catalogs) often classify these pairs and groups as single objects. Double and multiple galaxies Arp originally referred to these galaxies as "double galaxies", but many of these sources are more than two galaxies. Some of the objects consist of interacting galaxies, whereas other sources are actually groups of galaxies. The difference is that interacting galaxies will be distorted, whereas galaxies in groups are simply gravitationally bound to each other but not necessarily close enough to each other to induce major structural changes. Connected arms All of these galaxies are interacting pairs of galaxies except for NGC 5679 (Arp 274), which may be an interacting galaxy triplet. The connected arms described here are tidal bridge features that form between interacting galaxies. These bridges form early during galaxy interactions. Interacting galaxies Unlike many of the objects listed in the amorphous galaxies section, the interacting galaxies that comprise these objects are still distinguishable from each other. Infall and attraction This category contains an odd mixture of objects. Two of the objects are edge-on disk galaxies with smaller companion galaxies nearby. Two of the objects are connected by tidal bridges. The last two objects may simply be interacting with each other over long distance. Wind effects Although included in the double galaxies category, many of these objects are individual galaxies. The "wind effects" refer to the appearance, not the actual detection of high-velocity gas (such as is found in M82). In some cases, the appearance may be the result of interaction. In other cases, particularly NGC 3981 (Arp 289), the faint, extended emission may be related to the intrinsic nature of the galaxy itself and not interactions with other objects. Long filaments The long filaments in these systems are probably tidal tails or bridges that have been produced as the result of the gravitational interaction between the galaxies. Unclassified objects Arp did not give a subclassification for objects 298–310 in his atlas. These objects are mostly interacting galaxy pairs. Groups Chains Miscellaneous Brightest Arp galaxies for amateur astronomers Maynard Pittendreigh, an amateur astronomer and occasional writer, has compiled a list of the brightest Arp Galaxies that are most easily viewed by typical amateur astronomers. The galaxies on the list can be observed visually and do not require special photographic or imaging equipment. These include: See also Index Catalogue (IC) Messier object (M) New General Catalogue (NGC) Uppsala General Catalogue (UGC) Further reading J. Kanipe,