A mathematical model for the calculation of the optical and thermal properties of an ensemble of gold nanostars with different tip lengths L  =  10 to 20 nm in an aqueous environment is proposed. The heating of a nanoparticle ensemble under irradiation by a laser pulse of a nanosecond duration: resonant λ  =  808  nm and nonresonant λ  =  1064  nm is studied. A significant blue shift of surface plasmon resonance during the formation of a vapor bubble around the nanoparticle was detected and described. Based on the analysis of the kinetics of nanoparticle heating under the action of a laser pulse before and after the formation of a vapor bubble, a theoretical description of the experimentally observed threshold nature of particle photomodification is given. It was found that the laser intensity I_m necessary to achieve the melting temperature T_m (photomodification threshold) tends to increase with decreasing nanostar tip length and for an individual nanostar differs by an order of magnitude. However, up to 45% of the nanoparticles are heated to the melting temperature T_m by a single pulse with an intensity only 2.5 times higher than the minimum value of I_m. At a wavelength of 1064 nm, I_m is ∼6 times higher than at a wavelength of 808 nm.