Galaxy Morphology and Classification

Yogi Berra once said that ‘you can observe a lot by just watching’. The truth of this aphorism struck me when the first prints of the Palomar Sky Survey started to arrive at the Universitäts Sternwarte in Göttingen, where I was a graduate student in 1955. Just looking at this marvellous atlas immediately showed a number of interesting things that had not been so obvious on the smaller, and less homogeneous, databases that had previously been available: (1) The most luminous galaxies in clusters are ‘pretty’ because they have long well-defined spiral arms, whereas ‘ugly’ spirals of lower luminosity tend to exhibit short patchy arms. (2) Intrinsically faint galaxies generally have lower surface brightnesses than do luminous ones. (3) Galaxies in rich clusters sometimes exhibit peculiarities, like fuzzy spiral arms, that are rare among isolated field galaxies.

In the present volume, which is based on a series of lectures given at the University of Victoria in early 1997, I have tried to provide an up-to-date summary of current ideas on the morphology [morphe = shape] and classification of galaxies. I am indebted to Roberto Abraham for suggesting that I write this review. I also thank Ralf Bender, Scott Tremaine and Stephen Zepf for discussions on the interpretation of the classification of elliptical galaxies, and Guy Worthey and Masafumi Noguchi for discussion of the abundance ratio of elements to iron in normal and barred spirals.