A type of galaxy with bright arms of stars, gas, and dust that extend in a spiral pattern from a central hub; Hubble type S. There are usually two arms, which can wrap right around the galaxy, but four-armed and even three-armed examples are known. The arms can also be broken into many short sections. At the centre of a spiral galaxy is a spheroidal bulge of old stars of Population II. The bulge is large in type Sa spirals, which have tightly coiled arms, but much smaller and less conspicuous in types Sc and Sd, whose arms are coiled more loosely. Types Sa and Sb are, on average, intrinsically brighter and more massive than types Sc and Sd.
The spiral arms are sites of active star formation, and their appearance is dominated by bright, blue, massive young stars of Population I and gaseous H II regions. In mass they range from about 109 to 5 × 1011 solar masses, and in diameter from about 10 000 to over 300 000 l.y. 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 rich clusters, there are no spirals with masses as low as those of many irregulars and dwarf elliptical galaxies. The three brightest members of the Local Group of galaxies are spirals.
If there is an almost rectangular or cigar-shaped concentration of stars across the central region, the galaxy is termed a barred spiral. Nearly half of all bright spirals show an obvious bar, but most ordinary spirals probably have some degree of barred structure in their star or gas distribution.
Subjects: Astronomy and Astrophysics.