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Hubble Galaxy Classification
The concept of an S0 galaxy was introduced to astronomy in 1936, when Edwin Hubble introduced the "tuning fork" galaxy classification system, so called because its visual representation resembles a two-pronged tuning fork. The handle of the fork is composed of the elliptical galaxies, ranging from the nearly-round E0 at the end to the wildly oblong E7. The two prongs represent the range of unbarred and barred spiral galaxies, Sa to Sc and SBa to SBc, respectively. At the place where the three lines meet on the diagram are the S0 or lenticular galaxies.
Subdivisions
Though they only occupy one point on the tuning fork diagram, two astronomers have further classified S0 galaxies into subdivisions. According to de Vaucouleurs, S0- galaxies have traces of structure and perhaps a small nucleus; S0° galaxies have a distinct nucleus and envelope with weak traces of a ring at the outer edge of the lens; and S0+ galaxies have a well-defined ring that separates the nuclear bulge from the outer structure. Sandage classifies S0 galaxies into S01, which have flat outer envelopes; S02, which have circular absorption patterns; and S03, which have strong internal rings.
Examples
Also known as the Spindle Galaxy, NGC 3115 was discovered by William Hershel in 1787. From Earth, it is seen almost exactly edge-on and has been misclassified as an elliptical galaxy. MCG-6-30-15 is 130 million light years away from Earth. Its center is a spinning black hole as big around as the orbit of Mars.
Origin Theories
There are two main theories regarding the formation of S0 galaxies. The first is that they are former spiral galaxies that simply used up all their gas for star formation and is now collapsing into an elliptical galaxy. The second is that they are formed by two spiral galaxies merging together, which accounts for S0 galaxies' high frequency of globular clusters.