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Thanks to spectroscopy, a first star catalogue was published in 1887. With large additions in 1918 and 1924, this extraordinary work gathered 265.000 stars classified in 10 groups according to their surface temperature, (i.e. according to their spectral types). These 10 groups are designated by the following letters: O, B, A, F, G, K, M, R, NR, S, in order of decreasing temperatures.
Adding a digit from 0, 9... to the letters made it possible to define sub categories (thus the sun is classified G5 spectral type with its temperature being 5770 K).
Class O: the spectrum presents ionized helium lines. Temperature of surface: 35.000 K. Example: Ori delta. O stars are blue stars.
Class B: lines of neutral helium and hydrogen known as Balmer lines, temperature of surface: 13.000 K. Examples: Rigel, spica. Blue color also. These absorption lines of hydrogen (found in the atmosphere of almost every star) are of particular importance.
Class A: stronger Balmer lines, line K of ionized calcium. Temperature of surface: 8.000 K. Examples: Sirius, Véga. White or bluish color.
Class F: The hydrogen lines intensity decreases to the benefit of metal origin lines. Temperature of surface: 6.500 K. Example: Procyon. White or bluish color.
Class G: very rich in metal lines spectrum. Solar type stars with a temperature of surface of approximately 5.500 K. Examples: sun, Capella. Yellow color.
Class K: Maximum lines of calcium and metals, the lines of hydrogen disappear. Temperature of surface: 4.000 K. Examples: Arcturus, Aldebaran. These stars are red giants.
Class M: strong lines of molecular bands and neutral metals. Temperature of surface: 2.600 K. Examples: Antares, Béltegeuse. These stars are red super-giants.
Class R and N: not so trong line, same temperature as M stars. Their spectrum is mainly made of carbon. Quite rare stars
Class S: Same temperature as M Stars, lines of Zirconium in their spectrum, very rare.
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