They consist of a disc and a smaller bulge of variable size. About half also have well-defined 'bars' near the center, and these are called barred spirals (SBa, SBb, SBc above).Ī few disc galaxies (S0, SB0) do not have any spiral arms and these are called lenticular (or 'lens shaped') galaxies. Most disc galaxies (Sa, Sb, Sc above) also have spiral arms and are called spiral galaxies. (For the same reason an elliptical galaxy may well be a lot flatter in reality than it appears to us.) The greater the viewing angle, the more elliptical they appear (i.e. 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 disc is usually fairly close to being circular in shape. Apparent shapes range from almost circular (E0) to quite elliptical (E6) - these have the long axis four times the short axis.ĭisc 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. All that can usually be detected is a decrease in surface brightness as one move outwards from the center of the galaxy. Below is a diagrammatic representation of one commonly used simple modification of his diagram.Įlliptical galaxies (on the left) look roughly egg-shaped and are relatively featureless. 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. Hubble's original classification of galaxy types was published in 1936 in a book called ' The Realm of the Nebulae'. The Hubble classification, often called the tuning fork diagram, is still used today to describe galaxies. A large central bulge and broad central arms corresponds to a, while a small central bulge and well defined spiral arms corresponds to c. He classified spiral and barred spiral galaxies further according to the size of their central bulge and the texture of their arms. Edwin Hubble invented a classification of galaxies and grouped them into four classes: spirals, barred spirals, ellipticals and irregulars. Galaxies range from 1,000 to 100,000 parsecs in diameter and are usually separated by millions of parsecs. Galaxies come in many different shapes and sizes ranging from dwarf galaxies with as few as 10 7 stars, to giants with 10 12 stars.

Parts of a barred spiral galaxy