The classification of stars based on their spectral characteristics such as temperature and colour.
Class O stars, temp >=30,000 K, colour = blue
Class B stars, temp 10,000K to 30,000K, colour = blue white
Class A stars, temp 7,500K to 10,000K, colour = white
Class F stars, temp 6,000K to 7,500K, colour = yellow white
Class G stars, temp 5,200K to 6,000K, colour = yellow
Class K stars, temp 3,700K to 5,200K, colour = orange
Class M stars, temp 2,400K to 3,700K, colour = red