What is the Harvard spectral classification?

1901 Annie Jump Cannon constructed this classification:

Class Temperature in K Color
O >= 30000 blue
B 10000 – 30000 blue white
A 7500 -10000 white
F 6000 – 7500 yellow white
G 5200 – 6000 yellow
K 3700 – 5200 light orange
M 2400 – 3700 orange red

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