Galaxies come in all shapes and sizes, but astronomers divide them into just a few main types. More than half of all galaxies are elliptical, named because of their egg-like shape. They are made up of large numbers of old, red stars and have very little gas and dust with which to make new ones. Around a third of all galaxies are spiral in shape, like the Milky Way. Old stars are packed tightly in their centres, while new stars are continually being born from the large amounts of gas and dust in their spiral arms. Some spiral galaxies are classed separately because their nucleus is elongated into a bar. This bar is made up of stars in motion, and the spiral arms extend from the ends of the bar. Some galaxies cannot be classed as either spirals or ellipticals because they have no recognizable shape. These galaxies are called irregulars, and are full of gas in which new stars are forming.
In our universe are four identified types of galaxies. These galaxies are composed of stars, gas and dust held together with gravitational forces. Each galaxy type is named for its shape, making it easy to identify which type of galaxy you are looking at. You may already be familiar with the galaxies: spiral, barred spiral, elliptical and irregular. These fascinating galaxies were each formed from a unique mixture of gases and matter that came together, painting our magnificent galaxies, each one a unique work of artistic beauty.
Spiral Galaxy – Spiral galaxies are characterized by a distinct flattened spiral disk with a bright center called the nucleus. Our own Milky Way is a spiral galaxy. Spiral galaxies are represented by the letter S and are divided into four subgroups. These are S0, Sa, Sb, and Sc. S0 galaxies have a bright nucleus but have no spiral arms. Sa galaxies have spiral arms that are wound tightly around the nucleus while the arms of Sc galaxies are wound much more loosely.
Barred Spiral Galaxy – A barred spiral galaxy is very similar to a spiral with one important difference. The arms spiral out from a straight bar of stars instead of from the center. About one third of all spiral galaxies are barred spiral in shape. Barred spiral galaxies are represented by the letters SB and are arranges into three subgroups according to the openness of the arms. These subgroups are labeled SBa, SBb, and SBc. SBa galaxies have a short bar of stars extending from the center while SBc galaxies have a long, well-defined bar. Our own Milky Way galaxy is now believed to be a barred spiral galaxy.
Elliptical Galaxy – Elliptical galaxies vary in shape from completely round to extremely elongated ovals. Unlike spiral galaxies, they have no bright nucleus at their center. Elliptical galaxies are represented by the letter E and are divided into seven subgroups according to their shape. These subgroups are labeled E0 to E7. E0 galaxies nearly circular in shape while E7 galaxies are extremely elongated or stretched out.
Irregular Galaxy – A fourth type of galaxy is known as the irregular galaxy. These galaxies have no discernible shape or structure. Irregular galaxies are divided into two classes, Im and IO. Im class galaxies are the most common and show just a hint of structure. Sometimes the faint remnants of spiral arms can be seen. IO class galaxies are completely chaotic in form. The large and small Magellanic Clouds are examples of Im class irregular galaxies.
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