
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