Spectroscopy of B stars in the very young open star cluster IC 1805

Based on moderate-resolution spectra in the range 4200–5200 Å, we have studied late O and early B stars in the very young open star cluster IC 1805. The temperatures of the stars under the study have been derived by the differential method, in which simple parameters were chosen for a number of spectral lines and were then compared with the analogous parameters from an extensive sample of O–B stars. Accurate $T_{\rm eff}$ estimates have been obtained for them from model atmospheres. Applying this method for the sample of objects under the study has allowed the cluster age $t = 2 \pm 0.5$ Myr and distance modulus $(v$–$V)_{0} = 11.\!\!^{\rm m}5$ to be determined with confidence.