What is a Spiral Galaxy?

A spiral galaxy is one that contains a central bulge surrounded by a rotating flattened disk of young, bright stars and interstellar medium. Due to density waves, this rotating material separates itself into two or more spiraling arms, slowing the rotation of stars and molecular gas and compressing it to form new stars. The central bulge of these galaxies, composed of old dim stars, is thought in most cases to contain a supermassive blackhole. Although never directly observed, the presence of these blackholes can be inferred by detecting their effect on nearby matter such as stars and interstellar medium. Spiral galaxies can be classified into two groups, ordinary and barred. Ordinary, being the galaxies with arms deriving directly from the central bulge and barred, the galaxy type making up two thirds of all spiral galaxies containing a central bar structure composed of stars that the arms emerge from.

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