Spiral galaxies have a bulge at the center and a flattened disk containing spiral arms. Spiral galaxies have a variety of shapes and 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 numerous 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. The disks of spiral galaxies rotate somewhat like a hurricane or a whirlpool.
Barred spiral galaxies are spiral galaxies that have a bar-shaped collection of stars running across the center of the galaxy.
Elliptical galaxies do not have a disk or arms. Instead, they are characterized by a smooth, oval-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 to oval (baseball-shaped to football-shaped). In contrast to the disks of spirals, the stars in ellipticals do not all revolve around the center in an organized way. The stars move on randomly oriented orbits within the galaxy, like a swarm of bees.
Irregular galaxies are galaxies that are neither spiral nor elliptical. They tend to be smaller objects that are without definite shape, and tend to have very hot newer stars mixed in with lots of gas and dust.
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