Based on their spectral features, stars are divided into different spectral types according to the Harvard spectral classification scheme. These spectral types indicate the temperature of the star and form the sequence OBAFGKM (often remembered by the mnemonic ‘Oh Be A Fine Girl/Guy, Kiss Me’) running from the hottest stars to the coolest. Within each spectral type there are significant variations in the strengths of the absorption lines, and each type has been divided into 10 sub-classes numbered 0 to 9. Our Sun, with a temperature of about 5,700 Kelvin has the spectral type G2.