where is the number of communicative civilizations per galaxy, is the number of stars per galaxy, is the fraction of stars with planets, in the planets per star in the life zone for 4 billion years, is the fraction of suitable planets on which life begins, is the fraction of planets on which life forms evolve to intelligence, and , is the fraction of a star's lifetime for which a technological civilization survives. The equation is also called the Green Bank equation or Sagan equation.