t was in 1926 that the most widely used galaxy classification scheme today, known as the Hubble classification, was published in the Astrophysical Journal. The author was the American astronomer E.P. Hubble.
Hubble recognized three principal form families, and placed galaxies into one of three principal classes: there were elliptical galaxies, spiral galaxies and those with a rather chaotic optical appearance, known as irregular galaxies. Spiral galaxies themselves were separated into two separate families, the normal spirals and those with a central elongated feature, the barred spirals (Figure 111).
The recognition of a bar in spiral galaxies belongs to Heber D. Curtis. Curtis had recognized a class of spirals called the phi-type spirals, but Hubble later suggested that the Greek letter theta better represented this form. To be fair to Hubble, he did consider Curtis' classification to be "easily the most significant that has been proposed up to date  ..." The Curtis phi-class was fully recognized in Hubble's final classification system, but Hubble used the terminology of barred spirals instead. Full credit must be given to the great observer Heber Curtis, for being the first astronomer to recognize bars in spiral galaxies.
"Normal" and "barred" spirals were grouped into three principal morphological classes, of types a, b and c. Hubble used the prefix "S" for normal spiral and "SB" for barred spiral. From the multitudes of shapes and forms of galaxies presented on photographic plates, it became relatively simple to speak of three classes for normal spirals (Sa, Sb and Sc) and the Morphologist Extraordinaire three types of barred spiral galaxies (SBa, SBb, SBc). 183
The classification criteria for the Hubble system are threefold: firstly, the size of the central bulge compared to the flattened disk; secondly, the degree of openness or tightness of the spiral arms, and finally, the degree to which the eye can discern newly-born stars contained in clouds of hydrogen gas (HII regions in technical jargon). In general, Sa galaxies have large bulges, tightly wound spiral arms which are relatively smooth. In contrast, the bulges of Sc galaxies are invariably much smaller or even so minute, as to appear point-like as a star ("semi-stellar"); Sc galaxies spawn wide open spiral arm patterns, knotted with newly born stars flooding their environs with energetic ultraviolet photons of light. Galaxies of type a were spoken of by Hubble as early-type; those of type c, late-type.
In the pages to follow, we explore a fascinating question:
How much did other astronomers, working in the years before Hubble's epochal publication, influence Hubble's ideas on matters morphological?
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