Spiral galaxies, like our Milky Way Galaxy, look like flat disks when viewed from the side and are easily recognized for their bright arms which radiate out in circular patterns from the clusters of stars in their centers, known as NUCLEI. Just as the curves and lines of human fingerprints are different for every person, no two spiral galaxies are alike:
Photo. M31, also known as the Andromeda Galaxy, is one of two dozen galaxies near the Milky Way Galaxy. Courtesy of Carina Software, 12919 Alcosta Boulevard, Suite #7, San Ramon, CA 94583, (510) 355-1266. Used with permission.
Spirals rotate and are the birthplace of many of the new stars of the universe because these galaxies contain much gas and dust -- the materials of which stars are made.
Photo. The Whirlpool Galaxy (M51) lies 20 light-years from our Galaxy. The HST spotted a supernova (an exploded star that shines very brightly for a short time) in the galaxy. The bright spot marked by the white arrow, the supernova is 2000 light-years from the nucleus. Courtesy NASA, Robert P. Kirshner/Harvard-Smithsonian Center for Astrophysics.
When Edwin Hubble became the first to categorize galaxies, he clumped all spirals together. Since his time, astronomers have divided the spiral classification into two distinct types: standard -- or regular -- spirals and "barred spirals."
Barred spirals are easily distinguished from their regular counterparts: in the nucleus, there is a distinct bar from which the arms begin to curve around the galaxy.