Hubble sequence
The Hubble sequence is a morphological ordering scheme for galaxies that was developed by Edwin Hubble in 1936 and is still used to a limited extent today.
In this scheme (also tuning fork diagram ) stand
- E0 to E7 for elliptical galaxies with increasing flattening,
-
Sa to Sc for spiral galaxies ; The following applies to the subtypes that from a to c
- the size ratio of the central round bulge to the disk decreases,
- the spiral arms become more open and
- the proportion of dust and young stars increases,
- SBa to SBc for barred spiral galaxies and
- Ir or Irr for irregular galaxies .
- S0 and SB0 are the transition types of the lenticular (or lenticular) galaxies.
The individual galaxy types are described in more detail in the article Galaxie .
Hubble saw in this representation a possible evolutionary sequence for galaxies. Research in recent years has shown, however, that the evolution of galaxies is far more complex and cannot be represented by a simple sequence. Nevertheless, one sometimes still calls, actually incorrectly, elliptical galaxies as "early" and spiral galaxies as "late" galaxies.
Problems
The main problem with the Hubble sequence is that Hubble only had a very small amount of galaxies available (a few hundred) due to the insufficiently adequate observation technology at the time, all of which are very close (very low redshift ). As a result, this scheme is based on a data set that does not exhaust the diversity of the galaxy morphologies and is therefore not representative of the actual galaxy population . It is therefore said that the original data set and thus the classification scheme based on it show a bias , ie the galaxy types defined by Hubble are not a reliable description.
In addition, the Hubble sequence is “tailored” to the appearance of galaxies in the light visible to the human eye. Galaxy morphologies in other spectral ranges are usually clearly different from this. For example, most galaxies look extremely irregular in the UV or X-ray range , since in this spectral range, almost exclusively irregularly distributed star formation regions are visible. Hubble's classification scheme is therefore not suitable for other wavelength ranges. Again, this is a bias of this classification scheme.
These problems resulted in very limited use of the Hubble sequence nowadays, despite some efforts to extend the scheme, e.g. B. 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 galaxy morphologies. It is difficult to quantify them reliably. In contrast, there are reliable quantitative descriptions of galaxy spectra , which is why the classification of galaxies based on this has become more important.
Examples
| Type | Name of the example galaxy |
Right ascension | declination |
|---|---|---|---|
| E0 | M89 (NGC 4552) | 12h 35m 39,8073s | + 12 ° 33 '22.829 " |
| E1 | M105 (NGC 3379) | 10h 47m 49,600s | + 12 ° 34 '53.90 " |
| E2 | M60 (NGC 4649) | 12h 43m 39.66s | + 11 ° 33 '09.4 " |
| E3 | M86 (NGC 4406) | 12h 26m 11.743s | + 12 ° 56 '46.40 " |
| E4 | NGC 1700 | 04h 56m 56.33s | −04 ° 51 '56.7 " |
| E5 | M59 (NGC 4621) | 12h 42m 02.32s | + 11 ° 38 '48.9 " |
| E6 | NGC 821 | 02h 08m 21.14s | + 10 ° 59 '41.7 " |
| E7 | |||
| E-S0 | NGC 4379 | 12h 25m 14.7s | + 15 ° 36 '28 " |
| S0 | NGC 3 | 0h 7m 16.8s | + 8 ° 18 '5.9 " |
| SB0 | NGC 4386 | 12h 24m 77.7s | + 75 ° 31 '46 " |
| Sat | NGC 11 | 0h 8m 42.5s | + 37 ° 26 '52.5 " |
| Sb | M81 (NGC 3031) | 09h 55m 33.17s | + 69 ° 03 '55.1 " |
| Sc | M74 (NGC 628) | 01h 36m 41.77s | + 15 ° 47 '00.5 " |
| SBa | NGC 23 | 00h 09m 53.41s | + 25 ° 55 '25.6 " |
| SBb | M58 (NGC 4579) | 12h 37m 43.5223s | + 11 ° 49 '05.488 " |
| SBc | NGC 7 | 0h 8m 20.9s | −29 ° 54 '53.6 " |
| in the | NGC 4376 | 12h 25m 18.1s | + 05 ° 44 '28 " |
