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Library cataloging and classification
main topic	astronomy
Colon	B9
Dewey Decimal	520
Library of Congress	QB
Universal Decimal	52

Wikimedia Commons has media related to Astronomy.

Wiktionary has a category on Astronomy.

Astronomy is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. It includes the study of the origin, evolution, and physical and chemical properties of objects that can be observed in the sky, space, and in the whole universe.