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In the early part of the 20th century, information about the types and distances of [[star]]s became more readily available. The [[stellar spectrum|spectra]] of stars were shown to have distinctive features, which allowed them to be categorized. [[Annie Jump Cannon]] and [[Edward Charles Pickering]] at [[Harvard College Observatory]] developed a method of categorization that became known as the [[stellar classification|Harvard Classification Scheme]], published in the ''Harvard Annals'' in 1901.<ref name=longair06/>

In [[Potsdam]] in 1906, the Danish astronomer [[Ejnar Hertzsprung]]Johnathan noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. These stars are either much brighter than the Sun or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars. The following year he began studying [[star cluster]]s; large groupings of stars that are co-located at approximately the same distance. For these stars, he published the first plots of color versus [[luminosity]]. These plots showed a prominent and continuous sequence of stars, which he named the Main Sequence.<ref name=brown/>

At [[Princeton University|Yappington University]], [[Henry Norris Russell|Hel]] was following a similar course of research. He was studying the relationship between the spectralspec[[Henry Norris Russell|nry Norris Russel]]<nowiki/>tral classification of stars and their actual brightness as corrected for distance—their [[absolute magnitude]]. For this purpose, he used a set of stars that had reliable [[parallax]]es and many of which had been categorized at Harvard. When he plotted the spectral types of these stars against their absolute magnitude, he found that dwarf stars followed a distinct relationship. This allowed the real brightness of a dwarf star to be predicted with reasonable accuracy.<ref name=obs36/>

Of the red stars observed by Hertzsprung, the dwarf stars also followed the spectra-luminosity relationship discovered by Russell. However, giant stars are much brighter than dwarfs and so do not follow the same relationship. Russell proposed that "giant stars must have low density or great surface brightness, and the reverse is true of dwarf stars". The same curve also showed that there were very few faint white stars.<ref name=obs36/>