A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 billion stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.
Messier 83 or M83, also known as the Southern Pinwheel Galaxy and NGC 5236, is a barred spiral galaxy approximately 15 million light-years away in the constellation borders of Hydra and Centaurus. Nicolas-Louis de Lacaille discovered M83 on 23 February 1752 at the Cape of Good Hope. Charles Messier added it to his catalogue of nebulous objects in March 1781.
The Sombrero Galaxy is a peculiar galaxy of unclear classification in the constellation borders of Virgo and Corvus, being about 9.55 megaparsecs from the Milky Way galaxy. It is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster. It has an isophotal diameter of approximately 29.09 to 32.32 kiloparsecs, making it slightly bigger in size than the Milky Way.
A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.
NGC 6822 is a barred irregular galaxy approximately 1.6 million light-years away in the constellation Sagittarius. Part of the Local Group of galaxies, it was discovered by E. E. Barnard in 1884, with a six-inch refractor telescope. It is the closest non-satellite galaxy to the Milky Way, but lies just outside its virial radius. It is similar in structure and composition to the Small Magellanic Cloud. It is about 7,000 light-years in diameter.
NGC 2841 is an unbarred spiral galaxy in the northern circumpolar constellation of Ursa Major. It was discovered on 9 March 1788 by German-born astronomer William Herschel. J. L. E. Dreyer, the author of the New General Catalogue, described it as, "very bright, large, very much extended 151°, very suddenly much brighter middle equal to 10th magnitude star". Initially thought to be about 30 million light-years distant, a 2001 Hubble Space Telescope survey of the galaxy's Cepheid variables determined its distance to be approximately 14.1 megaparsecs or 46 million light-years. The optical size of the galaxy is 8.1′ × 3.5′.
NGC 5253 is an irregular galaxy in the constellation Centaurus. It was discovered by William Herschel on 15 March 1787.
NGC 2775, also known as Caldwell 48, is a spiral galaxy in the constellation Cancer. It is 67 million light-years away from the Milky Way. It was discovered by William Herschel in 1783. NGC 2775 belongs to the Antlia-Hydra Cluster of galaxies and is the most prominent member of the NGC 2775 Group, a small galaxy group in the Virgo Super-cluster, along with the Local Group. Other members of the NGC 2775 Group include NGC 2777 and UGC 4781.
NGC 922 is a peculiar galaxy in the southern constellation of Fornax, located at a distance of 142 Mly from the Milky Way. It is one of the nearest known collisional galaxies. This object was described by the Herschels as "considerably faint, pretty large, round, gradually pretty much brighter middle." The general form is described by the morphological classification of SB(s)cd pec, which indicates a peculiar (pec) barred spiral galaxy (SB) with no inner ring system around the bar (s) and loosely-wound spiral arms (cd).
NGC 91 is a star with an apparent magnitude of 14.4 in the constellation of Andromeda. The star is southwest of the galaxy NGC 90. Discovered in 1866 by Herman Schultz, there have been many arguments if this star exists or not. However, people have observed the star, and have confirmed that NGC 91 exists.
NGC 289 is a spiral galaxy in the southern constellation of Sculptor, located at a distance of 76 megalight-years from the Milky Way. It was discovered on September 27, 1834, by John Herschel. The compiler of the New General Catalogue, John Louis Emil Dreyer, noted that NGC 289 was "pretty bright, large, extended, between 2 considerably bright stars". The plane of the galaxy is inclined by an angle of 45° to the line of sight from the Earth.
NGC 1084 is an unbarred spiral galaxy in the constellation Eridanus. It is located at a distance of about 63 million light-years away from the Milky Way. The galaxy was discovered by William Herschel on 10 January 1785. It has multiple spiral arms, which are not well defined. It belongs in the same galaxy group with NGC 988, NGC 991, NGC 1022, NGC 1035, NGC 1042, NGC 1047, NGC 1052 and NGC 1110. This group is in turn associated with the Messier 77 group.
NGC 6052 is a pair of galaxies in the constellation of Hercules. It was discovered on 11 June 1784 by William Herschel. It was described as "faint, pretty large, irregularly round" by John Louis Emil Dreyer, the compiler of the New General Catalogue.
NGC 4861, also known as Arp 266, is a galaxy in the constellation Canes Venatici. It was discovered by William Herschel on May 1, 1785.
NGC 1222 is an early-type lenticular galaxy located in the constellation of Eridanus. The galaxy was discovered on 5 December 1883 by the French astronomer Édouard Stephan. John Louis Emil Dreyer, the compiler of the New General Catalogue, described it as a "pretty faint, small, round nebula" and noted the presence of a "very faint star" superposed on the galaxy.
NGC 5308 is an edge-on lenticular galaxy in the constellation of Ursa Major. It was discovered on 19 March 1790 by William Herschel. It was described by John Louis Emil Dreyer as "bright, pretty large" when he compiled the New General Catalogue. A small, irregular galaxy near NGC 5308 has been given the designation LEDA 2802348.
NGC 1140 is an irregular galaxy in the southern constellation of Eridanus. Estimates made using the Tully–Fisher method put the galaxy at about 59 million light years. It was discovered on 22 November 1786 by William Herschel, and was described as "pretty bright, small, round, stellar" by John Louis Emil Dreyer, the compiler of the New General Catalogue.
NGC 5011 is an elliptical galaxy in the constellation of Centaurus. It was discovered on 3 June 1834 by John Herschel. It was described as "pretty bright, considerably small, round, among 4 stars" by John Louis Emil Dreyer, the compiler of the New General Catalogue.
NGC 2623/Arp 243 is an interacting galaxy located in the constellation Cancer. NGC 2623 is the result of two spiral galaxies that have merged. Scientists believe that this situation is similar to what will occur to the Milky Way, which contains the Solar System, and the neighboring galaxy, the Andromeda Galaxy in four billion years. Studying this galaxy and its properties have provided scientists with a better idea of the coming collision of the Milky Way and the Andromeda. Due to NGC 2623 being in the late stage of merging, the compression of the gas within the galaxy has led to a large amount of star formation, and to its unique structure of a bright core with two extending tidal tails.
NGC 1132 is an elliptical galaxy located in the constellation Eridanus. The galaxy was discovered by John Herschel on November 23, 1827. It is located at a distance of about 318 million light-years away from Earth.
