|With the probabilities determined
so far, it is possible to make a recalculation of how
many civilizations could have existed since the formation
of the galaxy. From this, a hypothetical rate of
civilization emergence can be determined, which
can provide information about the frequency of
intelligent life in the galaxy.
R is the mean star formation rate per year in our galaxy. Depending on whether you are looking at galaxies, star clusters, or stellar nebulae, the value of R varies between 4 and 19. The mean is then 11,5. 
Tg is the age of our galaxy, at 13.2 billion years.
First, S is the number of stars that have formed since the galaxy was formed.
The next step is to determine the
maximum number of civilizations that could have been
possible since the galaxy was born. In doing so, the
distant earth-like planets are taken as a measure.
Then the number M of intelligent species formed in the time span Tg can be determined as follows
Equation 7.4.2 can still be
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