The Cometary Knots in the Helix Nebula have shown themselves to be formed of a head which is a photoionized surface, optically thick to Lyman continuum radiation, on the central star facing sides of neutral clumps and a cometary tails photionized by the diffuse radiation field. In this study we will present the results of a photoionization model of the heads and the tails using the observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. We obtain a value of the lumionsity of the central star and we find that there is no correlation between the size of the knot and the distance of the central star. We also verify the validity of the "on the spot" approximation usually assumed in planetary nebulae.