Spirals are disk-like, with arms of young stars and gas winding out from the nucleus.
Elliptical galaxies are spherical or egg-shaped and contain hardly any population I stars because they have essentially no dense interstellar matter from which new stars could form. Irregular galaxies lack a symmetry to their shape but are often extremely rich in interstellar matter and thus contain many young, population I stars. Galaxies form from the collapse of primordial gas clouds. Astronomers do not yet understand why some clouds become spiral and others become elliptical galaxies. Perhaps the initial rotation speed and the rate at which the collapsing gas cloud forms stars play a role.