A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.
The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented. In June 2019, however, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.
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.
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.
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.
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 Hydra. Nicolas Louis de Lacaille discovered M83 on February 23, 1752 at the Cape of Good Hope. Charles Messier added it to his catalogue of nebulous objects in March 1781. It is one of the closest and brightest barred spiral galaxies in the sky, and is visible with binoculars. Its nickname of the Southern Pinwheel derives from its resemblance to the Pinwheel Galaxy (M101).
Messier 61 is an intermediate barred spiral galaxy in the Virgo Cluster of galaxies. It was discovered by Barnaba Oriani on May 5, 1779. This was six days before Charles Messier observed the same galaxy, but had mistaken it as a comet.
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. However, galaxy classification and morphology are now largely done using computational methods and physical morphology.
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about half of all spiral galaxies. Bars 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 our own Solar System is located, is classified as a barred spiral galaxy.
Messier 58 is an intermediate barred spiral galaxy with a weak inner ring structure located within the constellation Virgo, approximately 68 million light-years away from Earth. It was discovered by Charles Messier on April 15, 1779 and is one of four barred spiral galaxies that appear in Messier's catalogue. M58 is one of the brightest galaxies in the Virgo Cluster. From 1779 it was arguably the farthest known astronomical object until the release of the New General Catalogue in the 1880s and even more so the publishing of redshift values in the 1920s.
Messier 77 or M77, also known as NGC 1068, is a barred spiral galaxy about 47 million light-years away in the constellation Cetus. Messier 77 was discovered by Pierre Méchain in 1780, who originally described it as a nebula. Méchain then communicated his discovery to Charles Messier, who subsequently listed the object in his catalog. Both Messier and William Herschel described this galaxy as a star cluster. Today, however, the object is known to be a galaxy.
Messier 108 is a barred spiral galaxy in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 or 1782. From the perspective of the Earth, this galaxy is seen almost edge-on.
An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. It is designated as SAB in the galaxy morphological classification system devised by Gerard de Vaucouleurs. Subtypes are labeled as SAB0, SABa, SABb, or SABc, following a sequence analogous to the Hubble sequence for barred and unbarred spirals. The subtype is based on the relative prominence of the central bulge and how tightly wound the spiral arms are.
NGC 5364 is a grand design spiral galaxy located 54.5 million light years away in the constellation Virgo. It is inclined to the line of sight from the Earth at an angle of 47° along a position angle of 25°.
Galaxy Zoo is a crowdsourced astronomy project which invites people to assist in the morphological classification of large numbers of galaxies. It is an example of citizen science as it enlists the help of members of the public to help in scientific research.
NGC 10 is a spiral galaxy located in the southern constellation of Sculptor. It was discovered by John Herschel on 25 September 1834. The galaxy is located at a distance of 346 Mly from the Sun. Its morphological classification in the De Vaucouleurs system is SAB(rs)bc, where the 'SAB' denotes a weak-barred spiral, '(rs)' indicates a slight ring-like structure, and 'bc' means the spiral arms are moderately to loosely wound. Paturel et al. (2003) assigned this galaxy a classification of SBbc, indicating a barred spiral galaxy.
Magellanic spiral galaxies are (usually) dwarf galaxies which are classified as the type Sm. They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy. They can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies.
The following outline is provided as an overview of and topical guide to galaxies:
NGC 266 is a massive barred spiral galaxy in the constellation Pisces, and is located at a distance of 197 megalight-years from the Milky Way. It was discovered on September 12, 1784 by William Herschel. The form of this barred galaxy is described by its morphological classification of SB(rs)ab, which indicates a quasi-ring-like structure (rs) and moderate-to-tightly wound spiral arms (ab). It is the dominant member of a small group with six low-mass galaxies.
