In the nearly 100 years since galaxy morphology became a topic of research, much has been learned about galactic structure and dynamics. Known only as "nebulae" at that time, galaxies were found to have a wide range of largely inexplicable forms whose relations to one another were a mystery. As data accumulated, it became clear that galaxies are fundamental units of matter in space, and an understanding of how they formed and evolved became one of the major goals of extragalactic studies. Even in the era of space observations, galaxy morphology continues to be the backbone of extragalactic research as modern instruments provide information on galactic structure across a wide range of distances and lookback times.
In spite of the advances in instrumentation and the explosion of data, classical galaxy morphology (i. e., the visual morphological classification in the style of Hubble and others) has not lost its relevance. The reasons for this are as follows:
1. Morphology is still a logical starting point for understanding galaxies. Sorting galaxies into their morphological categories is similar to sorting stars into spectral types, and can lead to important astrophysical insights. Any theory of galaxy formation and evolution will have to, at some point, account for the bewildering array of galactic forms.
2. Galaxy morphology is strongly correlated with galactic star formation history. Galaxies where star formation ceased gigayears ago tend to look very different from those where star formation continues at the present time. Classical morphology recognizes these differences in an ordered way.
3. Information on galaxy morphology, in the form of new types of galaxies, multi-wavelength views of previously known galaxy types, and higher resolution views of all or part of some galaxies, has exploded as modern instrumentation has superceded the old photographic plates that were once used exclusively for galaxy classification.
4. Galaxy classification has gone beyond the realm of a few thousand galaxies to that of a million galaxies through the Galaxy Zoo project. Not only this, but GalaxyZoo has taken morphology from the exclusive practice of a few experts to the public at large, thus facilitating citizen science at its best. Galaxy Zoo images are also in color, thus allowing the recognition of special galaxy types and features based on stellar populations or gaseous emission.
5. Finally, deep surveys with the Hubble Space Telescope have extended morphological studies well beyond the realm of the nearby galaxies that dominated early catalogues, allowing detailed morphology to be distinguished at unprecedented redshifts.
Now, more than ever, galaxy morphology is a vibrant subject that continues to provide surprises as more galaxies are studied for their morphological characteristics across the electromagnetic spectrum. It is clear that a variety of effects are behind observed morphologies, including environmental density and merger/interaction history, internal perturbations, gas accretions, nuclear activity, secular evolution, as well as the diversity in star formation histories, and that a global perspective based on large numbers of galaxies will improve theoretical models and give a more reliable picture of galactic evolution.
The goal of this article is to present the phenomenology of galaxy morphology in an organized way, and highlight recent advances in understanding what factors influence morphology and how various galaxy types are interpreted. The article is a natural follow-up to the excellent review of galaxy morphology and classification by Sandage (1975) in Volume IX of the classic Stars and Stellar Systems series. It also complements the recently published de Vaucouleurs Atlas of Galaxies (Buta, Corwin, & Odewahn 2007, hereafter the dVA), which provided a detailed review of the state and technique of galaxy classification up to about the year 2005. Illustrations are very important in a review of this nature, and the article draws on a large number of sources of images. For this purpose, the Sloan Digital Sky Survey (SDSS), the NASA/IPAC Extragalactic Database (NED), and the dVA have been most useful.