Classification of Galaxies

Galaxies can be classified according to their shapes: elliptical, spiral, or irregular. Edwin Hubble, after whom the Hubble Space Telescope is named, devised another famous classification scheme after galaxies. Hubble’s system included elliptical and spiral galaxies but excluded irregulars. Today, astronomers use three main galaxy classes: ellipticals, spirals, and irregulars.

Elliptical Galaxies: (“E”) An elliptical galaxy is characterized by a smooth, ball shaped appearance. Ellipticals contain old stars and possess little gas or dust. They are classified by the shape of the ball, which can range from round (E0) to oval (E7) (baseball-shaped to American football-shaped). The smallest elliptical galaxies (called dwarf ellipticals) are probably the most common type of galaxy in the nearby universe. In contrast to spirals, the stars in ellipticals do not revolve around the center in an organized way. The stars move in randomly oriented orbits within the galaxy.

Spiral Galaxies: (“S”) A spiral galaxy consists of a flattened disk containing spiral arms, a central bulge, and a halo. Spiral galaxies have a variety of forms, and they are classified according to the size of the bulge and the tightness and appearance of the arms. The spiral arms, which wrap around the bulge, contain many young blue stars and lots of gas and dust. Stars in the bulge tend to be older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy.

Irregular Galaxies: (“Irr”) An irregular galaxy is neither a spiral nor an elliptical. Irregular galaxies tend to be smaller objects without definite shape, and they typically have very hot newer stars mixed in with lots of gas and dust. These galaxies often have active regions of star formation.