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Science of The Small Magellanic Cloud (SMC)

The LMC and the SMC are close satellite systems to the Milky Way. The latest distance estimates put the LMC at 170,000 light years and the SMC at 195,000 light years.

The SMC is thought to be seen nearly end on, the side facing the Milky Way is estimated to be 10,000 LY across. The galaxy then stretches away from the Milky Way for almost 30,000 LY. Recent studies concerning the distribution of stars in the cloud have revealed that the outermost halo of the galaxy is roughly circular with a diameter of about 40,000 light years.

The visual brightness of the SMC is magnitude 2.2, considering the galaxy's distance of 195,000 light years the absolute magnitude derived is -17. This may seem bright but if you compare the absolute magnitude of the Milky Way at -21 you can conclude that the SMC is quite small as galaxies go. The estimated mass of the SMC ranges between 600,000,000 and 5,000,000,000 solar masses. The latest result of weighing the SMC puts this figure at almost 2,000,000,000 solar masses. The Low Mass and the low absolute magnitude of the SMC class it as a dwarf galaxy.

The SMC is located 20° away from any major dust clouds in the Milky Way and is therefore relatively unobscured. Many foreground stars are present across the face of the cloud. Astronomers are able to distinguish between foreground stars and actual SMC members by comparing radial velocities. The established radial velocity for stars in the SMC is approximately 170 km/sec whereas the radial velocities for galactic stars are 50 km/sec.

Observations of the SMC in the 21-cm line by the Parkes 210-foot radio telescope, show 3 distinct and clearly separated expanding shells of neutral hydrogen. It is believed that these expanding shells of HI are the result of Supernovae in the SMC's past some 10 to 20 million years ago. These observations also revealed a higher proportion of HI in the SMC. Almost 33% of the SMC's mass may be made up from this neutral hydrogen. The SMC is also enveloped in a cloud of neutral hydrogen, and the radio observations show that this cloud is being disturbed by the LMC. Possibly due to a close approach of the 2 galaxies some 2 million years ago a trail of neutral hydrogen also envelops the Milky Way, following the orbital path of the SMC and LMC around our galaxy. This torus of material is called the Magellanic Stream.

The brightest stars in the SMC are approximately 11th magnitude, giving these stars on absolute magnitude of -8. The spectra of these intrinsically bright stars show that they fall between the spectra class of O.B to early A. The red giants observed in the SMC are about 12th magnitude (-7 absolute magnitude) Wolf Rayet stars are also abundant in the SMC peaking at about 14.5 magnitude (-4.5 Absolute magnitude) if the sun were placed in the SMC it would shine at 24th magnitude.

The stellar content of the SMC is considerably different to that of the Milky Way or even the LMC. Population 1 stars, stars that are mainly found in the spiral arms of Galaxies, predominantly dominate the SMC. They are usually young blue giants. Spectroscopic studies of these SMC stars show that they are lacking the heavier elements of carbon, nitrogen and oxygen that are found in our own galaxy.

The chemical composition of galaxies changes with each successive stellar generation. The heavier elements are synthesized inside a star's core by the thermonuclear reactions. These elements are released during supernova explosions, where they are added into the interstellar medium to be incorporated in the next generation of stars. A number of expanding gas shells have been discovered, which may be the result of Supernovae explosions.

The SMC is an ever evolving system. The oldest Globular clusters associated with the galaxy appear to be about 12 billion years old, slightly younger than the Milky Way's 15 billion. While NGC330, an open cluster, appears to be only a few tens of millions of years old. The galaxy is also filled with numerous emission nebulae like NGC346.

The SMC does appear to be less evolved than the Milky Way or the LMC as it contains a much higher fraction of hydrogen gas. The relative rareness of heavier elements and the predominance of young metal-poor stars indicate that the SMC is a less evolved galaxy.

The classification of the SMC has also been difficult to determine. The galaxy was originally classed as an irregular and was used as the yardstick to classify other irregular galaxies. The latest research on the SMC has revised this classification to a Barred Spiral Galaxy of low surface brightness with some peculiar aspects, or SBsm Pec. The peculiar nature is derived from tidal interactions caused by the LMC and the Milky Way. Any attempt to classify the SMC is fraught with trouble as the galaxy is disturbed by these interactions.

A page this size requires many acknowledgements which are too numerous to list here.