On the giant, asymptotic and horizontal branches of globular clusters - I. Photographic photometry of M5

B, V photographic photometry has been obtained for stars in the globular cluster M5. All stars with $$B\leqslant18.5$$ and $$2\leqslant r \leqslant 5.6$$ arcmin have been measured. The reduction procedure used is based on a multicomponent bidimensional Gaussian fit to the data derived by scanning the plates with a PDS microphotometer. From the colour–magnitude diagram obtained with a photometric accuracy of about 0.04 mag down to B = 17.5, the following can be deduced: (a) no blue HB stars have been found fainter than V = 16.4; (b) the HB is steeper and slightly redder than the HB found by Arp (1962). Possible explanations for this difference and a detailed analysis of its implications are discussed in the light of evolutionary theories; (c) the RGB and AGB are clearly separated, but the separation in colour is smaller than that found by Simoda & Tanikawa; (d) it follows from the colour width of the RGB that the abundance spread of low ionization potentials elements is smaller than about 35 per cent; (e) R = N(HB)/N(RGB) = 1.20 gives for the helium abundance Y = 0.17 ± 0.03 (with semiconvection), while Y = 0.22 has been derived from the blue edge of the instability strip; (f) the ratios of the numbers of HB, RGB and AGB stars are in good agreement with the proposed evolutionary model taking into account semiconvection and mass loss; (g) structures of the branches and observed and theoretical luminosity functions have been analysed. Explanations for the existence of gaps and clumps on the RGB and AGB are suggested;(h) variables (two new candidates are presented), UV stars and some peculiar objects are also dealt with.