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In his famous Equation, Dr. Drake uses as the first variable R*, the rate of star formation in the Galaxy. In helping to film a SETI documentary recently, I was asked why we use the rate of star formation, since it seems more logical to simply use the total number of stars in our Galaxy. I had no answer. Do you? Lee, Arizona
The Doctor Responds:
Consider the Units in which each of the seven Drake Factors are expressed:
Performing a dimensional analysis, you can see that: (1/years) * (ratio) * (unitless number) * (ratio) * (ratio) * (ratio) * (years) = a unitless number. Notice that Years in the denominator of R* cancels Years in the numerator of L. So the answer for N is a pure number, and we have achieved dimensional consistency. If instead of R* we were to use R, the number of stars in the Milky Way Galaxy, then N would come out as the number of civilization years, which means something else entirely.

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