Most galaxies fall into three kinds: irregular galaxies are small and formless masses of stars, rich in dust and hydrogen and containing mostly younger stars (see below for star classes); conversely, elliptical galaxies don't have much hydrogen or dust, and their stars tend to be very old; finally, spiral galaxies (such as our Milky Way and our closest neighbour, the Andromeda Galaxy) have a complex structure similar to a pinwheel, with a mixture of both young and old stars. About half of the galaxies (but none of the spirals) are dwarf galaxies.