In Potsdam in 1906, the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups.
1906年在波茨坦，丹麥的天文學家埃希納·赫茨普龍注意到最紅的那些恆星－在哈佛系統的分類是K和M－可以分為兩個不同的群體。