The Small Magellanic Cloud (SMC) is a nearby galaxy appearing in the constellation Tucana. It forms a pair with the Large Magellanic Cloud (LMC), which is located 20 degrees to the east. Both are members of the Local Group, and among the most distant objects that can be seen with the naked eye.
Discovery and History
The Magellanic clouds have long been included in the lore of native southerners, including Pacific islanders and indigenous Australians. Like its larger apparent neighbor, the Large Magellanic Cloud, the Small Magellanic Cloud was probably mentioned by Amerigo Vespucci in a letter written about his third voyage during 1503-4.
European sailors may have first noticed the clouds during the Middle Ages, but their existence only became widely known to the West after Ferdinand Magellan's circumnavigation of the Earth in 1519-22. Johann Bayer's 1603 celestial atlas Uranometria named the smaller cloud "Nubecula Minor", meaning "Little Cloud" in Latin.
Between 1834 and 1838, John Herschel observed of the southern skies from the Cape of Good Hope. He described Nubecula Minor as a cloudy mass of light with an oval shape and a bright center, and catalogued 37 clusters and nebulae within it. Many of these clusters and nebulae were given their own NGC numbers in Dreyer's catalog, and the main body of the Small Magellanic Cloud was assigned NGC 292.
It was in the Small Magellanic Cloud where Henrietta Swan Leavitt discovered the period-luminosity relation of Cepheid variables, in 1908. Since then, this has been the most reliable method available for determining large cosmic distances.
Amateur Observation
To the naked eye, the SMC appears as a detached piece of the Milky Way, a hazy patch covering a 280' x 160' area about 3° across. With a mean declination of -73°, the SMC can only be viewed from the southern hemisphere or lower northern latitudes. The SMC's total visual magnitude of 2.3 makes it the second-brightest external galaxy in the sky (after the LMC). But since the LMC has a very low surface brightness, it is best viewed from a dark site, away from city lights.
The SMC contains several nebulae and star clusters which can be seen through telescopes and in photographs. Our small neighboring galaxy contains the same kinds of objects as our Milky Way; in particular open clusters, diffuse nebulae, supernova remnants, planetary nebulae, and one globular cluster, NGC 121. It is situated well outside the denser regions of the galaxy, slightly north of the foreground globular 47 Tucanae (NGC 104).
Properties, Structure, and Motion
At a distance of about 200,000 light-years, the SMC is the Milky Way's fourth nearest neighbor (after the Sagittarius Dwarf Elliptical discovered in 1994, the Canis Major Dwarf Galaxy, and the LMC). The SMC has a diameter of about 7,000 light-years, and contains several hundred million stars. It has a total mass of approximately 7 billion Suns.
Some speculate that the SMC was once a barred spiral galaxy that was disrupted by the Milky Way, becoming somewhat irregular. It still contains a central bar structure. Hubble Space Telescope observations released in 2007 showed that both the LMC and SMC are moving too fast to be gravitationally bound to the Milky Way. Instead, they are instead simply passing through our galactic neighborhood.