The term was originally coined in 1906 when 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 .
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 .