The Magellanic Clouds (see LMC) represent a spectacular binary system of dwarf irregular galaxies that orbit the Milky Way. Similar to other dwarf irregular galaxies they are rich in gas and dust and exhibit higher than normal levels of active star formation. The Magellanic Clouds and the Milky Way have a long history of mutual interaction which have profoundly affected the structure and star forming history of all three galaxies. Galaxy evolution is a complex multifactorial process with star formation as one of the supreme driving forces. Local group galaxies are the only galaxies where faint stars can be resolved and where recent and ancient star formation histories can be elucidated with reasonable accuracy. The Small Magellanic Cloud is extremely well suited to investigation of its star forming history because of its proximity, its lack of a bar or spiral structure leaving it no apparent internal mechanism for global star formation, and like its companion, the LMC, has large gas reservoirs capable of vigorously forming stars.
Astronomers have traced the star forming history of the SMC and have concluded that approximately 50% of the stars that ever formed in the SMC formed prior to 8.4 billion years ago. Similar to the LMC, the SMC then underwent a quiescent period and formed relatively few stars between 8.4 and 3.0 billion years ago. There has been a gradual rise in star formation during the last 3 billion years with bursts occurring at 2.5, 0.4, and 0.06 billion years. The star forming epochs nicely coincide with past perigalactic passages of the SMC close to the Milky Way (MW) providing strong evidence that tidal interactions with the MW represent a significant driving force for star formation in the SMC. The SMC experienced a relatively recent tidal encounter with the LMC some 200 million years ago. At that time a thick bridge of neutral gas formed that now connects the two dwarf galaxies and is referred to as the "Magellanic Bridge. A more ancient encounter with both the MW and LMC occurred some 1.5 billion years ago creating a huge tidal arc of gas that stretches across some 100 degrees of southern sky and is known as the Magellanic Stream (see LMC).
Studies of the gaseous infrastructure of the SMC at radio wavelengths have identified an extraordinarily high number of expanding shell structures. 509 expanding neutral hydrogen shells were identified throughout the SMC, all apparently formed at the same time within the last few million years, indicating a recent period of dramatically escalated star formation. Although most of the shells contain central star clusters or OB associations whose winds and supernovae are assumed to be the drivers of the shells, more than 10% of the shells are peculiarly empty. The empty shells are concentrated in the northwest region of the SMC where a peculiar "chimney feature" is known to exist that may have provided the energy driver for the empty shells. The chimney effect occurs when heated neutral gas is channeled out of a region through conduits formed from spatially aligned shells and filaments which leads to propagation of the gas into distant areas of a galaxy. This phenomenon is known to occur in several more distant galaxies.
The SMC contains some 30 billion stars and
its visible extent is over 10,000 light years.
Optically its structure is rich in HII regions, young star clusters, and supernova remnants, all indicators of sustained active star production over the last few million years. The SMC is an important object in astronomical history as the location in which Henrietta Leavitt discovered the period-luminosity relation of Cepheid variables. Also featured within the field of this image are two foreground Milky Way globular clusters, the larger one being 47 Tucanae, and the smaller NGC 362.