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.
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The "tuning fork" system
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, also called lenticular galaxies. It starts off with S0. The "S" means lenticular, 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 lenticular, 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
The lower branch of the diagram covers barred spiral galaxies given the symbol "SB". This branch starts with SBO galaxies which is followed by a subscript number that indicates how heavily defined the bar is. After that the branch continues with the SB galaxies which have lower case letters after them that indicates how heavily defined the bar is. They range from "a" to "c" having the following meanings:
- 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
The Milky Way Galaxy is now believed to be an SBb galaxy; previously, it was thought to be Sb like its giant companion, the Andromeda Galaxy.
The de Vaucouleurs system
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There is an extension to the Hubble sequence that is in wide use; these are 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.
See also
External links
- Galaxies and the Universe - an introduction to galaxy classification