Dwarf stars refers to several types of star. The term was originally used in 1906 by the Danish astronomer Ejnar Hertzsprung. He noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. They are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars. The dwarf stars were fainter and the giants brighter than the sun.