It has a radius of 0.95 arc minutes. The name is derived from the Arabic word rukbah, which means knee.. Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable. It has a mass of about 36 solar masses, which now puts it behind more than a dozen massive stars, including four members of the Arches Cluster. Sagittarius has seven stars brighter than magnitude 3.00 and three stars located within 10 parsecs (32.6 light years) of Earth. The galaxy was discovered on a photographic plate taken for the ESO Atlas in June 1977. NGC 6520 is an open star cluster with an apparent magnitude of 9.0. This may indicate that, like the better known Messier 54 and Omega Centauri, Terzan 5 is not a true globular cluster, but possibly the remaining core of a dwarf galaxy disrupted by the Milky Way in the distant past. It is located about a degree to the east of Messier 8, the more famous and much larger Lagoon Nebula. The neighboring constellations are Aquila, Capricornus, Corona Australis, Indus, Microscopium, Ophiuchus, Scutum, Scorpius, Serpens Cauda and Telescopium. It is also associated with Crotus, the satyr who kept company of the Muses on Mount Helicon. The object is actually a small white dwarf star undergoing a helium flash one of only a handful of examples of such an event ever witnessed by astronomers. Nunki has a faint (magnitude 9.5) companion about 5.2 arc minutes away. It has an apparent size of 4.75 by 4.75. Messier 25 is an open cluster with an apparent magnitude of 4.6, about 2,000 light years from Earth. Image: European Space Agency, A. Riera (Universitat Politecnica de Catalunya, Spain) and P. Garcia-Lario (European Space Agency ISO Data Centre, Spain). NGC 6540 is a globular cluster with an apparent magnitude of 9.30, located at a distance of 17,300 light years from Earth. In mythology, Centaurs were hard drinkers, unreliable, misleading, crude, wild, and adulterous. Sagittarius is usually depicted as a centaur holding a bow and arrow. Sagittarius is the 15th largest constellation in the sky. It is not fully understood how planetary nebulae form such a wide variety of shapes and structures; some appear to be spherical, some elliptical, others shoot material in waves from their polar regions, some look like hourglasses or figures of eight, and others resemble large, messy stellar explosions to name but a few. The cluster was discovered by William Herschel on August 7, 1784. Terzan 7 is the brightest of the six globulars discovered by Agop Terzan in 1968. The centaur is depicted as aiming an arrow toward the heart of the neighbouring constellation Scorpio, represented by the red supergiant star Antares. This galaxy is currently colliding with, and being absorbed by, the Milky Way, which is a monster in size when compared to this tiny one. Observations in 2009 revealed that the cluster contains at least two generations of stars, one with an estimated age of 12 billion years and another about 4.5 billion years old. The Delta Sagittarii system is known in fiction from William R. Forstchens 1969 novel Into the Sea of Stars. It cannot be seen visually, but must be observed in radio, X-ray and infrared bands. The burst was a result of a starquake, during which the magnetar released more energy in a tenth of a second than our Sun does in 100,000 years. The brightest companion star is an F class dwarf with an apparent magnitude of 7.77. Both centaurs and satyrs were well known to be wild, rowdy, lustful creatures that had little respect for authority and proper manners. Because it has an imposing size and energy, Sagittarius A East is believed to be a remnant of a star that came close to the central black hole and was gravitationally compressed.