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What causes the central bar in some spiral galaxies?

Dennis Shingler, Inverness, Florida
Central-bar
A galaxy's central bar may look like a solid structure, but it’s really a dense region that affects the galaxy as it rotates around the core. Gas near the galaxy’s center moves at faster speeds than that farther out. This, combined with density waves, helps create the bar. Astronomy: Roen Kelly, after Daisuke Namekata, et al.

Bars are also density waves that rotate around the disk with a speed different from the rotation speed of individual stars, much like the waves that create the spiral structure of these galaxies. Gravitational instabilities in the centers of galaxies, or gravitational disturbances from nearby galaxies, can cause density waves. As the waves rotate around the galaxy, they hold their shape like the blades of a fan.

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