def add_galaxy_model(initial_sky_model,model_galaxy):
    """Add an observed galaxy position to the model
    
    :param initial_sky_model: The sky model
    :param model_galaxy: The model of a 'real galaxy' consist a bunch of observations
    """
    initial_sky_model.galax_model_list.append(model_galaxy);
    
    return initial_sky_model;