Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. In Greek mythology, Centaurus represents a centaur; a creature that is half human, half horse. Notable stars include Alpha Centauri, the nearest star system to the Solar System, its neighbour in the sky Beta Centauri, and V766 Centauri, one of the largest stars yet discovered. The constellation also contains Omega Centauri, the brightest globular cluster as visible from Earth and the largest identified in the Milky Way, possibly a remnant of a dwarf galaxy.
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.
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.
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.
Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees, and also the longest at over 100 degrees. Its southern end borders Libra and Centaurus and its northern end borders Cancer. It was included among the 48 constellations listed by the 2nd century astronomer Ptolemy. Commonly represented as a water snake, it straddles the celestial equator.
Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
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 1097 is a barred spiral galaxy about 45 million light years away in the constellation Fornax. It was discovered by William Herschel on 9 October 1790. It is a severely interacting galaxy with obvious tidal debris and distortions caused by interaction with the companion galaxy NGC 1097A.
NGC 4261 is an elliptical galaxy located around 100 million light-years away in the constellation Virgo. It was discovered April 13, 1784, by the German-born astronomer William Herschel. The galaxy is a member of its own somewhat meager galaxy group known as the NGC 4261 group, which is part of the Virgo Cluster.
NGC 4889 is an E4 supergiant elliptical galaxy. It was discovered in 1785 by the British astronomer Frederick William Herschel I, who catalogued it as a bright, nebulous patch. The brightest galaxy within the northern Coma Cluster, it is located at a median distance of 94 million parsecs from Earth. At the core of the galaxy is a supermassive black hole that heats the intracluster medium through the action of friction from infalling gases and dust. The gamma ray bursts from the galaxy extend out to several million light years of the cluster.
The type-cD galaxy is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy. Characterized by a large halo of stars, they can be found near the centres of some rich galaxy clusters. They are also known as supergiant ellipticals or central dominant galaxies.
Arp 271 is a pair of similarly sized interacting spiral galaxies, NGC 5426 and NGC 5427, in the constellation of Virgo. It is not certain whether the galaxies are going to eventually collide or not. They will continue interacting for tens of millions of years, creating new stars as a result of the mutual gravitational attraction between the galaxies, a pull seen in the bridge of stars already connecting the two. Located about 130 million light-years away, the Arp 271 pair is about 130,000 light-years across. It was originally discovered in 1785 by William Herschel. It is speculated, that the Milky Way will undergo a similar collision in about five billion years with the neighbouring Andromeda Galaxy, which is currently located about 2.6 million light-years away.
DDO 190 is a dwarf irregular galaxy in the vicinity of the Milky Way, as it is relatively small and lacks clear structure. It is 9.10 million light-years (2.79 Mpc) away from Earth and lies out of the Local Group, determined by the tip of the red giant branch method. The outskirts of the galaxy are harbouring older (reddish) stars, while the centre is crowded with younger (bluish) stars. Heated gas is observed at several places. DDO 190 still experiences some active star formation. The galaxy is categorised as a Magellanic dwarf galaxy of morphological type Im. Its metallicity is [Fe/H] = −1.55 ± 0.12.
ESO 198-13 is a ring galaxy with multiple ring-like structures located about 240 million light-years away in the constellation Eridanus.
ESO 444-46 is a class E4 supergiant elliptical galaxy; the dominant and brightest member of the Abell 3558 galaxy cluster around 640 million light-years away in the constellation Centaurus. It lies within the core of the massive Shapley Supercluster, one of the closest neighboring superclusters. It is one of the largest galaxies in the local universe, and possibly contains one of the most massive black holes known. The black hole's mass is very uncertain, with estimates ranging from as low as 501 million M☉, to as high as 77.6 billion M☉.
ESO 325-G004 is an elliptical galaxy located approximately 416 million light-years away in the constellation Centaurus.
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.
NGC 2293 is a lenticular galaxy located in the constellation Canis Major. It is located at a distance of circa 100 million light years from Earth, which, given its apparent dimensions, means that NGC 2293 is about 160,000 light years across. It was discovered by John Herschel on January 20, 1835. NGC 2293 forms a pair with NGC 2292 and has an HI ring that surrounds both galaxies.
NeVe 1 is a supergiant elliptical galaxy, which is the central, dominant member and brightest cluster galaxy (BCG) of the Ophiuchus Cluster. It lies at a distance of about 411 million light-years away from Earth and is located behind the Zone of Avoidance region in the sky. It is the host galaxy of the Ophiuchus Supercluster eruption, the most energetic astronomical event known.
ESO 383-76 is an elongated, X-ray luminous supergiant elliptical galaxy, residing as the dominant, brightest cluster galaxy (BCG) of the Abell 3571 galaxy cluster, the sixth-brightest in the sky at X-ray wavelengths. It is located at the distance of 200.6 megaparsecs from Earth, and is possibly a member of the large Shapley Supercluster. With a diameter of about 540.89 kiloparsecs, it is one of the largest galaxies known.
