Spirals, on the other hand, are thin disks with a central bulge and arms that wind out from the center. Spirals are designated by the letter S , followed by other letters that describe the size and strength of the spiral arms, as well as the relative size of the nucleus. Irregulars have no trace of circular or rotational symmetry but have an irregular or chaotic appearance. Irregulars are denoted by the letters Irr . The easiest way to learn to distinguish the different types of galaxies is to examine some specific examples.
Lets start with the spiral galaxies, and then move onto the ellipticals and the irregulars.
Ellipticals Elliptical galaxies are spherical or ellipsoidal systems that are thought to consist almost entirely of old stars. They have no trace of spiral arms. They resemble the nucleus and halo components of spiral galaxies. The degree of flattness subdivides the classification into ellipticity groups. The observed classes range from E0 to E7 .