spiral galaxy

spiral galaxy

n.

A galaxy having a spiral structure.

---

spiral galaxy  (sprl)

A galaxy consisting of a rotating flattened disk with an ellipsoidal central bulge from which extend a pattern of two or more luminous spiral arms. Spiral galaxies range from large bulges with tightly wound arms (classified as Sa) to small bulges with loosely wound arms (classified as Sc and in some cases Sd). The majority of the mass of a spiral galaxy is contained in its bulge, made up mostly of old stars, while the arms are composed mostly of younger stars and large amounts of interstellar gas and dust. A spherical, relatively dust-free region known as a galactic halo surrounding a spiral galaxy may contain large amounts of dark matter. The Milky Way is a spiral galaxy. A spiral galaxy whose central bulge has the shape of a bar from whose ends the spiral arms emanate is called a barred spiral galaxy. About a third of spiral galaxies have this straight or lozenge-shaped bar of stars, gas, and dust extending out from the nucleus. Barred spiral galaxies are classified similarly to regular spirals, from SBa (large bulge, tightly wound arms) to SBc (smaller bulge, looser arms). Astronomers believe that some elliptical galaxies containing hints of a bar and spiral might once have been barred spiral galaxies. See illustration at galaxy. Compare elliptical galaxyirregular galaxylenticular galaxy See more at Hubble classification system.

Thesaurus

Noun

1.

spiral galaxy - a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center spiral nebula extragalactic nebula, galaxy - (astronomy) a collection of star systems; any of the billions of systems each having many stars and nebulae and dust; "`extragalactic nebula' is a former name for `galaxy'" Andromeda galaxy - a spiral galaxy in the constellation of Andromeda that is visible to the naked eye