While the width and strength of the hydrogen lines is a useful luminosity criterion at B1, the sensitivity of the O II lines (see O II 4070, 4348 and 4416), especially in ratio with the hydrogen lines and the He I lines (which tend to weaken with increasing luminosity) helps to increase the precision of luminosity classification at B1. Note the Si III 4553 line can be used to discriminate between dwarf (V) and (III) classes at this type.
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