Introduction
As one of the first steps towards a coherent theory of galaxy evolution, the American astronomer Edwin Hubble, developed a classification scheme of galaxies in 1926. Although this scheme, also known as the Hubble Tuning Fork Diagram, is now considered somewhat over simplified, the basic ideas still hold and the nomenclature is still used today. Galaxy evolution is a far more complex process than Hubble imagined, involving the conditions of the galaxy's initial collapse, collisions with other galaxies, and the ebb and flow of internal star birth.
Click image for full-size version.
Hubble Tuning Fork Diagram. (Source: HubbleSite.)
Classification
The diagram is roughly divided into two parts: elliptical galaxies (ellipticals) and spiral galaxies (spirals). Hubble gave the ellipticals numbers from zero to seven, which characterize the ellipticity of the galaxy - "E0" is almost round, "E7" is very elliptical.
The spirals were assigned letters from "a" to "c," which characterize the compactness of their spiral arms. "Sa" spirals, for example, are tightly wound whereas "Sc" spirals are more loosely wound. It is worth noting that the sizes of the round central regions in spirals - the so-called bulges - increase in size the more tightly the spiral arms are wound. There are indications pointing to a very close connection between the bulges of certain galaxies (Hubble types "S0", "Sa" and "Sb") and elliptical galaxies. They may very well be similar objects.
Spiral galaxies are sub-divided into two groups - normal spirals and barred spirals. The most important difference between these two groups is the bar of stars that runs through the central bulge in barred spirals. The spiral arms in barred spirals usually start at the end of the bar instead of from the bulge. Barred spirals have a "B" in their classification. An "SBc" is thus a loosely wound barred spiral galaxy. "S0," or lenticular galaxies, are in the transition zones between ellipticals and spirals and bridge these two types.
For a time the Hubble tuning fork was thought to be an evolutionary sequence - that galaxies might evolve from one type to another progressing from left to right across the tuning-fork diagram. Hence "Sa" and "SBa" galaxies were called "early-type," while "Sc" and "SBc" were called "late-type."
Hubble found that some galaxies are difficult to put in the context of the tuning fork diagram. Those include irregular galaxies which have odd shapes, dwarf galaxies which are very small, and giant elliptical galaxies which are very large elliptical galaxies residing in the centers of some clusters of galaxies.
A newer and more detailed version of the Hubble Tuning Fork.
(Source: NASA/JPL-Caltech/K. Gordon (Space Telescope Science Institute) and SINGS Team.)
In the composite image above, galaxies are organized by shape, according to the Hubble-Tuning Fork. In this structure, elliptical galaxies sit on the left side of the poster, creating the tuning fork's handle. They are designated by the letter "E", and given a number from zero to seven. An "E0" galaxy looks round, while an E7 galaxy is very long and thin. Spiral galaxies are located to the right side of the poster creating the fork's two prongs. The top prong is made up of regular spiral galaxies, and identified by the letter "S." Barred spiral galaxies make up the bottom prong, and are branded "SB." Meanwhile, letters – "a", "b", and "c" – indicate how tightly the spiral arms are wound. An "Sa" galaxy's arms are wound very tightly, while an "Sc" galaxy's spiral arms are very loosely wound.
Irregular galaxies are organized on bottom-left side of the poster because they were not represented in Hubble's original Tuning Fork.
The galaxies in this poster are three-color composites where blue depicts the galaxies at a light wavelength of 3.6 microns, while 8.0 microns is green, and 24 microns is red. Blue colors reveal light from an older population of stars. Tints of green represent organic molecules called polycyclic aromatic hydrocarbons, while red lumps show clouds of warm dust and gas heated by radiation from newborn stars. These galactic portraits were collected as part of the Spitzer Infrared Nearby Legacy project (SINGS),
Preview Image
"The Hubble Tuning Fork" from "Starry Bulges Yield Secrets to Galaxy Growth," HubbleSite News Release Number: STScI-1999-34, October 6, 1999.
Disclaimer: This article is taken wholly from, or contains information that was originally published by, HubbleSite. Topic editors and authors for the Encyclopedia of the Cosmos may have edited its content or added new information. The use of information from HubbleSite should not be construed as support for, or endorsement by, that organization for any new information added by EoC personnel, or for any editing of the original content.
Original content retrieved from "The Hubble Tuning Fork — Classification of Galaxies" from "Starry Bulges Yield Secrets to Galaxy Growth," HubbleSite News Release Number: STScI-1999-34, October 6, 1999.
Citation
Wallace, Matthew. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Hubble Classification - Hubble Tuning Fork Diagram." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published March 14, 2009.].
<http://www.cosmosportal.org/articles/view/138224/>
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