A galaxy is a cluster of stars, nebulae, dark matter, and other astronomical objects. Most galaxies are tens of thousands of light years in diameter, and contain billions of stars. Galaxies come in three primary shapes; Spiral Galaxies are thin disks, with spiral arms surrounding a central hub; Elliptical Galaxies are uniform, oval-shaped agglomerations; and Irregular Galaxies have little or no definite structure.
NGC 4535, Spiral Galaxy in the constellation Virgo
A spiral galaxy centers around a hub, which is roughly spherical and bulges outward from the disk. The hub is the gravitational center of the galaxy, and usually contains a supermassive black hole. Surrounding the hub are spiral arms, which are bunched-up waves of stars and gas orbiting the center. The spiral arms usually have more gas and dust, and they contain most of the newly formed stars, which make them appear blue in color imagery. A spiral galaxy may have a horizontal bar passing through its hub; our own galaxy, the Milky Way, is believed to contain a bar.
M87- NGC 4486, an Elliptical Galaxy in the constellation Virgo
Elliptical galaxies have a large, central hub, but they lack the arms of a spiral galaxy; they usually appear more yellowish in color imagery from old, reddish stars. Irregular galaxies often have no discernible structure, but often they can be recognized as having a distorted spiral or elliptical shape. Elliptical and irregular galaxies often result from collisions, which cause large bursts of star formation and distort the galaxies' structure due to the complex gravitational interactions. Eventually, the two galaxies usually merge to form one large galaxy, which has cast off most of its gas and dust clouds.
NGC 5477, Irregular Galaxy in the constellation Ursa Major
Irregular galaxies, labeled "IA," or "IB" if barred, do not fall into any other classification. Most are relatively small, like NGC 5477. Many have active star-forming regions.
NGC 3115, The Spindle Galaxy, a lenticular galaxy in the constellation Sextans
Lenticular galaxies, like Sextans's NGC 3115, the Spindle Galaxy, are an intermediate type between ellipticals and spirals. They are indicated by "SA0" or "SB0" if they display a bar. Lenticulars somewhat resemble spirals, having a central bulge and a disk, but they lack spiral arms. They are common in galaxy clusters.