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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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Hubble sequence
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The Hubble sequence is a classification of galaxy types developed by Edwin Hubble in 1936. It is also called the tuning-fork diagram due to 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
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.
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. However, there are reasons to believe that elliptical galaxies in general are older than spiral galaxies. For instance, elliptical E galaxies appear redder than S galaxies, which indicates that they consist of older, redder stars and stellar clusters. The fact that S galaxies usually seem bluer and brighter hints at star formation. Since stellar formation requires dust clouds to collapse gravitationally, we may think S galaxies to be younger than E galaxies where all necessary ingredients for star formation has already 'been used up'. Yet it needs to be mentioned that also through intergalactical interaction star formation (Balmer lines) is frequently observed. The all-encompassing evolutionary diagram of galaxies remains one of the unresolved challanges of astronomy today.
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 is related to eccentricity but is defined by
ight) where is the short axis and is the long axis. 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.
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.
From this, astronomers have constructed a theory of galaxy evolution which suggests that ellipticals are, in fact, the result of collisions between spiral and/or irregular galaxies, which strip out much of the gas and dust and randomize the orbits of the stars. See galaxy formation and evolution.
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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).
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 (or Morgan) scheme
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.
So, for exemple, the Andromeda Galaxy is classified as kS5.
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See also
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