The Magellanic Clouds are named for the explorer Ferdinand Magellan who noted their presence in 1519. The clouds represent a binary system of dwarf irregular galaxies that orbit the Milky Way. The Large Magellanic Cloud (LMC) measures some 20,000 light years across making it the fourth largest member of the Local group followed by the Milky Way, M31, and M33. The LMC is about 1/20th the diameter of our galaxy and contains about 1/10th the number of stars. Although it is an irregular galaxy it has traces of spiral structure and even contains an off-centered bar. Regarding the origin of the LMC one possible scenario is that the LMC was once a barred spiral that became disrupted in a close encounter with the Milky Way.
The LMC is the second nearest neighboring galaxy, the Sagittarius Dwarf galaxy being the nearest. It is full of remarkable objects including the giant HII region 30 Doradus (see 30 Doradus). On February 24, 1987 supernova 1987A occurred in the LMC. It was the nearest observed supernova since the invention of the telescope. The LMC is different than the Milky way in many ways. Massive sequential and triggered star formation is occurring on a large scale in the LMC. It contains a diverse population of young stars and HII regions preferentially located on the edges of supergiant shells of neutral gas formed from the winds of massive stars and supernovae. The internal structure of the LMC is peculiar. It has an off-centered bar with an inner disk that rotates counter to the outer disk. The inner disk whose radius extends some 3 degrees from the center was probably acquired through a merger event in the remote past. The galaxy shows various warps, rings, and tidal features, all indicators of remote and recent encounters with its neighbor, the Milky Way.
The LMC has played an important role in
astronomical research. It served a critical role in establishing
the extragalactic distance scale by allowing calibration of standard
candles such as Cepheid variables. Its proximity has advanced
our knowledge of the astrophysics of objects difficult to observe
in our own galaxy like giant HII regions and super clusters.
The relationship of the LMC to the Milky Way has advanced our knowledge about our own galaxy's history as well as its mass and dark matter content.
The Magellanic clouds are gravitationally
bound to each other and to the Milky Way.
Observations at radio wavelengths show that the two dwarf galaxies contain a remarkable network of neutral gas, enough in total to form 500 million suns. This is in contrast to another Milky Way dwarf galaxy, the Sagittarius Dwarf which contains almost no neutral gas. Both galaxies are embedded in a common envelope of neutral hydrogen gas stretching across several tens of degrees of sky. Several distinct features of the Magellanic clouds are assumed to be relics of past interactions of the clouds with each other and the Milky Way. A gaseous bridge forms an interface between the two galaxies, known as the Magellanic Bridge. This bridge is thought to have formed from the last major interaction between the two Magellanic clouds some 200 million years ago. An impressive filamentary gaseous structure exists which starts at the bridge and extends in a great 300,000 light year arc some 100 degrees across the southern sky where it crosses the south galactic pole of the Milky Way. It contains some 200 million solar masses of material and is known as the Magellanic Stream. This thick filament of neutral hydrogen gas was probably torn from the Magellanic system by the Milky Way some 1.5 billion years ago during a close encounter. In essence the Magellanic Clouds and the Milky Way are a spectacular set of interacting galaxies.