This may be a dumb question but why is the product of all of those numbers equal to the estimated number of alien civilizations? Specifically, why is the lifetime of communicating civilizations (as a raw value) a factor? All other factors make sense to me as they either are probabilities of an alien race making it to that point or are contributions to a rough estimate of the number of alien races out there, but the number of years of a lifetime of the race doesn’t seem to fall into either of the two categories. Am I thinking about this wrong?,(reply>), @bigmacman007 unit analysis is very helpful to approach this type of question...,(reply>), Many of the parameters are just guesses. How could we possibly know how many ET civilizations would choose to communicate? SETI is a non-scientific bunch of guesses. Only astronomers and cosmologists believed in it when SETI came out with the Drake Equation in 1960. Biologists were its harshest critics. Astrobiology has nothing to study except speculation. Look up Carbon and its atomic resonance. It is the only element capable of making the thousands of chemical combinations needed for life in a solvent of water. I'm a retired bio-engineer btw. Maybe another solvent will work it has been speculated, but none found here in a life form.,(reply>), Think of that variable as the duration of our civilization's lifetime, which has everything to do with our chances to encounter alien life someday. To provide an analogy: you're shipwrecked on a small island. The longer you survive, the better chance you have of getting rescued.,(reply>), One lens to look at this for me, is to keep in mind that for the final value to be a dimensionless quantity you need to multiply by time (i.e. variable L in the equation) to cancel out reciprocal time in R* (which is the rate of formation of stars suitable for the development of intelligent life _per year_).,(reply>), because the first value is # of stars born per YEAR. If you multiple by in lifetime of civilizations in years, then the "years" cancel out and you're left with number of civilizations.