Published online by Cambridge University Press: 25 May 2016
The origin of the Hubble sequence remains a long-standing puzzle in astronomy. Giant galaxies range from slowly-rotating dense ellipticals to thin late-type spiral disks. At the faint end, there are two distinct classes of “ellipticals/spheroids” that are easily separated in plots of nearly any two of their properties, such as central surface brightness versus luminosity (Ferguson and Binggeli 1994; Kormendy 1985). The elliptical class includes the bright giants and extends to the rare high surface brightness “dwarf ellipticals”, M32 being the prototype. The “spheroidal” galaxies have low surface brightnesses and are all ≳3 magnitudes fainter than L∗, the characteristic break in the luminosity function. The dwarf spheroidal galaxies (dSph) in our Local Group of galaxies with magnitudes in the range −8≳MB≳ - 12 are often considered to be the low luminosity extreme of this sequence, but nearly all other known galaxies in this class reside in clusters.