The one feature that correlates with the shape, appearance and color of a galaxy is the amount of
current star formation. Stars form when giant clouds of hydrogen gas and dust collapse under their
own gravity. As the cloud collapses it fragments into many smaller pieces, each section continues
to collapse until thermonuclear fusion begins.
Initial conditions for a galaxy determines its rate of star formation. For example, elliptical galaxies
collapse early and form stars quickly. The gas is used up in its early years and today has the
appearance of a smooth, red object with no current star formation.
Spirals, on the other hand, form slower, with lower rates of star formation. The gas that `fuels' star
formation is used slower and, thus, there is plenty around today to continue to form stars within
the spiral arms.
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