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Galaxy Classification

Types of Galaxies

Galaxies come in many different shapes and sizes ranging from dwarf galaxies with as few as 107 stars, to giants with 1012 stars. Galaxies range from 1,000 to 100,000 parsecs in diameter and are usually separated by millions of parsecs. Edwin Hubble invented a classification of galaxies and grouped them into four classes: spirals, barred spirals, ellipticals and irregulars. He classified spiral and barred spiral galaxies further according to the size of their central bulge and the texture of their arms. A large central bulge and broad central arms corresponds to a, while a small central bulge and well defined spiral arms corresponds to c. The Hubble classification, often called the tuning fork diagram, is still used today to describe galaxies.

Hubble's original classification of galaxy types was published in 1936 in a book called "The Realm of the Nebulae". Since then several people have suggested modifications and additions to his original scheme, but the basic idea of his "tuning fork diagram" has continued to be useful for astronomers. Below is a diagrammatic representation of one commonly used simple modification of his diagram.

Hubble Tuning Fork diagram. On the left are the Ellipticals - E0, E2 and E5. In the center is S0 (lenticular galaxies), the in between point of ellipticals and spirals. Then there are two branches coming out from S0 - Normal spirals (Sa, Sb, and Sc) and Barred spirals (SBa, SBb and SBc).

Hubble Tuning Fork diagram. Credit: LCO

Elliptical galaxies (on the left) look roughly egg-shaped and are relatively featureless. All that can usually be detected is a decrease in surface brightness as one move outwards from the center of the galaxy. Apparent shapes range from almost circular (E0) to quite elliptical (E6) - these have the long axis four times the short axis.

Disc galaxies have a bulge in the center, which is very similar to an elliptical galaxy, but outside of that they have a thin disc of stars. The disc is usually fairly close to being circular in shape. However, because galaxies are randomly oriented relative to our line of sight, we see most of them tilted, giving them a more or less elliptical shape in the sky, somewhere between face-on and edge-on. The greater the viewing angle, the more elliptical they appear (i.e. a viewing angle of zero means face-on). (For the same reason an elliptical galaxy may well be a lot flatter in reality than it appears to us.)

Most disc galaxies (Sa, Sb, Sc above) also have spiral arms and are called spiral galaxies. About half also have well-defined 'bars' near the center, and these are called barred spirals (SBa, SBb, SBc above).

A few disc galaxies (S0, SB0) do not have any spiral arms and these are called lenticular (or 'lens shaped') galaxies. They consist of a disc and a smaller bulge of variable size. The S0 and SB0 diagrams above are just diagrammatic representations - in practice it is hard to tell lenticular galaxies at various viewing angles from elliptical galaxies (because a face-on lenticular would look like an E0 elliptical, while one inclined at 66 degrees would appear like an E6, for example).

A very few galaxies show no obvious symmetry  and do not fall into any of these categories.  We will simply call all of these irregular galaxies (Irr), although  astronomers have identified many different types which have been given different names.

As already remarked, different astronomers have invented slightly differing versions of the above basic classification, in order to fit in with the particular aspects of galaxies that they have been researching.  The above classification is a basic one which will do very well for work with the LCOGT network.

Hubble originally arranged the different shapes of galaxy in the form of a tuning fork, because he noticed a gradual variation in visual appearance between the different galaxies that he observed. When Hubble devised his classification scheme, he thought that spiral galaxies evolved from elliptical ones. This idea has long since been recognised to be false, but confusingly the terms 'early type' and 'late type' are still often used.  Thus astronomers often refer to an Sa galaxy as an 'early type spiral' and to an Sc galaxy as a 'late type' spiral.

We now know that the tuning fork diagram is an arrangement of galaxies according to their rotation. Spiral galaxies rotate rapidly, while elliptical ones have little or no rotation. We also know that elliptical galaxies probably form as a result of spiral galaxies colliding

As we move along the top prong of the tuning fork from Sa to Sc, or along the bottom from SBa to SBc, the following changes generally occur:

  • the disc to bulge ratio increases,
  • the openness of the spiral arms increases (i.e. the pitch angle increases),
  • individual stars and pink emission nebulae (HII regions) become easier to pick out, and the overall colour of the galaxy gets bluer as the spiral arms contain more young bright bluish stars,
  • the hydrogen gas content of the disc increases