Hubble sequence
The Hubble sequence is a morphological classification scheme for galaxies, which was developed in 1936 by Edwin Hubble and uses limited to date.
In this scheme (also provided tuning fork diagram )
- E0 to E7 for elliptical galaxies with increasing flattening
- Sa to Sc for spiral galaxies; this applies to the subtypes that from a to c each the size ratio of central roundish bulge to the disk decreases.
- The spiral arms are open.
- The proportion of dust and young stars increases.
The individual galaxy types are described in the article galaxy closer
Hubble saw in this representation may be a developmental sequence for galaxies. The researches in recent years have shown that galaxy evolution is much more complex and can not be represented by a simple sequence. Nevertheless, it is still sometimes referred to, is actually incorrect, elliptical galaxies as "early" and spiral galaxies as "late" galaxies.
Problems
Main problem of the Hubble sequence is that Hubble was only a very small amount of galaxies available from the observation technique at that time weak enough (a few hundred), all of which are very close ( very low redshift ). This is based on this scheme a record that does not exhaust the variety of galaxy morphologies and therefore is not representative of the real galaxy population. It is said, therefore, that the original record, and therefore the classification scheme founded on it have a bias, ie defined by Hubble types of galaxies are no reliable description.
Add to that the Hubble sequence is "tailored" to the appearance of galaxies in the optical range. Galaxy morphologies in other spectral regions are usually significantly different. For example, see the Most galaxies in the UV or X-ray range from extremely irregular, as almost exclusively irregularly distributed regions of star formation are visible in this spectral range. Hubble's classification scheme is therefore not suitable for other wavelength ranges. Again, this is a classification scheme of this bias.
These problems led to the fact that the Hubble sequence is now used only to a very limited, despite some efforts to extend the schema, eg by Gérard Henri de Vaucouleurs. A major reason why the Hubble sequence is still used, is the lack of a more modern and better classification scheme for galaxies morphologies. It is difficult to describe galaxy morphologies quantitatively reliable. In contrast, there are indeed reliable quantitative descriptions of galaxy spectra, and therefore the classification of galaxies based thereon has gained greater importance.