Astron. Astrophys. 319, 747-756 (1997)
2. Traditional morphologists' classification
Most of our present knowledge of galaxy morphology is based on the
pioneering works of a few observers who classified thousands of
galaxies. The observational material available for the classification
(better material allows a more detailed scheme) and the aims of the
classification work generally govern the choice of the classification
scheme. The details of these schemes depend on the authors;
nevertheless, some points are common to most authors.
- The morphological 'system is usually defined by a set of
standards or prototypes' (Buta 1990), and the galaxies are classified
according the resemblance to these standards (although it is not
always the case; see e.g. Kormendy 1979, Kennicutt 1981 or Schombert
- The observational material used for classifying galaxies is very
often photographic plates (or copies of them). Historical reasons and
the large angular extent of plates with respect to CCDs made this
choice obligatory for large samples of galaxies as well as for
galaxies larger than a few arcmin.
- Almost all galaxies are on the same plate or on very similar
plates exposed under similar observing conditions (seeing, sky level,
etc.). The observational data are thus uniform, i.e. the quality of
the observational material is the same for the whole sample.
- Structural components (disk, bar, etc.) are not measured by most
morphologists (Hubble's definition of types does not require such a
measurement), but only visually estimated. Up to now, it was not
reasonable, in terms of computer time, to measure such structural
components for large samples of galaxies: morphologists take 30 sec.
to classify a galaxy (Naim et al. 1995), whereas, for example, the
determination of the Hubble type by means of structural components
(see Sect. 5) may reasonably take 30 min. per galaxy.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998