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.
A starburst galaxy is one undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formation rate of the Milky Way galaxy is approximately 3 M☉/yr, while starburst galaxies can experience star formation rates of 100 M☉/yr or more. In a starburst galaxy, the rate of star formation is so large that the galaxy will consume all of its gas reservoir, from which the stars are forming, on a timescale much shorter than the age of the galaxy. As such, the starburst nature of a galaxy is a phase, and one that typically occupies a brief period of a galaxy's evolution. The majority of starburst galaxies are in the midst of a merger or close encounter with another galaxy. Starburst galaxies include M82, NGC 4038/NGC 4039, and IC 10.
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 3982, also known as UGC 6918, is an intermediate spiral galaxy approximately 68 million light-years away in the constellation Ursa Major. It was discovered by William Herschel on April 14, 1789, and misclassified as a planetary nebula. NGC 3982 is a part of the M109 Group.
The Sculptor Galaxy is an intermediate spiral galaxy in the constellation Sculptor. The Sculptor Galaxy is a starburst galaxy, which means that it is currently undergoing a period of intense star formation.
I Zwicky 18 is a blue compact dwarf galaxy located about 59 million light years away in the constellation Ursa Major. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies.
NGC 1569 is a dwarf irregular galaxy in Camelopardalis. The galaxy is relatively nearby and consequently, the Hubble Space Telescope can easily resolve the stars within the galaxy. The distance to the galaxy was previously believed to be only 2.4 Mpc. However, in 2008 scientists studying images from Hubble calculated the galaxy's distance at nearly 11 million light-years away, about 4 million light-years farther than previous thought, meaning it is a member of the IC 342 group of galaxies.
NGC 1705 is a peculiar lenticular galaxy and a blue compact dwarf galaxy (BCD) in the southern constellation of Pictor, positioned less than a degree to the east of Iota Pictoris, and is undergoing a starburst. With an apparent visual magnitude of 12.6 it requires a telescope to observe. It is estimated to be approximately 17 million light-years from the Earth, and is a member of the Dorado Group.
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 1309 is a spiral galaxy located approximately 120 million light-years away, appearing in the constellation Eridanus. It is about 75,000 light-years across, and is about 3/4s the width of the Milky Way. Its shape is classified as SA(s)bc, meaning that it has moderately wound spiral arms and no ring. Bright blue areas of star formation can be seen in the spiral arms, while the yellowish central nucleus contains older-population stars. NGC 1309 is one of over 200 members of the Eridanus Group of galaxies.
NGC 5253 is an irregular galaxy in the constellation Centaurus. It was discovered by William Herschel on 15 March 1787.
NGC 4214 is a dwarf barred irregular galaxy located around 10 million light-years away in the constellation Canes Venatici. NGC 4214 is a member of the M94 Group.
NGC 2366 is a Magellanic barred irregular dwarf galaxy located in the constellation Camelopardalis.
NGC 1427A is an irregular galaxy in the constellation Fornax. Its distance modulus has been estimated using the globular cluster luminosity function to be 31.01 ± 0.21 which is about 52 Mly. It is the brightest dwarf irregular member of the Fornax cluster and is in the foreground of the cluster's central galaxy NGC 1399.
NGC 4449, also known as Caldwell 21, is an irregular Magellanic type galaxy in the constellation Canes Venatici, being located about 13 million light-years away. It is part of the M94 Group or Canes Venatici I Group that is relatively close to the Local Group hosting our Milky Way galaxy.
The NGC 5679 group, also known as Arp 274, is a triplet of galaxies, MCG+1-37-36, MCG+1-37-35 and MCG+1-37-34, spanning about 200000 light-years and at some 400 million light-years from Earth in the constellation Virgo. Arp 247 refers to the Atlas of Peculiar Galaxies, compiled by Halton Arp in 1966. Galaxies 269 through 274 in his catalogue are galaxies that appear to have connected arms.
IC 2233, also known as UGC 4278, is a spiral galaxy lying in the constellation of Lynx. IC 2233 is located between 26 and 40 million light-years away from Earth. A comparatively quiet galaxy with a low rate of star formation, it was long thought to be possibly interacting with the Bear's Paw galaxy. However, this is now considered highly unlikely as radio observations with the Very Large Array showed the two galaxies lie at different distances. This galaxy was discovered by British astronomer Isaac Roberts in 1894.
NGC 660 is a peculiar and unique polar-ring galaxy located approximately 45 million light-years from Earth in the Pisces constellation. It is the only such galaxy having, as its host, a "late-type lenticular galaxy". It was probably formed when two galaxies collided a billion years ago. However, it may have first started as a disk galaxy that captured matter from a passing galaxy. This material could have, over time, become "strung out" to form a rotating ring.
NGC 2276 is an intermediate spiral galaxy in the constellation Cepheus. The galaxy lies 120 million light-years away from Earth. NGC 2276 has an asymmetrical appearance, most likely caused by gravitational interactions with its neighbor, elliptical galaxy NGC 2300. One of the many starburst spiral arms contains an intermediate mass black hole with 50,000 times the mass of the Sun, named NGC 2276-3c. NGC 2276-3c has produced two jets: a large-scale radio jet, approximately 2,000 light years long, and an "inner jet" about 6 light years long. The galaxy shows an enhanced rate of star formation that may have been triggered by a collision with a dwarf galaxy, or by the gravitational interaction with its neighbor compressing gas and dust.
NGC 5264, also known as DDO 242, is an irregular galaxy in the constellation Hydra. It is part of the M83 subgroup of the Centaurus A/M83 Group, located some 15 million light years away. The galaxy was discovered on 30 March 1835 by John Herschel, and it was described as "very faint, pretty large, round, very little brighter middle" by John Louis Emil Dreyer, the compiler of the New General Catalogue.
