The Large Magellanic Cloud (LMC) is a nearby irregular galaxy, once thought to be a satellite of our own. It is visible in the night sky from the southern hemisphere, on the border between the constellations Dorado and Mensa.
History and Discovery
The LMC and its apparent neighbor, the Small Magellanic Cloud, were certainly known to ancient southerners. But the first recorded mention of the Large Magellanic Cloud was by Persian astronomer Abd Al-Rahman Al Sufi, in 964 A.D. In his Book of Fixed Stars, he called it "Al Bakr", the "White Ox", and pointed out that while invisible from Northern Arabia and Baghdad, it could be seen from the Strait of Babd al Mandab (at 12° 15' north latitude).
The next recorded observation was by Amerigo Vespucci in a letter written about his third voyage during 1503-4. In this letter he mentioned "three Canopes, two bright and one obscure"; Amerigo's bright "Canopes" are thought to be the Magellanic Clouds, while the obscure one was probably the Coalsack dark nebula.
Ferdinand Magellan was the first to bring the LMC into common Western knowledge, on his voyage of 1519. The galaxy now bears his name.
Amateur Observation
The LMC is is visible as a faint "cloud" in the night sky from the southern hemisphere, looking like a separated piece of the Milky Way to the naked eye. It has a total visual magnitude of 0.1, spanning an area of 650' x 550' between Dorado and Mensa.
The LMC is full of interesting objects, especially the Tarantula Nebula (NGC 2070) a giant diffuse nebula. Surveys of the galaxy have found roughly 60 globular clusters, 400 planetary nebulae, and 700 open clusters, along with hundreds of thousands of giant and supergiant stars. Robert Burnham described the LMC as an "astronomical treasure-house, a great celestial laboratory for the study of the growth and evolution of the stars."
On February 24, 1987, supernova 1987A occurred in the Large Magellanic Cloud. This was the most recent supernova visible to the naked eye, and nearest observed since the invention of the telescope. This peculiar type II supernova was one of the most interesting events for astrophysicists of the 20th century.
Properties and Structure
Using Cepheid variable observations and other methods, the distance to the LMC has been measured at 157,000 light years. The LMC was long thought to be the nearest external galaxy, until 1994, when the Sagittarius Dwarf Elliptical Galaxy was discovered. In fact, the LMC is the third closest galaxy to the Milky Way, after the Sagittarius Dwarf Elliptical (52,000 light years away) and Canis Major Dwarf Galaxy (42,000).
The LMC has a diameter of about 14,000 light-years. It contains about 10 billion solar masses, making it roughly 1/10 as massive as the Milky Way. The LMC is the fourth largest galaxy in the Local Group, after the Andromeda Galaxy (M 31), our own Milky Way, and the Triangulum Galaxy (M 33).
While the LMC is often considered an irregular galaxy, it contains a very prominent warped bar at its center, and a spiral arm tipped at an inclination of 35° (a face on galaxy has an inclination of 0°). The LMC may have previously been a barred spiral galaxy, whose irregular appearance was caused by tidal interactions with both the Milky Way, and the Small Magellanic Cloud.
Origin and Evolution
Like many irregular galaxies, the LMC is rich in gas and dust, and it is currently undergoing vigorous star formation activity. It contains the Tarantula Nebula, the most active star-forming region in the Local Group; as well as "blue populous" clusters, which resemble compact, young globulars of a type unseen in our own galaxy. An absence of intermediate-age clusters suggests that the LMC has experienced several bursts of star formation, rather than forming stars continuously like the Milky Way.
For more than a century, both Magellanic Clouds were thought to orbit the Milky Way. However, in 2007, astronomers using the Hubble Space Telescope determined that the Magellanic Clouds were moving too fast to be gravitationally bound to our galaxy. Instead, the Magellanic Clouds are recent visitors to our neighborhood, arriving only 1 - 3 billion years ago. The measurements also suggested that the clouds are not even bound to each other, which may explain why they did not merge long ago. These results forced astronomers to revisit their understanding of the history and evolution of both the Magellanic Clouds and the Milky Way itself.
From a viewpoint inside the LMC, the Milky Way would be a spectacular sight as it passed by - over 14 times brighter than the LMC appears to us, and spanning 36° of sky (the width of over 70 full Moons!) Furthermore, because of the LMC's low galactic latitude, an observer there would get an oblique view of our entire galaxy, free from the interstellar dust which makes studying the Milky Way so difficult from Earth. The Small Magellanic Cloud would appear at magnitude 0.6, substantially brighter than it appears from here.