In this paper we adopt the theory of Shields and Wheeler (1976) to explain the long-term period and use the Shields and Wheeler’s storage model to calculate the mass of the center black hole for NGC 4151, which shows a period of 14. years (Fan & Su 1998). The mass of the center black hole that the new method gets,～10 6M ⊙,is consistent with other authors. We find the energy of the stable phase, which utilize the period of 14. years to calculate, is closed equal to the burst energy. Other period variation mechanisms have been discussed.
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