A glance at any star map reveals cryptic references to objects such as M31, NGC 4565, and IC 434. What do they mean?
In the late 1700s, the French astronomer Charles Messier compiled the Catalogue of Nebulous Objects and Star Clusters, which remains the most popular list of bright deep-sky wonders. Number 31 on Messier's list, for example, is the Andromeda Galaxy (M31). Modern versions ot the list contain 110 entries, ;md seeing all of them is a challenging goal for amateur astronomers.
By the mid-1800s, astronomers had discovered many more fuzzy telescope targets in the night sky. In 1864, the English astronomer John Herschel published the General Catalogue of Nebulae, a list of more than 5,000 objects discovered mostly by him and his father, William Herschel (famous for discovering the planet Uranus).
In 1888.J. L.E.Dreyer published a master list, the New General Catalogue of Nebulae and Clusters of Stars. Supplementary Index Catalogues (IC) were added in 1895 and 1908.
Astronomers such as Wilhelm and Otto Struve and S. W. Burnham compiled catalogs of double stars in the nineteenth century. For example, stars designated with the Greek letter sigma are from Wilhelm Struve's catalogs. Collinder, Melotte, Ruprecht, Stock, Trumpler, and many others in the twentieth century have cataloged star clusters and lent their names to clusters that Messier and the NGC authors missed. In the 1950s, George Abell cataloged clusters of galaxies—distant Abell clusters are challenging targets for owners of large telescopes.