Other fainter members of the Pleiades ordered (from top to bottom) by brightness. The B8 spectral type is characterised by He I 4471 AA and Mg II 4481 Å displaying similar strengths (18 Tau is the primary standard), but their ratio changes strongly over this temperature range, allowing the definition of spectral type B8.5 (see Appendix B.2). HD 23873 is used as a slowly rotating B9.5 V standard in Fig. E.6. When compared to HD 23873, HD 23568 displays strongly the effects of rotation, although it is still a moderate rotator.