Spiral galaxy

Spiral galaxies are named for the (usually two-armed)ructures that extend from the center into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot  that inhabit them. Roughly half of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. Our own  has recently (in the 1990s) been confirmed to be a although the bar itself is difficult to observe from our position within the Galactic disk. The most convincing evidence for its existence comes from a recentperformed by the of stars in the Galactic center.

Together with , spiral galaxies make up approximately 60% of galaxies in the local They are mostly found in low-density regions and are rare in the centers of galaxy clusters

A spiral galaxy is a f  originally described by in his 1936 work The Realm of the Nebulae and, as such, forms part of theSpiral galaxies consist of a flat, rotating containingand a central concentration of stars known as the . These are surrounded by a much fainter of stars, many of which reside in