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Part of Hall of the Universe.
AMNH/D. Finnin
The shape of a galaxy is a result of the individual orbits and their collective rotation. The force of gravity from all the stars, gas, dust, and dark matter holds a galaxy together.
The flattened disk of a spiral galaxy is evidence of the rapid revolution of stars, gas, and dust around the center. The orbits are nearly circular and share a common plane.
If, however, the orbits are tipped at all angles to one another, the galaxy has a round and puffy shape. Such an elliptical galaxy rotates very slowly.