Edwin Hubble’s study of galaxies led him to a classification system based on shape and composition into regular and irregular forms.
He found that 97 percent of the regular galaxies had elliptical or spiral shapes. He divided spiral galaxies into two types: normal and barred.
The arms of a normal spiral stretch out from a central, circular core, while those of a barred spiral stretch out from an elongated, bar-shaped core.
Hubble found only 3 percent of the galaxies he studied to be irregular, or having no regular shape or internal structure.
It is now thought that these galaxies lost their shape after a collision with another galaxy.
Hubble knew that the system was not exact because some galaxies have the characteristics of two or more different types.
Hubble delayed accepting a Job at Mount Wilson Observatory for three years while he served in the U.S. army during World War I.
He also served for six years during World War II as a weapons expert.