Horizontal-Branch Stars: Their nature and their absolute magnitude
  Horizontal-branch stars play an important role in defining the absolute distance scale in the universe. For that one has to know Mv at relevant B-V with well defined [M/H] for each type of horizontal-branch star. The nature of the horizontal-branch stars is reviewed with emphasis on all the open questions regarding evolution and structure, questions which show that these stars are not the easiest objects in the distance scale discussions.   Using the HB stars with the best Hipparcos parallaxes, the HB at (B-V)o = 0.20 and [Fe/H] = -1.5 has Mv = +0.71. Assuming the [Fe/H] to Mv relation for RR Lyrae stars to be valid for HB stars too, one finds Mv 1.00 for [Fe/H] = 0. 
Klaas S. de Boer (Sternwarte, University of Bonn, Bonn, Germany)