Galaxy classification
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Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). The system of galaxy classification is called the Hubble "tuning fork" diagram, and is the Hubble sequence.
Hubble sequence
The Hubble sequence is a classification of galaxy types developed by Edwin Hubble in 1936. It is also called the tuning-fork diagram as a result of the shape of its graphical representation.
The Hubble "tuning fork" diagram starts from the left with elliptical galaxies as its base. Elliptical galaxies can be named from E0 to E7. E stands for elliptical while the number indicates how oval-shaped the ellipse is with 0 being ball shape (in other words, a giant globular cluster) to 7 being discus shape. Technically speaking, the number is ten times the eccentricity. For example, an E7 galaxy has an eccentricity of 0.7.
After the elliptical galaxies the diagram splits into two branches. The upper branch covers spiral galaxies. It starts off with S0, also called lenticular galaxies. The "S" means spiral, the "0" means no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc. On the same branch are the next 3 types which all have spiral arms. The "S" here also means spiral, but the lower case letter after it tell how wound up the arms are. They range from "a" to "d" having the following meanings:
- Sa - tightly-wound, smooth arms, and a bright central disc
- Sb - better defined spiral arms than Sa
- Sc - much more loosely wound spiral arms than Sb
- Sd - very loose arms, most of the luminosity is in the arms and not the disc
- SBa - a bright center and tight spirals
- SBb - better defined arms than SBa galaxy and are more loosely wound
- SBc - even looser arms, and a much dimmer central portion of the galaxy
Contrary to popular opinion, the galactic tuning fork has nothing to do with the evolution of galaxies. For example, S0 galaxies do not split into two groups, one which turns into regular spirals and one which becomes barred. Likewise, spiral or barred-spiral galaxies do not evolve into ellipticals.
Galaxy types are divided as follows:
- An elliptical galaxy (E0-7) has an ellipsoidal form, with a fairly even distribution of stars throughout. The number gives the degree of eccentricity: E0 galaxies are nearly round, while E7 are greatly flattened. The number indicates only how the galaxy appears on the sky, not its true geometry.
- A lenticular galaxy (S0 and SB0) appears to have a disk-like structure with a central spherical bulge projecting from it, and does not show any spiral structure.
- A spiral galaxy (Sa-d) has a central bulge and an outlying disk containing spiral arms. The arms are centered around the bulge, and vary from tightly wound (Sa) to very loose (Sc and Sd). The latter also have less pronounced central bulges.
- A barred spiral galaxy (SBa-d) has a similar sort of spiral structure to spiral galaxies, but instead of emanating from the bulge, the arms project out from the ends of a "bar" running through the bulge, like ribbons on either end of a baton. Again, SBa to SBd refer to how "tightly wound" these arms are.
- An irregular galaxy (Irr) can be of type Irr-I, which shows spiral structure but is deformed in some way, and Irr-II for any other galaxy that does not fit into another category.
| Galaxy Type | Mass (Solar Masses) | Luminosity (Solar Luminosity) | Diameter (kpc) | Stellar Populations | Percentage of Observed Galaxies |
|---|---|---|---|---|---|
| Spiral / Barred Spiral | 109 to 1011 | 108 to 1010 | 5-250 | disk: Population I halo:Population II | 77% |
| Elliptical | 105 to 1013 | 105 to 1011 | 1-205 | Population II | 20% |
| Irregular | 108 to 1010 | 107 to 109 | 1-10 | Population I | 3% |
Hubble based his classification on photographs of the galaxies through the telescopes of the time. He originally believed that elliptical galaxies were an early form, which might have later evolved into spirals; our current understanding suggests that the situation is roughly opposite, however, this early belief left its imprint in the astronomers' jargon, who still speak of "early type" or "late type" galaxies according to whether a galaxy's type appears to the left or to the right in the diagram.
More modern observations of galaxies have given us the following information about these types:
- Elliptical galaxies are generally fairly low in gas and dust, and are composed mostly of older stars.
- Spiral galaxies generally have plentiful supplies of gas and dust, and have a broad mix of older and younger stars.
- Irregular galaxies are fairly rich in gas, dust, and young stars.
| Name | Right Ascension | Declination | Hubble Type |
|---|---|---|---|
| M49 (NGC 4472) | 12h 29.8m | 8° 00' | E4 |
| M59 (NGC 4621) | 12h 42.0m | 11° 39' | E3 |
| M60 (NGC 4649) | 12h 43.7m | 11° 33' | E1 |
| M84 (NGC 4374) | 12h 25.1m | 12° 53' | E1 |
| M86 (NGC 4406) | 12h 26.2m | 12° 57' | E3 |
| M89 (NGC 4552) | 12h 35.7m | 12° 33' | E0 |
| M110 (NGC 205) | 00h 40.4m | 41° 41' | E6 |
The de Vaucouleurs system
There is an extension to the Hubble sequence that widely used: 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).
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].
See also
External links
- [Galaxies and the Universe] - an introduction to galaxy classification
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