Galaxies consist of hundreds of millions of stars and interstellar dust and gas. According to the Hubble classication (named after its inventor, the American astronomer Edwin Hubble (1889-1953)), galaxies can be classified as spiral, elliptical, or irregular. Among the galaxies shown below, M100 is a good example of a spiral galaxy, M104 is a good example of an elliptical galaxy, and M33 is a good example of an irregular galaxy.

The numbers M51, M101, etc. refer to the Messier catalog of celestial objects, compiled by the 18th century French astronomer Charles Messier. Messier compiled his catalog of diffuse objects so that they would not be confused with comets. The objects in Messier's catalog are now known to be among the brightest nebulae, galaxies, and star clusters visible to us.