Audio Guide：

Stellar temperature cannot be measured by a thermometer. Therefore, astronomers can only approximately infer the surface temperature of stars by observing their colors, and then confirm their inference through specific absorption spectrum. In early 20th century, the astronomical observatory of Harvard University conducted a spectral study and classification, which classifies stars into seven types, namely O, B, A, F, G, K and M, according to the colors ranging from the hot blue white stars to the cold red stars.