Stellar classification

Stellar Classification was used to assign classes to stars based on certain characteristics.

Human classification
The Tenth Doctor knew of and used the human designed 'Harvard Spectral Classification' method. Accidentally arriving on the planet Sunday, the Doctor wondered why Arkon (his intended destination) had an orange star, instead of a hot, white class F star. This led him to believe that either the Arkonides had experimented with stellar manipulation, or that he was on the wrong planet. (NSA: Wetworld)

Behind the scenes

 * NSA: Wetworld uses a classification system that includes classes from the Harvard System; however, at no point is it directly referred to as such.