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Though it was discovered as a non-stellar object in 1784 by William Herschel , its status as a globular cluster was not ascertained until 1932 , when Clyde Tombaugh looked at photographic plates taken of the region near Pi Hydrae on 12 May 1931 .
 person: William Herschel; person: Clyde Tombaugh; astronomical object: Pi Hydrae
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
 Entities: William Herschel, Clyde Tombaugh, Pi Hydrae 
Answer:
 person: William Herschel; person: Clyde Tombaugh; astronomical object: Pi Hydrae