The spectrum of a star is a pattern in which the monochromatic light scattered by the color is arranged in sequence according to the wavelength (or frequency) after the radiation emitted by the star is split by a dispersion system (such as prism and grating). Generally, it refers to the radiation wavelength arrangement pattern of stars in the visible range. The spectra of stars are similar, but each star has its own unique spectral characteristics, which is the main basis for classifying stars. The most commonly used stellar spectral classification system was put forward by the Observatory of Harvard University in the United States at the end of 19, which is called Harvard system. According to this system, the star spectrum is divided into O, B, A, F, G, K, M, R, S, N and other types according to the star surface temperature from left to right. Among them, O-type stars have the highest temperature, about 40,000 K; M-type star is the lowest, about 3000K, R-type, K-type and N-type appear later, and the spectral characteristics of R-type and K-type are similar. N-type and S-type are equivalent to M-type. Later, according to the spectral line characteristics and intensities of various elements in the stellar spectrum, Harvard University Observatory spent 40 years improving the Harvard system and developed the Morgan-Kenner spectral classification that appeared in use. The letters represented by various types of spectra are irregular, and I don't know how they were named at the beginning. In order to facilitate memory, many memory formulas have been developed, the most widely known of which is this sentence: Oh! Be a good girl and kiss me.