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Galaxy morphological classification
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Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gerard de Vaucouleurs and Allan Sandage.
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The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936.[1] It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented. Hubbles’s scheme divides galaxies into 3 broad classes based on their visual appearance (originally on photographic plates):
- Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky.
- Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol S. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB.
- Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.
These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxy, such as irregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).
The Hubble sequence is often represented in the form of a two-pronged fork, with the ellipticals on the left (with the degree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the two parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”.
To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.
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There is a widely used extension to the Hubble sequence: the de Vaucouleurs extensions. The distinction between the de Vaucouleurs and Hubble classification systems lies primarily with spiral galaxies. While the Hubble type describes spiral galaxies based upon the two criteria of tightness of spiral and barredness, de Vaucouleurs adds a third descriptor, internal ring.
- Spiralness: galaxies range from E, through S0, through the other spirals, to Im.
- Barredness: galaxies are described as being A (ordinary), B (barred), or AB (intermediate).
- Ringedness: galaxies are described as being s-shaped (no ring), r-shaped (ring), or sr (intermediate).
Therefore, a galaxy may be described as being SAB(rs)c - Sc spiral, between barred and ordinary, and between ringed and no ring.
Visually, the de Vaucouleurs system is often represented in three dimensions, with spiralness on the x-axis, barredness on the y-axis, and ringedness on the z-axis. A cross-section of one spiralness (eg: Sb) will yield a representation in two dimensions with ringedness on the x-axis and barredness on the y-axis. Pictures are available here.
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The Yerkes classification involves using the spectrum of the stars in the galaxy and the shape, real and apparent, and the degree of the its central concentration.
| Spectral Type | Explanation |
|---|---|
| a | Prominent A stars |
| af | Prominent A-F stars |
| f | Prominent F stars |
| fg | Prominent F-G stars |
| g | Prominent G stars |
| gk | Prominent G-K stars |
| k | Prominent K stars |
| Galactic Shape | Explanation |
|---|---|
| B | Barred spiral |
| D | Rotational symmetry without pronounced spiral or elliptical structure |
| E | Elliptical |
| Ep | Elliptical with dust absorption |
| I | Irregular |
| L | Low surface brightness |
| N | small bright nucleus |
| S | Spiral |
| Inclination | Explanation |
|---|---|
| 1 | Galaxy is "Face-on" |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
| 6 | |
| 7 | Galaxy is "Edge-on" |
So, for example, the Andromeda Galaxy is classified as kS5.
- ^ Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 36018182.
- Galaxies and the Universe - an introduction to galaxy classification
- Near-Infrared Galaxy Morphology Atlas, T.H. Jarrett
- The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project