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Arp Many galaxies are better known by a name than by their catalog numbers. For example, M-51 is better known as the Whirlpool Galaxy, and NGC 5128 is better known as Centaurus A. Clicking on "Common Name" leads to a menu listing some of these objects. For each of the following catalogs, you can click on the menu item for that catalog and enter a designation. Guide will then recenter on that object. The PGC (Principal Galaxy Catalog) lists extensive data for over 160,000 galaxies, and is the fundamental galaxy catalog used by Guide. The LEDA data is a recent extension to the PGC. "More info" on galaxies comes (mostly) from these catalogs. ("PGC" and "PGC/LEDA" will be used interchangeably. ) The UGC (Uppsala Galaxy Catalog) is an older catalog of over 10,000 galaxies. The MCG (Morphological Catalog of Galaxies), another older catalog, contains data for over 30,000 galaxies. MCG designations consist of three numbers, with spaces or dashes between them; the first number can be negative. A letter may be added on, as in "MCG -04-12-133A". You can use either dashes or spaces to separate the numbers; for example, "-4 12 133a" will work as well as "-04-12-133A". The ESO/Uppsala (European Southern Observatory) catalog extends the Uppsala catalog to the southern sky; the CGCG (Catalog of Galaxies and Clusters of Galaxies) is commonly used for labelling northern galaxies. In both of these catalogs, galaxies are designated by two numbers, again separated by a space or dash, as in "ESO 34-12" or "CGCG 17 41". Again, as with the MCG, a letter is sometimes added to this. Again, you can use either a space or dash to separate the two numbers. The Markarian catalog lists around 1,400 galaxies "active galaxies", such as Seyfert galaxies and quasars: galaxies that emit prodigious amounts of energy. The Zwicky and Abell catalogs list "clusters of galaxies". These objects are not very easy to find, unless you have access to an unusually large telescope. These catalogs were assembled because if you study enough of them, it is possible to determine information about the size and age of the universe. The Zwicky catalog lists 9,134 clusters in the northern sky. The Abell catalog lists over 5,000 clusters in all parts of the sky. Don't confuse the Abell catalog of clusters of galaxies with the Abell catalog of planetary nebulae (p. 12). One can also find a galaxy by its Hickson designation. The Hickson catalog lists 100 compact clusters of galaxies, similar to Copeland's Septet and other close groupings. The Hickson designation consists
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