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Classifying galaxies is done according to a system initiated by astronomer Edwin Hubble. His system broke the known galaxies into four major categories. This system is still used today. He named the galaxy categories spiral, barred spiral, elliptical and irregular. Each is defined below.
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.
Barred spiral galaxies are an additional way to distinguish spiral galaxies. These galaxies have a bar-shaped collection of stars running across the center of the galaxy.
Elliptical galaxies lack a disk and arms. They typically have a smooth, oval-shaped appearance. Ellipticals contain old stars and possess little gas or dust. They are recognized by the shape of the ball, which can range from round to oval. The stars in an elliptical do not all revolve around the center in an organized way. The stars move in random orbits within the galaxy.
Irregular galaxies are neither spiral nor elliptical. They tend to be smaller objects without definite shape. Also, they tend to have very hot newer stars mixed in with lots of gas and dust.
Astronomers have added subcategories to the method of classifying galaxies, but every galaxy that we can currently observe clearly falls with these major types.
We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.