Defining the limits of galaxy clusters is imprecise as many clusters are still forming. In particular, clusters close to the Milky Way tend to be classified as galaxy clusters even when they are much smaller than more distant clusters.
In this collision between two clusters of galaxies, the stars pass between each other unhindered, while the hot, diffuse gas experiences friction and is left behind between the clusters. The gas dominates the visible mass budget of the clusters, being several times more massive than all the stars. Yet the regions with the stars show more gravitational lensing than the gas region, indicating that they are more massive than the gas. Some dark (since we don't see it), collision-less (or it would have been slowed, like the gas) matter is inferred to be present to account for the extra lensing around otherwise low-mass regions.
Named in comparison with the Bullet Cluster, being of similar formation, except smaller
Also has a systematic catalogue name SL2S J08544-0121. As of 2014, it was the lowest mass object that showed separation between the concentrations of dark matter and baryonic matter in the object.
Coined by Tom Lorenzin (author of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing") to honor Deer Lick Gap in the mountains of North Carolina, from which he had especially fine views of the galaxy group.
Named after its discoverer, Carl Seyfert. At the time it appeared to contain six external nebulae. It is also called the NGC 6027 Sextet, after its brightest member.
There are actually only five galaxies in the sextet, and only four galaxies in the compact group. One of the galaxies is an ungravitationally bound background object. The other "galaxy" is instead an extension of the interacting system — a tidal stream caused by the merger. The group is, therefore, more properly called HCG 79; the name refers to the visual collection and not the group. HCG 79 lies 190 million light-years away in the Serpens Caput constellation.
There are actually only four galaxies in the compact group, the other galaxy is a foreground galaxy. The group is therefore more properly called HCG 92, because the name refers to a visual collection and not a group. Thus, the real group is also called Stephan's Quartet
The major nearby groups and clusters are generally named after the constellation they lie in. Many groups are named after the leading galaxy in the group. This represents an ad hoc systematic naming system.
Groups visible to the unaided eye
The Local Group contains the largest number of visible galaxies with the naked eye. However, its galaxies are not visually grouped together in the sky, except for the two Magellanic Clouds. The IC342/Maffei Group, the nearest galaxy group, would be visible by the naked eye if it were not obscured by the stars and dust clouds in the Milky Way's spiral arms.
Mpc represents millions of parsecs, a measure of distance (1 Mpc = 3.26 Mly).
z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion. In this very nearby context, however, the observed redshift and recessional velocity are due to the Doppler shifting of the light.
Distances are measured from Earth, with Earth being at zero.
Attempts at measuring the redshift of the brightest cluster galaxy of this Hydra Cluster had been attempted for years before it had been successfully achieved. The BCG was also the most distant galaxy of the time.
This was the first noted cluster of "nebulae" that would become galaxies. The first redshifts to galaxies in the cluster were measured in the 1910s. Galaxies were not identified as such until the 1920s. The distance to the Virgo Cluster would have to wait until the 1930s.
In 1995 and 2001, the cluster around 3C 294 was announced, at z=1.786 
In 1992, observations of the field of cluster Cl 0939+4713 found what appears to be a background cluster near a quasar, also in the background. The quasar was measured at z=2.055 and it was assumed that the cluster would be as well.
In 1975, 3C 123 and its galaxy cluster was incorrectly determined to lie at z=0.637 (actually z=0.218) 
In 1958, cluster Cl 0024+1654 and Cl 1447+2619 were estimated to have redshifts of z=0.29 and z=0.35 respectively. However, they were not spectroscopically determined.
z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion.
Distances are measured from Earth, with Earth being at zero.
In 2002, a very large, very rich protocluster, or the most distant protosupercluster was found in the field of galaxy cluster MS 1512+36, around the gravitationally lensed galaxy MS 1512-cB58, at z=2.724 
Sometimes clusters are put forward that are not genuine clusters or superclusters. Through the researching of member positions, distances, peculiar velocities, and binding mass, former clusters are sometimes found to be the product of a chance line-of-sight superposition.
^F. Gastaldello; M. Limousin; G. Foëx; R. P. Muñoz; T. Verdugo; V. Motta; A. More; R. Cabanac; D. A. Buote; D. Eckert; S. Ettori; A. Fritz; S. Ghizzardi; P. J. Humphrey; M. Meneghetti; M. Rossetti (22 Apr 2014). "Dark matter-baryons separation at the lowest mass scale: the Bullet Group". Monthly Notices of the Royal Astronomical Society: Letters (published July 2014). 442 (1): L76-L80. arXiv:1404.5633. Bibcode:2014MNRAS.442L..76G. doi:10.1093/mnrasl/slu058.
^ abMAX-PLANCK-INSTITUT FÜR EXTRATERRESTRISCHE PHYSIK, GARCHING, GERMANY ; DEPARTMENT OF ASTRONOMY, UNIVERSITY OF MICHIGAN, ANN ARBOR, USA ; ESO, ASTROPHYSIKALISCHES INSTITUT, POTSDAM, GERMANY ; "GALAXY CLUSTER ARCHAEOLOGY"(PDF). Archived from the original(PDF) on 2006-06-22. ; HANS BÖHRINGER, CHRISTOPHER MULIS, PIERO ROSATI, GEORG LAMER, RENE FASSBENDER, AXEL SCHWOPE, PETER SCHUECKER
^Observational Cosmology. Astronomical Society of the Pacific Conference Series, Volume 51; Proceedings of an International Symposium; held in Milano; Italy; 21–25 September 1992; San Francisco: Astronomical Society of the Pacific (ASP); |c1993; edited by Guido L. Chincarini, Angela Iovino, Tommaso Maccacaro, and Dario Maccagni, p.225 ; The Spectra and Morphology of Galaxies in High-Redshift Clusters ; 1993ASPC...51..225D