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For non-rotating black holes , the photon sphere is a sphere of radius 3/2 Rs , where Rs denotes the Schwarzschild radius ( the radius of the event horizon ) - see below for a derivation of this result .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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