Merge pull request #3519 from cgobat/add-astronomy-topic

Add "astronomy" topic

Author: MikeMcQuaid

topics/astronomy/index.md

@@ -0,0 +1,9 @@
+---
+display_name: Astronomy
+related: astrophysics, physics
+short_description: Astronomy is the study of celestial objects and phenomena.
+topic: astronomy
+wikipedia_url: https://en.wikipedia.org/wiki/Astronomy
+---
+
+Astronomy is an academic discipline and major branch of science that deals with the study of matter, objects, and phenomena in [outer space](https://en.wikipedia.org/wiki/Outer_space) and the Universe. Subfields include astrophysics, planetary science, astrobiology, and astrochemistry.