Galaxies are classified using a very simple code that describes their basic shape. “E” is used to describe an elliptical galaxy, and a number from 0 to 7 is added to further define its form. An EO galaxy looks like a ball, whereas an E7 galaxy resembles a short, fat sausage. Spiral galaxies are defined by the letter “S”, and barred spiral galaxies by the letters “BS”. Both these forms of galaxy are given further definition by the addition of a letter a, b, .c or d. Galaxies with tightly wound arms are labelled Sa, and galaxies with looser arms are labelled Sd.
Galaxies are classified by shape. There are three general types: elliptical, spiral, and irregular. Perhaps the most familiar kind of galaxy is spiral galaxies. They have a distinctive shape with spiral arms in a relatively flat disk and a central “bulge”. The bulge has a large concentration of stars. The arms and bulge are surrounded by a faint halo of stars. The bulge and halo consist mainly of older stars, where spiral arms have more gas, dust and younger stars. Our Milky Way Galaxy is a spiral galaxy.
Some spiral galaxies are what we call “barred spirals” because the central bulge looks elongated – like a bar. In barred spirals, the spiral arms of the galaxy appear to spring out of the ends of the bar.
As their name suggests, elliptical galaxies are round or oval, with stars distributed fairly uniformly throughout. They have a bulge and halo, like spiral galaxies, but don’t have the flat disk of stars. The stars in ellipticals tend to be older.
Irregular galaxies have no identifiable shape or structure to them. They are often chaotic in appearance, without a bulge or any trace of spiral arms. The different shapes and orientation of galaxies are a result of their history, which may have included interactions with other galaxies.
Artist’s concept of Edwin Hubble’s galaxy classification system, created to classify galaxies depending on their appearance, This system is sometimes called Hubble’s Tuning Fork. At the left are elliptical galaxies, which are classified depending on how round they appear. The scale goes from E0 (the roundest) to E7, the most elliptical. Further to the right are lenticular galaxies, which are an intermediary class between ellipticals and spirals (classified SA0 or SB0, depending on if they have a bar at the core). To the right are the spiral galaxies, and they are classified depending on how tightly coiled (Sa, Sb and Sc) the spiral arms are (top branch), and if their core hosts a barred shape (bottom branch). The barred galaxies get the classification SBa, SBb and SBc, where SBa has the most tightly coiled arms.
Picture Credit : Google