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 million stars, range in size from dwarfs with less than a thousand 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.
The Hubble sequence is a morphological classification scheme for galaxies published by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tuning fork. It was invented by John Henry Reynolds and Sir James Jeans.
An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.
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.
A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.
Dwarf elliptical galaxies (dEs) are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies.
Messier 32 is a dwarf "early-type" galaxy about 2,650,000 light-years (810,000 pc) from the Solar System, appearing in the constellation Andromeda. M32 is a satellite galaxy of the Andromeda Galaxy (M31) and was discovered by Guillaume Le Gentil in 1749.
The Canis Major Overdensity or Canis Major Dwarf Galaxy is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major.
NGC 1300 is a barred spiral galaxy located about 61 million light-years away in the constellation Eridanus. The galaxy is about 110,000 light-years across. It is a member of the Eridanus Cluster, a cluster of 200 galaxies. It was discovered by John Herschel in 1835.
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 2683 is a field spiral galaxy in the northern constellation of Lynx. It was nicknamed the "UFO Galaxy" by the Astronaut Memorial Planetarium and Observatory. It was discovered by the astronomer William Herschel on February 5, 1788.
NGC 4216 is a metal-rich intermediate spiral galaxy located not far from the center of the Virgo Cluster of galaxies, roughly 55 million light-years away. It is seen nearly edge-on.
A dwarf spiral galaxy is the dwarf version of a spiral galaxy. Dwarf galaxies are characterized as having low luminosities, small diameters, low surface brightnesses, and low hydrogen masses. The galaxies may be considered a subclass of low-surface-brightness galaxies.
The Sérsic profile is a mathematical function that describes how the intensity of a galaxy varies with distance from its center. It is a generalization of de Vaucouleurs' law. José Luis Sérsic first published his law in 1963.
LEDA 74886, also known by its 2MASX designation 2MASX J03404323-1838431, and sometimes known as the Emerald-cut Galaxy, is a dwarf galaxy with a rare rectangular shape. It is located at a distance of about 70,000,000 light-years (21 Mpc) in the Eridanus constellation. The galaxy was detected in a wide field of view image taken by the Subaru Telescope using the Subaru Prime Focus Camera (Suprime-Cam). Using the Keck Telescope, a thin disc with a side on orientation was confirmed to be lurking at the center of LEDA 74886, and spinning at a speed of 33 km/s at the orbital radius of half a kpc. LEDA 74886 has a mass of around 109M☉ (Compared to the Milky Way's mass of about 1012M☉).
NGC 1277 is a lenticular galaxy in the constellation of Perseus. It is a member of the Perseus Cluster of galaxies and is located approximately 73 Mpc (megaparsecs) or 220 million light-years from the Milky Way. It has an apparent magnitude of about 14.7. It was discovered on December 4, 1875 by Lawrence Parsons, 4th Earl of Rosse.
NGC 128 is a lenticular galaxy in the constellation Pisces. It is approximately 190 million light-years from the Sun and has a diameter of about 165,000 light-years.
NGC 4488 is a lenticular galaxy located about 60 million light-years away in the constellation of Virgo. The galaxy was discovered by astronomer William Herschel on December 28, 1785. NGC 4488 is a member of the Virgo Cluster.
NGC 4469 is a nearly edge-on spiral galaxy located about 55 million light-years away in the constellation of Virgo. It is also classified as a LINER galaxy. NGC 4469 was discovered by astronomer William Herschel on April 15, 1784. It is a member of the Virgo Cluster.
NGC 1271 is a compact elliptical or lenticular galaxy located about 250 million light-years away in the constellation Perseus. The galaxy was discovered by astronomer Guillaume Bigourdan on November 14, 1884. NGC 1271 is a member of the Perseus Cluster and has a nuclear dust disk in its center. It also has an edge-on, intermediate-scale disk and has a central bulge. Like NGC 1277, NGC 1271 is a candidate "relic galaxy".
