The Magellanic Clouds are members of the Local Group of
galaxies. Named in honor of the Portuguese navigator Ferdinand Magellan,
they are visible to Southern Hemisphere observers even with the unaided eye. Until the early 1980s two
galaxies were recognized: the Large Magellanic Cloud (LMC), at a distance of 150,000 light-years; and the Small Magellanic Cloud (SMC), at a distance of 173,000 light-years. Later studies, however, indicated that a third
galaxy, the Mini Magellanic Cloud (MMC), lies behind the SMC and was torn from it about 200 million years ago by a near collision between the LMC and the SMC. Although superimposed in the Earth's sky, the SMC and the MMC are actually separated by a distance of some 20,000 light-years and are continuing to separate.The Large Magellanic Cloud, at declination ç69¡ south of the equator, is in the constellation Doradus and has a diameter of 31,000 light-years. The Small Magellanic Cloud is in Tucana, at declination ç73¡, and has a diameter of 24,000 light-years. Both are classified as Irregular I extragalactic systems but are sometimes considered to be loose barred spirals. The visual magnitudes of 0.9 and 2.5 correspond to absolute magnitudes of ç17.4 and ç16.1 for the LMC and SMC, respectively. The SMC is apparently oriented so as to show maximum surface brightness to Earth viewers, and the galaxy appears to be disintegrating rapidly on a galactic time scale. The LMC contains considerable interstellar dust, but the SMC is relatively dust free. A cloud of hydrogen gas envelops both Magellanic Clouds.Because the Magellanic Clouds are relatively close, the stars and other celestial objects they contain are easier to observe than those in more distant galaxies. This has been one of the main reasons for establishing large telescopes in the Southern Hemisphere. In 1987 astronomers were thereby able to observe closely a rare supernova that occurred in the LMC. The brightest known supergiant stars that have been studied spectroscopically in great detail are in the Magellanic Clouds as well. The brightest stars in these galaxies can be sorted out from intervening stars in our own Galaxy because their radial velocitiesÑthe component of their velocities directly away from us, whatever their velocities as a wholeÑare much greater than the radial velocities of the stars in our Galaxy.The LMC and SMC have both been tidally distorted by the gravitational attraction of our Galaxy. Conversely, in its outer regions, the plane of neutral hydrogen gas that defines the plane of our Galaxy (galactic latitude = 0¡) has been substantially distorted from that plane by the Magellanic Clouds.Because the objects in the Magellanic Clouds are all at essentially the same distance from the Sun, relative sizes and luminosities reflect true differences in sizes and absolute magnitudes. Thus the Magellanic Clouds are ideal regions for studying sizes, absolute magnitudes, and the genetic relationships existing among various types of objects. The period-luminosity relation, the most accurate method for determining large distances in the universe, was discovered in the SMC by Henrietta Leavitt in 1912 from plates taken at the Harvard southern observing station in Peru.