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.
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
The Pinwheel Galaxy is a face-on spiral galaxy 21 million light-years away from Earth in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 and was communicated that year to Charles Messier, who verified its position for inclusion in the Messier Catalogue as one of its final entries.
Dwingeloo 1 is a barred spiral galaxy about 10 million light-years away from the Earth, in the constellation Cassiopeia. It lies in the Zone of Avoidance and is heavily obscured by the Milky Way. The size and mass of Dwingeloo 1 are comparable to those of Triangulum Galaxy.
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where the Solar System is located, is classified as a barred spiral galaxy.
NGC 1, also occasionally referred to as GC 1, UGC 57, PGC 564 or Holm 2a is an intermediate spiral galaxy of the morphological type Sbc, located approximately 210 to 215 million light-years from the Solar System in the constellation Pegasus. It was discovered on 30 September 1861 by Heinrich d'Arrest.
The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called the Scutum–Centaurus Arm. The Perseus Arm begins from the distal end of the long Milky Way central bar. Previously thought to be 13,000 light-years away, it is now thought to lie 6,400 light years from the Solar System.
NGC 1365, also known as the Great Barred Spiral Galaxy, is a double-barred spiral galaxy about 75 million light-years away in the constellation Fornax.
The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος, meaning "milky circle". From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, which may lead to a galaxy merger.
NGC 3370 (also known as UGC 5887 or Silverado Galaxy) is a spiral galaxy about 82.2 ± 5.9 million light-years (25.2 ± 1.8 megaparsecs) away in the constellation Leo. It is nearly comparable to our Milky Way both in diameter with a D25 isophotal size about 77,300 ly (23.69 kpc) comparing to the Milky Way Galaxy's 87,400 ly (26.8 kpc) diameter, and as well as in mass (1011 M☉). NGC 3370 exhibits an intricate spiral arm structure surrounding a poorly defined nucleus. It is a member of the NGC 3370 Group of galaxies, which is a member of the Leo II Groups, a series of galaxies and galaxy clusters strung out from the right edge of the Virgo Supercluster.
NGC 6744 is an intermediate spiral galaxy about 30 million light-years away in the constellation Pavo (Peacock). It is considered as a Milky Way mimic in the immediate vicinity, displaying flocculent (fluffy) arms and an elongated core. It also has at least one distorted companion galaxy superficially similar to one of the Magellanic Clouds. It was discovered from Parramatta in Australia by Scottish astronomer James Dunlop on 30 June 1826.
David Dunlap Observatory Catalogue, known as the DDO or A Catalogue of Dwarf Galaxies, is a catalogue of dwarf galaxies that was compiled by Sidney van den Bergh and published by the David Dunlap Observatory in 1959.
NGC 5754 is a barred spiral galaxy located 218 million light years away in the constellation Boötes. It is a member of the Arp 297 interacting galaxies group, which consists of NGC 5752, NGC 5753, NGC 5754, NGC 5755. Along with NGC 2718 and UGC 12158, NGC 5754 is often considered a Milky Way-twin.
Malin 1 is a giant low surface brightness (LSB) spiral galaxy. It is located 1.19 billion light-years (366 Mpc) away in the constellation Coma Berenices, near the North Galactic Pole. As of February 2015, it is the largest known spiral galaxy, with an approximate diameter of 650,000 light-years (200,000 pc), thus over six times the diameter of our Milky Way. It was discovered by astronomer David Malin in 1986 and is the first LSB galaxy verified to exist. Its high surface brightness central spiral is 30,000 light-years (9,200 pc) across, with a bulge of 10,000 light-years (3,100 pc). The central spiral is a SB0a type barred-spiral.
UGC 2885 is a large barred spiral galaxy of type SA(rs)c in the constellation Perseus. It is 232 million light-years (71 Mpc) from Earth and measures 463,000 ly (142,000 pc) across, making it one of the largest known spiral galaxies. It is also a possible member of the Pisces-Perseus supercluster.
UGC 12158 or PGC 69533 is an Sb-type barred spiral galaxy located approximately 384,000,000 ly (118 Mpc) away from Earth in the constellation of Pegasus. Its tight spiral disk spans approximately 140,000 ly (40 kpc), whose scale at heliocentric distance is about 36.9 kiloparsecs per arcminute. It is also often stated to resemble the Milky Way in appearance, with a similar central bar and spiral arm structure.
NGC 7640 is a barred spiral galaxy in the constellation of Andromeda. Discovered on October 17, 1786 by the English astronomer William Herschel, NGC 7640 is an 11th magnitude barred spiral galaxy. Also known as LEDA 71220, UGC 12554 and IRAS 23197+4034, this galaxy is located in the constellation Andromeda, 38.8 million light-years from Earth. Galaxies of this type are recognizable by their spiral arms, which fan out not from a circular core, but from an elongated bar cutting through the galaxy’s centre. NGC 7640 might not look much like a spiral, but this is due to the orientation of the galaxy with respect to an observer on Earth. There is evidence that this galaxy has experienced an interaction with another galaxy in the (astronomically) recent past.
NGC 3506 is a spiral galaxy in the constellation Leo. It is located at a distance of circa 300 million light years from Earth, which, given its apparent dimensions, means that NGC 3506 is about 115,000 light years across. The galaxy has two main spiral arms, with high surface brightness, which can be traced for half a revolution before they fade. One arm splits into four spiral arcs.
NGC 721 is a barred spiral galaxy located in the constellation Andromeda about 250 million light years from the Milky Way. It was discovered by the Prussian astronomer Heinrich d'Arrest in 1862.
