The most common galaxies in the universe are dwarf ellipticals and irregulars, but by far the largest proportion of bright galaxies—about 75 percent—are spirals. These systems range from 15,000 to 150,000 light-years in diameter and may contain several hundred billion stars in a flattened disk.
Within the disk, spiral arms appear to emerge from a bright central nucleus, traced out by young, hot stars and bright emission nebulas, like lights around a Christmas tree. Open star clusters and interstellar dust and gas are distributed throughout the disk. The Andromeda (M31) and the Whirlpool (M51) galaxies are typical spirals.
In a barred spiral, the bright stars and ionized gas of the nucleus extend for thousands of light-years from each side of the center in a straight "bar." From the end of each bar, the arms wrap back around the nucleus, as normal spiral arms do. In obvious cases, each bar looks something like a scythe blade. Approximately one-third of the spirals exhibit a bar-like structure, and some astronomers suspect that all spirals contain at least a weak bar running through the disk. NGC 1365, in Fornax, is sometimes known as the Great Barred Spiral.