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 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.
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 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 borders of Hydra and Centaurus. Nicolas-Louis de Lacaille discovered M83 on 23 February 1752 at the Cape of Good Hope. Charles Messier added it to his catalogue of nebulous objects in March 1781.
Messier 63 or M63, also known as NGC 5055 or the seldom-used Sunflower Galaxy, is a spiral galaxy in the northern constellation of Canes Venatici with approximately 400 billion stars. M63 was first discovered by the French astronomer Pierre Méchain, then later verified by his colleague Charles Messier on June 14, 1779. The galaxy became listed as object 63 in the Messier Catalogue. In the mid-19th century, Anglo-Irish astronomer Lord Rosse identified spiral structures within the galaxy, making this one of the first galaxies in which such structure was identified.
Messier 65 is an intermediate spiral galaxy about 35 million light-years away in the constellation Leo, within its highly equatorial southern half. It was discovered by Charles Messier in 1780. With M66 and NGC 3628, it forms the Leo Triplet, a small close group of galaxies.
NGC 4565 is an edge-on spiral galaxy about 30 to 50 million light-years away in the constellation Coma Berenices. It lies close to the North Galactic Pole and has a visual magnitude of approximately 10. It is known as the Needle Galaxy for its narrow profile. First recorded in 1785 by William Herschel, it is a prominent example of an edge-on spiral galaxy.
NGC 88 is a barred spiral galaxy exhibiting an inner ring structure located about 160 million light years from the Earth in the Phoenix constellation. NGC 88 is interacting with the galaxies NGC 92, NGC 87 and NGC 89. It is part of a family of galaxies called Robert's Quartet discovered by astronomer John Herschel in the 1830s.
NGC 92 is a highly warped interacting unbarred spiral galaxy in Robert's Quartet; it is interacting with three neighbouring galaxies NGC 87, NGC 88 and NGC 89.
NGC 4911 is a disturbed, warped spiral galaxy with a bright prominent central starburst ring and located deep within the Coma Cluster of galaxies, which lies some 300 million light years away in the northern constellation Coma Berenices. NGC 4911 is believed to be interacting with its warped, barred lenticular companion, producing the enhanced star formation and shell-like appearance seen in optical images. The galaxy contains rich lanes of dust and gas near its centre. The existence of clouds of hydrogen within the galaxy indicates ongoing star formation. It is rare for a spiral galaxy to be situated at the heart of a cluster.
NGC 134 is a barred spiral galaxy that resembles the Milky Way with its spiral arms loosely wrapped around a bright, bar-shaped central region. Its loosely bound spiral arms categorize it as Hubble-type Sbc. It is 60 million light years away, and part of the Sculptor constellation.
NGC 428 is a barred spiral galaxy in the constellation of Cetus, with its spiral structure distorted and warped, possibly the result of the collision of two galaxies. There appears to be a substantial amount of star formation occurring within NGC 428 and it lacks well defined arms — a telltale sign of a galaxy merger. In 2015 the Hubble Space Telescope made a close-up shot of the galaxy with its Advanced Camera for Surveys and its Wide Field and Planetary Camera 2. The structure of NGC 428 has been compared to NGC 5645.
ESO 198-13 is a ring galaxy with multiple ring-like structures located about 240 million light-years away in the constellation Eridanus.
NGC 470 is a spiral galaxy in the constellation Pisces. Located approximately 91 million lightyears from Earth, it was discovered by Friedrich Wilhelm Herschel in 1784. The galaxy also weakly interacts with NGC 474.
ESO 269-57 is a large spiral galaxy located about 150 million light-years away in the constellation Centaurus. ESO 269-57 has a diameter of about 200,000 light-years. It is part of group of galaxies known as LGG 342. which is also known as the NGC 5064 Group which is part of the Centaurus Supercluster.
NGC 6040 is a spiral galaxy located about 550 million light-years away in the constellation Hercules. NGC 6040 was discovered by astronomer Édouard Stephan on June 27, 1870. NGC 6040 is interacting with the lenticular galaxy PGC 56942. As a result of this interaction, NGC 6040's southern spiral arm has been warped in the direction toward PGC 56942. NGC 6040 and PGC 56942 are both members of the Hercules Cluster.
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 235-58 is a galaxy in the constellation of Indus.
NGC 2280 is a spiral galaxy located in the constellation Canis Major. It is located at a distance of circa 75 million light years from Earth, which, given its apparent dimensions, means that NGC 2280 is about 135,000 light years across. It was discovered by John Herschel on February 1, 1835. One supernova has been observed in NGC 2280, SN 2001fz, a type II supernova discovered by the Beijing Astronomical Observatory Supernova Survey on November 15, 2001. It had a peak magnitude of 17.4.
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.
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.
