spiral galaxy

NGC 1309

The spiral galaxy NGC 1309.

A spiral galaxy is a galaxy with a flattened disk and a nuclear bulge from which emanate luminous spiral arms. As well as stars, the disk harbors extensive interstellar clouds of gas and dust. Surrounding the disk is a large galactic halo. There are two main types of spiral galaxy – barred spirals (type SB) and ordinary spirals (type S), each of which is divided into subtypes a (tightly wound arms, large bulge), b, c, and (sometimes) d (loosely wound arms, small bulge). The trend from a to d is toward decreasing brightness and mass. Spiral galaxies and lenticular galaxies together make up the class known as disk galaxies.


In diameter, spirals range from about 10,000 to over 300,000 light-years, and in mass from about one billion to 500 billion solar masses. Spiral structure can apparently exist only in disk galaxies above a certain size and, although spirals represent 80% of the bright galaxies in regions outside of rich clusters, there are no spirals with masses as low as those of many irregular galaxies and dwarf ellipticals.


Our own Milky Way Galaxy, together with the nearby Andromeda Galaxy, are both large spiral galaxies. Other relatively close and well known spirals include the Triangulum Galaxy, the Whirlpool Galaxy, the Sunflower Galaxy, the Black Eye Galaxy, the Sombrero Galaxy, M74, M81, and M101.