- Star: average rate of star formation in our galaxy.
Star with two planets: fraction of those stars that have planets.
Three planets: average number of planets that can potentially support life per star that has planets.
Cell: fraction of planets that could support life that develop life at some point.
Gear: fraction of planets with life that develop intelligent life.
Radio signal: fraction of civilizations that develop a technology that send detectable signs of their existence into space.
Clock: length of time civilizations broadcast detectable signals into space.
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