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human
[Star Wars] About the construction of the Death Star

In Episode II we see the Separatist Alliance on Geonosis with plans to build the Death Star, but in Rogue One we're told Galen Erso was responsible for it. Is it a plot hole or is there an actual explanation?
gpt
The station itself is rather mundane and just existing technology on a larger scale. What made the death star truly a work of sublime engineering was it's main laser. More powerful than nearly any other weapon devised prior it was the ultimate expression of destruction. The separatists designed it's bones. The empire funded it's construction. Krenik envisioned it's final form. Tarkin put it to use. But Erso was the one who created her power systems of reactors and the exact amount of energy needed to turn Kyber crystals into a planet destroying engine rather than a chaotic mess. 

The separatists wanted a massive battle station but Palpatine and tarkin and krenik wanted more. Galen Erso was the one who made it more than the sum of its parts and ultimately the one who destroyed it and the trillions of credits and dozen years and millions of lives spent to create it.