Barred spiral galaxy

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where the Solar System is located, is classified as a barred spiral galaxy.

Edwin Hubble classified spiral galaxies of this type as "SB" (spiral, barred) in his Hubble sequence and arranged them into 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 the two.

Although theoretical models of galaxy formation and evolution had not previously expected galaxies becoming stable enough to host bars very early in the universe's history, evidence has recently emerged of the existence of numerous spiral galaxies in the early universe.

Bars

Further information: Density wave theory § Galactic spiral arms

Many spiral galaxies have a special feature called a bar. This bar is a straight group of stars in the center of the galaxy. Scientists think bars form when a density wave moves from the center, changing the paths of stars and making the bar grow over time.

Bars help new stars form by pulling material toward the center of the galaxy. They may also help create bright areas in the center, called active galactic nuclei. Bars don’t last forever; they can change shape or disappear over billions of years. Most spiral galaxies, including our own Milky Way, have bars at some point in their lives.

Grades

Barred spiral galaxies are classified with the general label "SB", short for spiral barred. The differences between types depend on how tightly or loosely the galaxy's spiral arms are wound. SBa types have tightly bound arms, while SBc types have loosely bound arms. SBb types sit in the middle, and SBm describes somewhat irregular barred spirals. Another related type is SB0, which refers to a barred lenticular galaxy.