Astro is a pet acquired through

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Astro
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Pet ID: melvorF:Astro
Source:
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Astrology
Drop Chance: See: Acquiring Pets
Effect: +1% chance to gain Golden Stardust in Astrology
Part of 100% Completion: Yes
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Astrology

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There is no best constellation to study to acquire Astro, all constellations are equivalent.