Abstract: The grand unified theory of particle physics is applied to the supermassive stars,whose energy scale is large enough to take an infinitely collapsing process.A new model is proposed that after nucleon-decays,a supermassive star will convert nearly all its mass into energy,and become a lepton star that might possibly be substable or unstable.According to the model the ultrahigh energy cosmic rays and these puzzles in astrophysics at high energy:quasars and gamma-ray bursts,etc.,may be explained.A lepton star is probably a true white hole.