spiral galaxy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A galaxy consisting of a rotating flattened disk with a central bulge made up mostly of old stars, from which extend two or more spiral arms made up mostly of younger stars and large amounts of interstellar gas and dust.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy A galaxy having a number of arms of younger stars that spiral out from the centre containing older ones

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center