Magellanic Clouds (măjˌəlănˈĭk) [key], two dwarf galaxies located in the far southern sky and visible to the unaided eye; they are classified as irregular because they show no definite symmetry or nucleus. The larger of the two, known as the Large Magellanic Cloud (LMC), is located mostly in the constellation Dorado; its angular diameter measures approximately 7°. The Small Cloud (SMC) is almost completely in the constellation Tucana, and measures approx. 4° in diameter. The LMC is some 160,000 light-years from the earth; the SMC, some 200,000 light-years. They are part of the Local Group of galaxies, which includes our own galaxy (the Milky Way) and the Andromeda Galaxy, and are the nearest extragalactic objects.
The Magellanic Clouds, named for the Portuguese navigator Magellan, were first studied in detail by Sir John Herschel in the 19th cent. While studying Cepheid variable stars in the SMC, Henrietta Leavitt discovered (1912) the period-luminosity relation. This relation offered a technique for measuring the distances of stars and galaxies. In Feb., 1987, Supernova 1987A erupted in the LMC. The first supernova visible without a telescope since 1604, this star gradually brightened over the next few months and remains under careful observation as it fades. The diffuse nebulae in both the LMC and the SMC appear to have fewer "metals" (elements heavier than helium); the deficiency is much more pronounced in the SMC.
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