Drake Equation  Green Bank Equation  An equation devised by
astronomer and SETI researcher Frank Drake to estimate the
number of civilizations in our Galaxy that might be detectable
across interstellar distances. There are several extant
variations of the equation, but the original form was as
follows:  N = R fp ne fl fi fc L  The main body of the equation
computes the number of technologically advanced civilizations
likely to come into existence in a single year. The final 'L'
factor estimates these civilizations' likely lifespans, and so
approximates the total number of such civilizations that might
be in existence at any particular point. With the exception of R
and fp, all of these factors are highly speculative, but it is
possible to at least produce a range of estimates, and this
process can throw up some interesting considerations.  The
Factors   The annual rate of stellar formation within the
Galaxy. We can place this figure fairly safely in the range 10
to 20, with observational evidence favouring th